1 .. _whatisrcu_doc: 1 .. _whatisrcu_doc:
2 2
3 What is RCU? -- "Read, Copy, Update" 3 What is RCU? -- "Read, Copy, Update"
4 ====================================== 4 ======================================
5 5
6 Please note that the "What is RCU?" LWN series 6 Please note that the "What is RCU?" LWN series is an excellent place
7 to start learning about RCU: 7 to start learning about RCU:
8 8
9 | 1. What is RCU, Fundamentally? https://l !! 9 | 1. What is RCU, Fundamentally? http://lwn.net/Articles/262464/
10 | 2. What is RCU? Part 2: Usage https://l !! 10 | 2. What is RCU? Part 2: Usage http://lwn.net/Articles/263130/
11 | 3. RCU part 3: the RCU API https://l !! 11 | 3. RCU part 3: the RCU API http://lwn.net/Articles/264090/
12 | 4. The RCU API, 2010 Edition https://l !! 12 | 4. The RCU API, 2010 Edition http://lwn.net/Articles/418853/
13 | 2010 Big API Table https://l !! 13 | 2010 Big API Table http://lwn.net/Articles/419086/
14 | 5. The RCU API, 2014 Edition https://l !! 14 | 5. The RCU API, 2014 Edition http://lwn.net/Articles/609904/
15 | 2014 Big API Table https://l !! 15 | 2014 Big API Table http://lwn.net/Articles/609973/
16 | 6. The RCU API, 2019 Edition https://l <<
17 | 2019 Big API Table https://l <<
18 <<
19 For those preferring video: <<
20 <<
21 | 1. Unraveling RCU Mysteries: Fundamentals <<
22 | 2. Unraveling RCU Mysteries: Additional U <<
23 16
24 17
25 What is RCU? 18 What is RCU?
26 19
27 RCU is a synchronization mechanism that was ad 20 RCU is a synchronization mechanism that was added to the Linux kernel
28 during the 2.5 development effort that is opti 21 during the 2.5 development effort that is optimized for read-mostly
29 situations. Although RCU is actually quite si !! 22 situations. Although RCU is actually quite simple once you understand it,
30 of it requires you to think differently about !! 23 getting there can sometimes be a challenge. Part of the problem is that
31 of the problem is the mistaken assumption that !! 24 most of the past descriptions of RCU have been written with the mistaken
32 describe and to use RCU. Instead, the experie !! 25 assumption that there is "one true way" to describe RCU. Instead,
33 people must take different paths to arrive at !! 26 the experience has been that different people must take different paths
34 depending on their experiences and use cases. !! 27 to arrive at an understanding of RCU. This document provides several
35 several different paths, as follows: !! 28 different paths, as follows:
36 29
37 :ref:`1. RCU OVERVIEW <1_whatisRCU>` 30 :ref:`1. RCU OVERVIEW <1_whatisRCU>`
38 31
39 :ref:`2. WHAT IS RCU'S CORE API? <2_wha 32 :ref:`2. WHAT IS RCU'S CORE API? <2_whatisRCU>`
40 33
41 :ref:`3. WHAT ARE SOME EXAMPLE USES OF 34 :ref:`3. WHAT ARE SOME EXAMPLE USES OF CORE RCU API? <3_whatisRCU>`
42 35
43 :ref:`4. WHAT IF MY UPDATING THREAD CAN 36 :ref:`4. WHAT IF MY UPDATING THREAD CANNOT BLOCK? <4_whatisRCU>`
44 37
45 :ref:`5. WHAT ARE SOME SIMPLE IMPLEMENT 38 :ref:`5. WHAT ARE SOME SIMPLE IMPLEMENTATIONS OF RCU? <5_whatisRCU>`
46 39
47 :ref:`6. ANALOGY WITH READER-WRITER LOC 40 :ref:`6. ANALOGY WITH READER-WRITER LOCKING <6_whatisRCU>`
48 41
49 :ref:`7. ANALOGY WITH REFERENCE COUNTIN !! 42 :ref:`7. FULL LIST OF RCU APIs <7_whatisRCU>`
50 <<
51 :ref:`8. FULL LIST OF RCU APIs <8_whati <<
52 43
53 :ref:`9. ANSWERS TO QUICK QUIZZES <9_wh !! 44 :ref:`8. ANSWERS TO QUICK QUIZZES <8_whatisRCU>`
54 45
55 People who prefer starting with a conceptual o 46 People who prefer starting with a conceptual overview should focus on
56 Section 1, though most readers will profit by 47 Section 1, though most readers will profit by reading this section at
57 some point. People who prefer to start with a 48 some point. People who prefer to start with an API that they can then
58 experiment with should focus on Section 2. Pe 49 experiment with should focus on Section 2. People who prefer to start
59 with example uses should focus on Sections 3 a 50 with example uses should focus on Sections 3 and 4. People who need to
60 understand the RCU implementation should focus 51 understand the RCU implementation should focus on Section 5, then dive
61 into the kernel source code. People who reaso 52 into the kernel source code. People who reason best by analogy should
62 focus on Section 6 and 7. Section 8 serves as !! 53 focus on Section 6. Section 7 serves as an index to the docbook API
63 API documentation, and Section 9 is the tradit !! 54 documentation, and Section 8 is the traditional answer key.
64 55
65 So, start with the section that makes the most 56 So, start with the section that makes the most sense to you and your
66 preferred method of learning. If you need to 57 preferred method of learning. If you need to know everything about
67 everything, feel free to read the whole thing 58 everything, feel free to read the whole thing -- but if you are really
68 that type of person, you have perused the sour 59 that type of person, you have perused the source code and will therefore
69 never need this document anyway. ;-) 60 never need this document anyway. ;-)
70 61
71 .. _1_whatisRCU: 62 .. _1_whatisRCU:
72 63
73 1. RCU OVERVIEW 64 1. RCU OVERVIEW
74 ---------------- 65 ----------------
75 66
76 The basic idea behind RCU is to split updates 67 The basic idea behind RCU is to split updates into "removal" and
77 "reclamation" phases. The removal phase remov 68 "reclamation" phases. The removal phase removes references to data items
78 within a data structure (possibly by replacing 69 within a data structure (possibly by replacing them with references to
79 new versions of these data items), and can run 70 new versions of these data items), and can run concurrently with readers.
80 The reason that it is safe to run the removal 71 The reason that it is safe to run the removal phase concurrently with
81 readers is the semantics of modern CPUs guaran 72 readers is the semantics of modern CPUs guarantee that readers will see
82 either the old or the new version of the data 73 either the old or the new version of the data structure rather than a
83 partially updated reference. The reclamation 74 partially updated reference. The reclamation phase does the work of reclaiming
84 (e.g., freeing) the data items removed from th 75 (e.g., freeing) the data items removed from the data structure during the
85 removal phase. Because reclaiming data items 76 removal phase. Because reclaiming data items can disrupt any readers
86 concurrently referencing those data items, the 77 concurrently referencing those data items, the reclamation phase must
87 not start until readers no longer hold referen 78 not start until readers no longer hold references to those data items.
88 79
89 Splitting the update into removal and reclamat 80 Splitting the update into removal and reclamation phases permits the
90 updater to perform the removal phase immediate 81 updater to perform the removal phase immediately, and to defer the
91 reclamation phase until all readers active dur 82 reclamation phase until all readers active during the removal phase have
92 completed, either by blocking until they finis 83 completed, either by blocking until they finish or by registering a
93 callback that is invoked after they finish. O 84 callback that is invoked after they finish. Only readers that are active
94 during the removal phase need be considered, b 85 during the removal phase need be considered, because any reader starting
95 after the removal phase will be unable to gain 86 after the removal phase will be unable to gain a reference to the removed
96 data items, and therefore cannot be disrupted 87 data items, and therefore cannot be disrupted by the reclamation phase.
97 88
98 So the typical RCU update sequence goes someth 89 So the typical RCU update sequence goes something like the following:
99 90
100 a. Remove pointers to a data structure, s 91 a. Remove pointers to a data structure, so that subsequent
101 readers cannot gain a reference to it. 92 readers cannot gain a reference to it.
102 93
103 b. Wait for all previous readers to compl 94 b. Wait for all previous readers to complete their RCU read-side
104 critical sections. 95 critical sections.
105 96
106 c. At this point, there cannot be any rea 97 c. At this point, there cannot be any readers who hold references
107 to the data structure, so it now may s 98 to the data structure, so it now may safely be reclaimed
108 (e.g., kfree()d). 99 (e.g., kfree()d).
109 100
110 Step (b) above is the key idea underlying RCU' 101 Step (b) above is the key idea underlying RCU's deferred destruction.
111 The ability to wait until all readers are done 102 The ability to wait until all readers are done allows RCU readers to
112 use much lighter-weight synchronization, in so 103 use much lighter-weight synchronization, in some cases, absolutely no
113 synchronization at all. In contrast, in more 104 synchronization at all. In contrast, in more conventional lock-based
114 schemes, readers must use heavy-weight synchro 105 schemes, readers must use heavy-weight synchronization in order to
115 prevent an updater from deleting the data stru 106 prevent an updater from deleting the data structure out from under them.
116 This is because lock-based updaters typically 107 This is because lock-based updaters typically update data items in place,
117 and must therefore exclude readers. In contra 108 and must therefore exclude readers. In contrast, RCU-based updaters
118 typically take advantage of the fact that writ 109 typically take advantage of the fact that writes to single aligned
119 pointers are atomic on modern CPUs, allowing a 110 pointers are atomic on modern CPUs, allowing atomic insertion, removal,
120 and replacement of data items in a linked stru 111 and replacement of data items in a linked structure without disrupting
121 readers. Concurrent RCU readers can then cont 112 readers. Concurrent RCU readers can then continue accessing the old
122 versions, and can dispense with the atomic ope 113 versions, and can dispense with the atomic operations, memory barriers,
123 and communications cache misses that are so ex 114 and communications cache misses that are so expensive on present-day
124 SMP computer systems, even in absence of lock 115 SMP computer systems, even in absence of lock contention.
125 116
126 In the three-step procedure shown above, the u 117 In the three-step procedure shown above, the updater is performing both
127 the removal and the reclamation step, but it i 118 the removal and the reclamation step, but it is often helpful for an
128 entirely different thread to do the reclamatio 119 entirely different thread to do the reclamation, as is in fact the case
129 in the Linux kernel's directory-entry cache (d 120 in the Linux kernel's directory-entry cache (dcache). Even if the same
130 thread performs both the update step (step (a) 121 thread performs both the update step (step (a) above) and the reclamation
131 step (step (c) above), it is often helpful to 122 step (step (c) above), it is often helpful to think of them separately.
132 For example, RCU readers and updaters need not 123 For example, RCU readers and updaters need not communicate at all,
133 but RCU provides implicit low-overhead communi 124 but RCU provides implicit low-overhead communication between readers
134 and reclaimers, namely, in step (b) above. 125 and reclaimers, namely, in step (b) above.
135 126
136 So how the heck can a reclaimer tell when a re 127 So how the heck can a reclaimer tell when a reader is done, given
137 that readers are not doing any sort of synchro 128 that readers are not doing any sort of synchronization operations???
138 Read on to learn about how RCU's API makes thi 129 Read on to learn about how RCU's API makes this easy.
139 130
140 .. _2_whatisRCU: 131 .. _2_whatisRCU:
141 132
142 2. WHAT IS RCU'S CORE API? 133 2. WHAT IS RCU'S CORE API?
143 --------------------------- 134 ---------------------------
144 135
145 The core RCU API is quite small: 136 The core RCU API is quite small:
146 137
147 a. rcu_read_lock() 138 a. rcu_read_lock()
148 b. rcu_read_unlock() 139 b. rcu_read_unlock()
149 c. synchronize_rcu() / call_rcu() 140 c. synchronize_rcu() / call_rcu()
150 d. rcu_assign_pointer() 141 d. rcu_assign_pointer()
151 e. rcu_dereference() 142 e. rcu_dereference()
152 143
153 There are many other members of the RCU API, b 144 There are many other members of the RCU API, but the rest can be
154 expressed in terms of these five, though most 145 expressed in terms of these five, though most implementations instead
155 express synchronize_rcu() in terms of the call 146 express synchronize_rcu() in terms of the call_rcu() callback API.
156 147
157 The five core RCU APIs are described below, th 148 The five core RCU APIs are described below, the other 18 will be enumerated
158 later. See the kernel docbook documentation f 149 later. See the kernel docbook documentation for more info, or look directly
159 at the function header comments. 150 at the function header comments.
160 151
161 rcu_read_lock() 152 rcu_read_lock()
162 ^^^^^^^^^^^^^^^ 153 ^^^^^^^^^^^^^^^
163 void rcu_read_lock(void); 154 void rcu_read_lock(void);
164 155
165 This temporal primitive is used by a r !! 156 Used by a reader to inform the reclaimer that the reader is
166 reclaimer that the reader is entering !! 157 entering an RCU read-side critical section. It is illegal
167 section. It is illegal to block while !! 158 to block while in an RCU read-side critical section, though
168 critical section, though kernels built !! 159 kernels built with CONFIG_PREEMPT_RCU can preempt RCU
169 can preempt RCU read-side critical sec !! 160 read-side critical sections. Any RCU-protected data structure
170 data structure accessed during an RCU !! 161 accessed during an RCU read-side critical section is guaranteed to
171 is guaranteed to remain unreclaimed fo !! 162 remain unreclaimed for the full duration of that critical section.
172 critical section. Reference counts ma !! 163 Reference counts may be used in conjunction with RCU to maintain
173 with RCU to maintain longer-term refer !! 164 longer-term references to data structures.
174 <<
175 Note that anything that disables botto <<
176 or interrupts also enters an RCU read- <<
177 Acquiring a spinlock also enters an RC <<
178 sections, even for spinlocks that do n <<
179 as is the case in kernels built with C <<
180 Sleeplocks do *not* enter RCU read-sid <<
181 165
182 rcu_read_unlock() 166 rcu_read_unlock()
183 ^^^^^^^^^^^^^^^^^ 167 ^^^^^^^^^^^^^^^^^
184 void rcu_read_unlock(void); 168 void rcu_read_unlock(void);
185 169
186 This temporal primitives is used by a !! 170 Used by a reader to inform the reclaimer that the reader is
187 reclaimer that the reader is exiting a !! 171 exiting an RCU read-side critical section. Note that RCU
188 section. Anything that enables bottom !! 172 read-side critical sections may be nested and/or overlapping.
189 or interrupts also exits an RCU read-s <<
190 Releasing a spinlock also exits an RCU <<
191 <<
192 Note that RCU read-side critical secti <<
193 overlapping. <<
194 173
195 synchronize_rcu() 174 synchronize_rcu()
196 ^^^^^^^^^^^^^^^^^ 175 ^^^^^^^^^^^^^^^^^
197 void synchronize_rcu(void); 176 void synchronize_rcu(void);
198 177
199 This temporal primitive marks the end !! 178 Marks the end of updater code and the beginning of reclaimer
200 beginning of reclaimer code. It does !! 179 code. It does this by blocking until all pre-existing RCU
201 all pre-existing RCU read-side critica !! 180 read-side critical sections on all CPUs have completed.
202 have completed. Note that synchronize !! 181 Note that synchronize_rcu() will **not** necessarily wait for
203 necessarily wait for any subsequent RC !! 182 any subsequent RCU read-side critical sections to complete.
204 sections to complete. For example, co !! 183 For example, consider the following sequence of events::
205 sequence of events:: <<
206 184
207 CPU 0 CPU 1 185 CPU 0 CPU 1 CPU 2
208 ----------------- --------------- 186 ----------------- ------------------------- ---------------
209 1. rcu_read_lock() 187 1. rcu_read_lock()
210 2. enters synchron 188 2. enters synchronize_rcu()
211 3. 189 3. rcu_read_lock()
212 4. rcu_read_unlock() 190 4. rcu_read_unlock()
213 5. exits synchron 191 5. exits synchronize_rcu()
214 6. 192 6. rcu_read_unlock()
215 193
216 To reiterate, synchronize_rcu() waits 194 To reiterate, synchronize_rcu() waits only for ongoing RCU
217 read-side critical sections to complet 195 read-side critical sections to complete, not necessarily for
218 any that begin after synchronize_rcu() 196 any that begin after synchronize_rcu() is invoked.
219 197
220 Of course, synchronize_rcu() does not 198 Of course, synchronize_rcu() does not necessarily return
221 **immediately** after the last pre-exi 199 **immediately** after the last pre-existing RCU read-side critical
222 section completes. For one thing, the 200 section completes. For one thing, there might well be scheduling
223 delays. For another thing, many RCU i 201 delays. For another thing, many RCU implementations process
224 requests in batches in order to improv 202 requests in batches in order to improve efficiencies, which can
225 further delay synchronize_rcu(). 203 further delay synchronize_rcu().
226 204
227 Since synchronize_rcu() is the API tha 205 Since synchronize_rcu() is the API that must figure out when
228 readers are done, its implementation i 206 readers are done, its implementation is key to RCU. For RCU
229 to be useful in all but the most read- 207 to be useful in all but the most read-intensive situations,
230 synchronize_rcu()'s overhead must also 208 synchronize_rcu()'s overhead must also be quite small.
231 209
232 The call_rcu() API is an asynchronous !! 210 The call_rcu() API is a callback form of synchronize_rcu(),
233 synchronize_rcu(), and is described in !! 211 and is described in more detail in a later section. Instead of
234 section. Instead of blocking, it regi !! 212 blocking, it registers a function and argument which are invoked
235 argument which are invoked after all o !! 213 after all ongoing RCU read-side critical sections have completed.
236 critical sections have completed. Thi !! 214 This callback variant is particularly useful in situations where
237 particularly useful in situations wher !! 215 it is illegal to block or where update-side performance is
238 or where update-side performance is cr !! 216 critically important.
239 217
240 However, the call_rcu() API should not 218 However, the call_rcu() API should not be used lightly, as use
241 of the synchronize_rcu() API generally 219 of the synchronize_rcu() API generally results in simpler code.
242 In addition, the synchronize_rcu() API 220 In addition, the synchronize_rcu() API has the nice property
243 of automatically limiting update rate 221 of automatically limiting update rate should grace periods
244 be delayed. This property results in 222 be delayed. This property results in system resilience in face
245 of denial-of-service attacks. Code us 223 of denial-of-service attacks. Code using call_rcu() should limit
246 update rate in order to gain this same 224 update rate in order to gain this same sort of resilience. See
247 checklist.rst for some approaches to l !! 225 checklist.txt for some approaches to limiting the update rate.
248 226
249 rcu_assign_pointer() 227 rcu_assign_pointer()
250 ^^^^^^^^^^^^^^^^^^^^ 228 ^^^^^^^^^^^^^^^^^^^^
251 void rcu_assign_pointer(p, typeof(p) v 229 void rcu_assign_pointer(p, typeof(p) v);
252 230
253 Yes, rcu_assign_pointer() **is** imple !! 231 Yes, rcu_assign_pointer() **is** implemented as a macro, though it
254 it would be cool to be able to declare !! 232 would be cool to be able to declare a function in this manner.
255 (And there has been some discussion of !! 233 (Compiler experts will no doubt disagree.)
256 to the C language, so who knows?) <<
257 234
258 The updater uses this spatial macro to !! 235 The updater uses this function to assign a new value to an
259 RCU-protected pointer, in order to saf 236 RCU-protected pointer, in order to safely communicate the change
260 in value from the updater to the reade !! 237 in value from the updater to the reader. This macro does not
261 opposed to temporal) macro. It does n !! 238 evaluate to an rvalue, but it does execute any memory-barrier
262 but it does provide any compiler direc !! 239 instructions required for a given CPU architecture.
263 instructions required for a given comp !! 240
264 Its ordering properties are that of a !! 241 Perhaps just as important, it serves to document (1) which
265 that is, any prior loads and stores re !! 242 pointers are protected by RCU and (2) the point at which a
266 structure are ordered before the store !! 243 given structure becomes accessible to other CPUs. That said,
267 to that structure. <<
268 <<
269 Perhaps just as important, rcu_assign_ <<
270 (1) which pointers are protected by RC <<
271 a given structure becomes accessible t <<
272 rcu_assign_pointer() is most frequentl 244 rcu_assign_pointer() is most frequently used indirectly, via
273 the _rcu list-manipulation primitives 245 the _rcu list-manipulation primitives such as list_add_rcu().
274 246
275 rcu_dereference() 247 rcu_dereference()
276 ^^^^^^^^^^^^^^^^^ 248 ^^^^^^^^^^^^^^^^^
277 typeof(p) rcu_dereference(p); 249 typeof(p) rcu_dereference(p);
278 250
279 Like rcu_assign_pointer(), rcu_derefer 251 Like rcu_assign_pointer(), rcu_dereference() must be implemented
280 as a macro. 252 as a macro.
281 253
282 The reader uses the spatial rcu_derefe !! 254 The reader uses rcu_dereference() to fetch an RCU-protected
283 an RCU-protected pointer, which return !! 255 pointer, which returns a value that may then be safely
284 then be safely dereferenced. Note tha !! 256 dereferenced. Note that rcu_dereference() does not actually
285 does not actually dereference the poin !! 257 dereference the pointer, instead, it protects the pointer for
286 protects the pointer for later derefer !! 258 later dereferencing. It also executes any needed memory-barrier
287 executes any needed memory-barrier ins !! 259 instructions for a given CPU architecture. Currently, only Alpha
288 CPU architecture. Currently, only Alp !! 260 needs memory barriers within rcu_dereference() -- on other CPUs,
289 within rcu_dereference() -- on other C !! 261 it compiles to nothing, not even a compiler directive.
290 volatile load. However, no mainstream <<
291 address dependencies, so rcu_dereferen <<
292 which, in combination with the coding <<
293 rcu_dereference.rst, prevent current c <<
294 these dependencies. <<
295 262
296 Common coding practice uses rcu_derefe 263 Common coding practice uses rcu_dereference() to copy an
297 RCU-protected pointer to a local varia 264 RCU-protected pointer to a local variable, then dereferences
298 this local variable, for example as fo 265 this local variable, for example as follows::
299 266
300 p = rcu_dereference(head.next) 267 p = rcu_dereference(head.next);
301 return p->data; 268 return p->data;
302 269
303 However, in this case, one could just 270 However, in this case, one could just as easily combine these
304 into one statement:: 271 into one statement::
305 272
306 return rcu_dereference(head.ne 273 return rcu_dereference(head.next)->data;
307 274
308 If you are going to be fetching multip 275 If you are going to be fetching multiple fields from the
309 RCU-protected structure, using the loc 276 RCU-protected structure, using the local variable is of
310 course preferred. Repeated rcu_derefe 277 course preferred. Repeated rcu_dereference() calls look
311 ugly, do not guarantee that the same p 278 ugly, do not guarantee that the same pointer will be returned
312 if an update happened while in the cri 279 if an update happened while in the critical section, and incur
313 unnecessary overhead on Alpha CPUs. 280 unnecessary overhead on Alpha CPUs.
314 281
315 Note that the value returned by rcu_de 282 Note that the value returned by rcu_dereference() is valid
316 only within the enclosing RCU read-sid 283 only within the enclosing RCU read-side critical section [1]_.
317 For example, the following is **not** 284 For example, the following is **not** legal::
318 285
319 rcu_read_lock(); 286 rcu_read_lock();
320 p = rcu_dereference(head.next) 287 p = rcu_dereference(head.next);
321 rcu_read_unlock(); 288 rcu_read_unlock();
322 x = p->address; /* BUG!!! */ 289 x = p->address; /* BUG!!! */
323 rcu_read_lock(); 290 rcu_read_lock();
324 y = p->data; /* BUG!!! */ 291 y = p->data; /* BUG!!! */
325 rcu_read_unlock(); 292 rcu_read_unlock();
326 293
327 Holding a reference from one RCU read- 294 Holding a reference from one RCU read-side critical section
328 to another is just as illegal as holdi 295 to another is just as illegal as holding a reference from
329 one lock-based critical section to ano 296 one lock-based critical section to another! Similarly,
330 using a reference outside of the criti 297 using a reference outside of the critical section in which
331 it was acquired is just as illegal as 298 it was acquired is just as illegal as doing so with normal
332 locking. 299 locking.
333 300
334 As with rcu_assign_pointer(), an impor 301 As with rcu_assign_pointer(), an important function of
335 rcu_dereference() is to document which 302 rcu_dereference() is to document which pointers are protected by
336 RCU, in particular, flagging a pointer 303 RCU, in particular, flagging a pointer that is subject to changing
337 at any time, including immediately aft 304 at any time, including immediately after the rcu_dereference().
338 And, again like rcu_assign_pointer(), 305 And, again like rcu_assign_pointer(), rcu_dereference() is
339 typically used indirectly, via the _rc 306 typically used indirectly, via the _rcu list-manipulation
340 primitives, such as list_for_each_entr 307 primitives, such as list_for_each_entry_rcu() [2]_.
341 308
342 .. [1] The variant rcu_dereference_protec 309 .. [1] The variant rcu_dereference_protected() can be used outside
343 of an RCU read-side critical section a 310 of an RCU read-side critical section as long as the usage is
344 protected by locks acquired by the upd 311 protected by locks acquired by the update-side code. This variant
345 avoids the lockdep warning that would 312 avoids the lockdep warning that would happen when using (for
346 example) rcu_dereference() without rcu 313 example) rcu_dereference() without rcu_read_lock() protection.
347 Using rcu_dereference_protected() also 314 Using rcu_dereference_protected() also has the advantage
348 of permitting compiler optimizations t 315 of permitting compiler optimizations that rcu_dereference()
349 must prohibit. The rcu_dereference_pr 316 must prohibit. The rcu_dereference_protected() variant takes
350 a lockdep expression to indicate which 317 a lockdep expression to indicate which locks must be acquired
351 by the caller. If the indicated protec 318 by the caller. If the indicated protection is not provided,
352 a lockdep splat is emitted. See Desig !! 319 a lockdep splat is emitted. See Documentation/RCU/Design/Requirements/Requirements.rst
353 and the API's code comments for more d 320 and the API's code comments for more details and example usage.
354 321
355 .. [2] If the list_for_each_entry_rcu() i 322 .. [2] If the list_for_each_entry_rcu() instance might be used by
356 update-side code as well as by RCU rea 323 update-side code as well as by RCU readers, then an additional
357 lockdep expression can be added to its 324 lockdep expression can be added to its list of arguments.
358 For example, given an additional "lock 325 For example, given an additional "lock_is_held(&mylock)" argument,
359 the RCU lockdep code would complain on 326 the RCU lockdep code would complain only if this instance was
360 invoked outside of an RCU read-side cr 327 invoked outside of an RCU read-side critical section and without
361 the protection of mylock. 328 the protection of mylock.
362 329
363 The following diagram shows how each API commu 330 The following diagram shows how each API communicates among the
364 reader, updater, and reclaimer. 331 reader, updater, and reclaimer.
365 :: 332 ::
366 333
367 334
368 rcu_assign_pointer() 335 rcu_assign_pointer()
369 +--------+ 336 +--------+
370 +---------------------->| reader | 337 +---------------------->| reader |---------+
371 | +--------+ 338 | +--------+ |
372 | | 339 | | |
373 | | 340 | | | Protect:
374 | | 341 | | | rcu_read_lock()
375 | | 342 | | | rcu_read_unlock()
376 | rcu_dereference() | 343 | rcu_dereference() | |
377 +---------+ | 344 +---------+ | |
378 | updater |<----------------+ 345 | updater |<----------------+ |
379 +---------+ 346 +---------+ V
380 | 347 | +-----------+
381 +--------------------------------- 348 +----------------------------------->| reclaimer |
382 349 +-----------+
383 Defer: 350 Defer:
384 synchronize_rcu() & call_rcu() 351 synchronize_rcu() & call_rcu()
385 352
386 353
387 The RCU infrastructure observes the temporal s !! 354 The RCU infrastructure observes the time sequence of rcu_read_lock(),
388 rcu_read_unlock(), synchronize_rcu(), and call 355 rcu_read_unlock(), synchronize_rcu(), and call_rcu() invocations in
389 order to determine when (1) synchronize_rcu() 356 order to determine when (1) synchronize_rcu() invocations may return
390 to their callers and (2) call_rcu() callbacks 357 to their callers and (2) call_rcu() callbacks may be invoked. Efficient
391 implementations of the RCU infrastructure make 358 implementations of the RCU infrastructure make heavy use of batching in
392 order to amortize their overhead over many use 359 order to amortize their overhead over many uses of the corresponding APIs.
393 The rcu_assign_pointer() and rcu_dereference() <<
394 spatial changes via stores to and loads from t <<
395 question. <<
396 360
397 There are at least three flavors of RCU usage 361 There are at least three flavors of RCU usage in the Linux kernel. The diagram
398 above shows the most common one. On the update 362 above shows the most common one. On the updater side, the rcu_assign_pointer(),
399 synchronize_rcu() and call_rcu() primitives us 363 synchronize_rcu() and call_rcu() primitives used are the same for all three
400 flavors. However for protection (on the reader 364 flavors. However for protection (on the reader side), the primitives used vary
401 depending on the flavor: 365 depending on the flavor:
402 366
403 a. rcu_read_lock() / rcu_read_unlock() 367 a. rcu_read_lock() / rcu_read_unlock()
404 rcu_dereference() 368 rcu_dereference()
405 369
406 b. rcu_read_lock_bh() / rcu_read_unlock_b 370 b. rcu_read_lock_bh() / rcu_read_unlock_bh()
407 local_bh_disable() / local_bh_enable() 371 local_bh_disable() / local_bh_enable()
408 rcu_dereference_bh() 372 rcu_dereference_bh()
409 373
410 c. rcu_read_lock_sched() / rcu_read_unloc 374 c. rcu_read_lock_sched() / rcu_read_unlock_sched()
411 preempt_disable() / preempt_enable() 375 preempt_disable() / preempt_enable()
412 local_irq_save() / local_irq_restore() 376 local_irq_save() / local_irq_restore()
413 hardirq enter / hardirq exit 377 hardirq enter / hardirq exit
414 NMI enter / NMI exit 378 NMI enter / NMI exit
415 rcu_dereference_sched() 379 rcu_dereference_sched()
416 380
417 These three flavors are used as follows: 381 These three flavors are used as follows:
418 382
419 a. RCU applied to normal data structures. 383 a. RCU applied to normal data structures.
420 384
421 b. RCU applied to networking data structu 385 b. RCU applied to networking data structures that may be subjected
422 to remote denial-of-service attacks. 386 to remote denial-of-service attacks.
423 387
424 c. RCU applied to scheduler and interrupt 388 c. RCU applied to scheduler and interrupt/NMI-handler tasks.
425 389
426 Again, most uses will be of (a). The (b) and 390 Again, most uses will be of (a). The (b) and (c) cases are important
427 for specialized uses, but are relatively uncom !! 391 for specialized uses, but are relatively uncommon.
428 RCU-Tasks-Rude, and RCU-Tasks-Trace have simil <<
429 their assorted primitives. <<
430 392
431 .. _3_whatisRCU: 393 .. _3_whatisRCU:
432 394
433 3. WHAT ARE SOME EXAMPLE USES OF CORE RCU API 395 3. WHAT ARE SOME EXAMPLE USES OF CORE RCU API?
434 ---------------------------------------------- 396 -----------------------------------------------
435 397
436 This section shows a simple use of the core RC 398 This section shows a simple use of the core RCU API to protect a
437 global pointer to a dynamically allocated stru 399 global pointer to a dynamically allocated structure. More-typical
438 uses of RCU may be found in listRCU.rst and NM !! 400 uses of RCU may be found in :ref:`listRCU.rst <list_rcu_doc>`,
>> 401 :ref:`arrayRCU.rst <array_rcu_doc>`, and :ref:`NMI-RCU.rst <NMI_rcu_doc>`.
439 :: 402 ::
440 403
441 struct foo { 404 struct foo {
442 int a; 405 int a;
443 char b; 406 char b;
444 long c; 407 long c;
445 }; 408 };
446 DEFINE_SPINLOCK(foo_mutex); 409 DEFINE_SPINLOCK(foo_mutex);
447 410
448 struct foo __rcu *gbl_foo; 411 struct foo __rcu *gbl_foo;
449 412
450 /* 413 /*
451 * Create a new struct foo that is the 414 * Create a new struct foo that is the same as the one currently
452 * pointed to by gbl_foo, except that 415 * pointed to by gbl_foo, except that field "a" is replaced
453 * with "new_a". Points gbl_foo to th 416 * with "new_a". Points gbl_foo to the new structure, and
454 * frees up the old structure after a 417 * frees up the old structure after a grace period.
455 * 418 *
456 * Uses rcu_assign_pointer() to ensure 419 * Uses rcu_assign_pointer() to ensure that concurrent readers
457 * see the initialized version of the 420 * see the initialized version of the new structure.
458 * 421 *
459 * Uses synchronize_rcu() to ensure th 422 * Uses synchronize_rcu() to ensure that any readers that might
460 * have references to the old structur 423 * have references to the old structure complete before freeing
461 * the old structure. 424 * the old structure.
462 */ 425 */
463 void foo_update_a(int new_a) 426 void foo_update_a(int new_a)
464 { 427 {
465 struct foo *new_fp; 428 struct foo *new_fp;
466 struct foo *old_fp; 429 struct foo *old_fp;
467 430
468 new_fp = kmalloc(sizeof(*new_f 431 new_fp = kmalloc(sizeof(*new_fp), GFP_KERNEL);
469 spin_lock(&foo_mutex); 432 spin_lock(&foo_mutex);
470 old_fp = rcu_dereference_prote 433 old_fp = rcu_dereference_protected(gbl_foo, lockdep_is_held(&foo_mutex));
471 *new_fp = *old_fp; 434 *new_fp = *old_fp;
472 new_fp->a = new_a; 435 new_fp->a = new_a;
473 rcu_assign_pointer(gbl_foo, ne 436 rcu_assign_pointer(gbl_foo, new_fp);
474 spin_unlock(&foo_mutex); 437 spin_unlock(&foo_mutex);
475 synchronize_rcu(); 438 synchronize_rcu();
476 kfree(old_fp); 439 kfree(old_fp);
477 } 440 }
478 441
479 /* 442 /*
480 * Return the value of field "a" of th 443 * Return the value of field "a" of the current gbl_foo
481 * structure. Use rcu_read_lock() and 444 * structure. Use rcu_read_lock() and rcu_read_unlock()
482 * to ensure that the structure does n 445 * to ensure that the structure does not get deleted out
483 * from under us, and use rcu_derefere 446 * from under us, and use rcu_dereference() to ensure that
484 * we see the initialized version of t 447 * we see the initialized version of the structure (important
485 * for DEC Alpha and for people readin 448 * for DEC Alpha and for people reading the code).
486 */ 449 */
487 int foo_get_a(void) 450 int foo_get_a(void)
488 { 451 {
489 int retval; 452 int retval;
490 453
491 rcu_read_lock(); 454 rcu_read_lock();
492 retval = rcu_dereference(gbl_f 455 retval = rcu_dereference(gbl_foo)->a;
493 rcu_read_unlock(); 456 rcu_read_unlock();
494 return retval; 457 return retval;
495 } 458 }
496 459
497 So, to sum up: 460 So, to sum up:
498 461
499 - Use rcu_read_lock() and rcu_read_unloc 462 - Use rcu_read_lock() and rcu_read_unlock() to guard RCU
500 read-side critical sections. 463 read-side critical sections.
501 464
502 - Within an RCU read-side critical secti 465 - Within an RCU read-side critical section, use rcu_dereference()
503 to dereference RCU-protected pointers. 466 to dereference RCU-protected pointers.
504 467
505 - Use some solid design (such as locks o !! 468 - Use some solid scheme (such as locks or semaphores) to
506 keep concurrent updates from interferi 469 keep concurrent updates from interfering with each other.
507 470
508 - Use rcu_assign_pointer() to update an 471 - Use rcu_assign_pointer() to update an RCU-protected pointer.
509 This primitive protects concurrent rea 472 This primitive protects concurrent readers from the updater,
510 **not** concurrent updates from each o 473 **not** concurrent updates from each other! You therefore still
511 need to use locking (or something simi 474 need to use locking (or something similar) to keep concurrent
512 rcu_assign_pointer() primitives from i 475 rcu_assign_pointer() primitives from interfering with each other.
513 476
514 - Use synchronize_rcu() **after** removi 477 - Use synchronize_rcu() **after** removing a data element from an
515 RCU-protected data structure, but **be 478 RCU-protected data structure, but **before** reclaiming/freeing
516 the data element, in order to wait for 479 the data element, in order to wait for the completion of all
517 RCU read-side critical sections that m 480 RCU read-side critical sections that might be referencing that
518 data item. 481 data item.
519 482
520 See checklist.rst for additional rules to foll !! 483 See checklist.txt for additional rules to follow when using RCU.
521 And again, more-typical uses of RCU may be fou !! 484 And again, more-typical uses of RCU may be found in :ref:`listRCU.rst
522 and NMI-RCU.rst. !! 485 <list_rcu_doc>`, :ref:`arrayRCU.rst <array_rcu_doc>`, and :ref:`NMI-RCU.rst
>> 486 <NMI_rcu_doc>`.
523 487
524 .. _4_whatisRCU: 488 .. _4_whatisRCU:
525 489
526 4. WHAT IF MY UPDATING THREAD CANNOT BLOCK? 490 4. WHAT IF MY UPDATING THREAD CANNOT BLOCK?
527 -------------------------------------------- 491 --------------------------------------------
528 492
529 In the example above, foo_update_a() blocks un 493 In the example above, foo_update_a() blocks until a grace period elapses.
530 This is quite simple, but in some cases one ca 494 This is quite simple, but in some cases one cannot afford to wait so
531 long -- there might be other high-priority wor 495 long -- there might be other high-priority work to be done.
532 496
533 In such cases, one uses call_rcu() rather than 497 In such cases, one uses call_rcu() rather than synchronize_rcu().
534 The call_rcu() API is as follows:: 498 The call_rcu() API is as follows::
535 499
536 void call_rcu(struct rcu_head *head, r 500 void call_rcu(struct rcu_head *head, rcu_callback_t func);
537 501
538 This function invokes func(head) after a grace 502 This function invokes func(head) after a grace period has elapsed.
539 This invocation might happen from either softi 503 This invocation might happen from either softirq or process context,
540 so the function is not permitted to block. Th 504 so the function is not permitted to block. The foo struct needs to
541 have an rcu_head structure added, perhaps as f 505 have an rcu_head structure added, perhaps as follows::
542 506
543 struct foo { 507 struct foo {
544 int a; 508 int a;
545 char b; 509 char b;
546 long c; 510 long c;
547 struct rcu_head rcu; 511 struct rcu_head rcu;
548 }; 512 };
549 513
550 The foo_update_a() function might then be writ 514 The foo_update_a() function might then be written as follows::
551 515
552 /* 516 /*
553 * Create a new struct foo that is the 517 * Create a new struct foo that is the same as the one currently
554 * pointed to by gbl_foo, except that 518 * pointed to by gbl_foo, except that field "a" is replaced
555 * with "new_a". Points gbl_foo to th 519 * with "new_a". Points gbl_foo to the new structure, and
556 * frees up the old structure after a 520 * frees up the old structure after a grace period.
557 * 521 *
558 * Uses rcu_assign_pointer() to ensure 522 * Uses rcu_assign_pointer() to ensure that concurrent readers
559 * see the initialized version of the 523 * see the initialized version of the new structure.
560 * 524 *
561 * Uses call_rcu() to ensure that any 525 * Uses call_rcu() to ensure that any readers that might have
562 * references to the old structure com 526 * references to the old structure complete before freeing the
563 * old structure. 527 * old structure.
564 */ 528 */
565 void foo_update_a(int new_a) 529 void foo_update_a(int new_a)
566 { 530 {
567 struct foo *new_fp; 531 struct foo *new_fp;
568 struct foo *old_fp; 532 struct foo *old_fp;
569 533
570 new_fp = kmalloc(sizeof(*new_f 534 new_fp = kmalloc(sizeof(*new_fp), GFP_KERNEL);
571 spin_lock(&foo_mutex); 535 spin_lock(&foo_mutex);
572 old_fp = rcu_dereference_prote 536 old_fp = rcu_dereference_protected(gbl_foo, lockdep_is_held(&foo_mutex));
573 *new_fp = *old_fp; 537 *new_fp = *old_fp;
574 new_fp->a = new_a; 538 new_fp->a = new_a;
575 rcu_assign_pointer(gbl_foo, ne 539 rcu_assign_pointer(gbl_foo, new_fp);
576 spin_unlock(&foo_mutex); 540 spin_unlock(&foo_mutex);
577 call_rcu(&old_fp->rcu, foo_rec 541 call_rcu(&old_fp->rcu, foo_reclaim);
578 } 542 }
579 543
580 The foo_reclaim() function might appear as fol 544 The foo_reclaim() function might appear as follows::
581 545
582 void foo_reclaim(struct rcu_head *rp) 546 void foo_reclaim(struct rcu_head *rp)
583 { 547 {
584 struct foo *fp = container_of( 548 struct foo *fp = container_of(rp, struct foo, rcu);
585 549
586 foo_cleanup(fp->a); 550 foo_cleanup(fp->a);
587 551
588 kfree(fp); 552 kfree(fp);
589 } 553 }
590 554
591 The container_of() primitive is a macro that, 555 The container_of() primitive is a macro that, given a pointer into a
592 struct, the type of the struct, and the pointe 556 struct, the type of the struct, and the pointed-to field within the
593 struct, returns a pointer to the beginning of 557 struct, returns a pointer to the beginning of the struct.
594 558
595 The use of call_rcu() permits the caller of fo 559 The use of call_rcu() permits the caller of foo_update_a() to
596 immediately regain control, without needing to 560 immediately regain control, without needing to worry further about the
597 old version of the newly updated element. It 561 old version of the newly updated element. It also clearly shows the
598 RCU distinction between updater, namely foo_up 562 RCU distinction between updater, namely foo_update_a(), and reclaimer,
599 namely foo_reclaim(). 563 namely foo_reclaim().
600 564
601 The summary of advice is the same as for the p 565 The summary of advice is the same as for the previous section, except
602 that we are now using call_rcu() rather than s 566 that we are now using call_rcu() rather than synchronize_rcu():
603 567
604 - Use call_rcu() **after** removing a da 568 - Use call_rcu() **after** removing a data element from an
605 RCU-protected data structure in order 569 RCU-protected data structure in order to register a callback
606 function that will be invoked after th 570 function that will be invoked after the completion of all RCU
607 read-side critical sections that might 571 read-side critical sections that might be referencing that
608 data item. 572 data item.
609 573
610 If the callback for call_rcu() is not doing an 574 If the callback for call_rcu() is not doing anything more than calling
611 kfree() on the structure, you can use kfree_rc 575 kfree() on the structure, you can use kfree_rcu() instead of call_rcu()
612 to avoid having to write your own callback:: 576 to avoid having to write your own callback::
613 577
614 kfree_rcu(old_fp, rcu); 578 kfree_rcu(old_fp, rcu);
615 579
616 If the occasional sleep is permitted, the sing !! 580 Again, see checklist.txt for additional rules governing the use of RCU.
617 be used, omitting the rcu_head structure from <<
618 <<
619 kfree_rcu_mightsleep(old_fp); <<
620 <<
621 This variant almost never blocks, but might do <<
622 synchronize_rcu() in response to memory-alloca <<
623 <<
624 Again, see checklist.rst for additional rules <<
625 581
626 .. _5_whatisRCU: 582 .. _5_whatisRCU:
627 583
628 5. WHAT ARE SOME SIMPLE IMPLEMENTATIONS OF RC 584 5. WHAT ARE SOME SIMPLE IMPLEMENTATIONS OF RCU?
629 ---------------------------------------------- 585 ------------------------------------------------
630 586
631 One of the nice things about RCU is that it ha 587 One of the nice things about RCU is that it has extremely simple "toy"
632 implementations that are a good first step tow 588 implementations that are a good first step towards understanding the
633 production-quality implementations in the Linu 589 production-quality implementations in the Linux kernel. This section
634 presents two such "toy" implementations of RCU 590 presents two such "toy" implementations of RCU, one that is implemented
635 in terms of familiar locking primitives, and a 591 in terms of familiar locking primitives, and another that more closely
636 resembles "classic" RCU. Both are way too sim 592 resembles "classic" RCU. Both are way too simple for real-world use,
637 lacking both functionality and performance. H 593 lacking both functionality and performance. However, they are useful
638 in getting a feel for how RCU works. See kern 594 in getting a feel for how RCU works. See kernel/rcu/update.c for a
639 production-quality implementation, and see: 595 production-quality implementation, and see:
640 596
641 https://docs.google.com/document/d/1X0 !! 597 http://www.rdrop.com/users/paulmck/RCU
642 598
643 for papers describing the Linux kernel RCU imp 599 for papers describing the Linux kernel RCU implementation. The OLS'01
644 and OLS'02 papers are a good introduction, and 600 and OLS'02 papers are a good introduction, and the dissertation provides
645 more details on the current implementation as 601 more details on the current implementation as of early 2004.
646 602
647 603
648 5A. "TOY" IMPLEMENTATION #1: LOCKING 604 5A. "TOY" IMPLEMENTATION #1: LOCKING
649 ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ 605 ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
650 This section presents a "toy" RCU implementati 606 This section presents a "toy" RCU implementation that is based on
651 familiar locking primitives. Its overhead mak 607 familiar locking primitives. Its overhead makes it a non-starter for
652 real-life use, as does its lack of scalability 608 real-life use, as does its lack of scalability. It is also unsuitable
653 for realtime use, since it allows scheduling l 609 for realtime use, since it allows scheduling latency to "bleed" from
654 one read-side critical section to another. It 610 one read-side critical section to another. It also assumes recursive
655 reader-writer locks: If you try this with non 611 reader-writer locks: If you try this with non-recursive locks, and
656 you allow nested rcu_read_lock() calls, you ca 612 you allow nested rcu_read_lock() calls, you can deadlock.
657 613
658 However, it is probably the easiest implementa 614 However, it is probably the easiest implementation to relate to, so is
659 a good starting point. 615 a good starting point.
660 616
661 It is extremely simple:: 617 It is extremely simple::
662 618
663 static DEFINE_RWLOCK(rcu_gp_mutex); 619 static DEFINE_RWLOCK(rcu_gp_mutex);
664 620
665 void rcu_read_lock(void) 621 void rcu_read_lock(void)
666 { 622 {
667 read_lock(&rcu_gp_mutex); 623 read_lock(&rcu_gp_mutex);
668 } 624 }
669 625
670 void rcu_read_unlock(void) 626 void rcu_read_unlock(void)
671 { 627 {
672 read_unlock(&rcu_gp_mutex); 628 read_unlock(&rcu_gp_mutex);
673 } 629 }
674 630
675 void synchronize_rcu(void) 631 void synchronize_rcu(void)
676 { 632 {
677 write_lock(&rcu_gp_mutex); 633 write_lock(&rcu_gp_mutex);
678 smp_mb__after_spinlock(); 634 smp_mb__after_spinlock();
679 write_unlock(&rcu_gp_mutex); 635 write_unlock(&rcu_gp_mutex);
680 } 636 }
681 637
682 [You can ignore rcu_assign_pointer() and rcu_d 638 [You can ignore rcu_assign_pointer() and rcu_dereference() without missing
683 much. But here are simplified versions anyway 639 much. But here are simplified versions anyway. And whatever you do,
684 don't forget about them when submitting patche 640 don't forget about them when submitting patches making use of RCU!]::
685 641
686 #define rcu_assign_pointer(p, v) \ 642 #define rcu_assign_pointer(p, v) \
687 ({ \ 643 ({ \
688 smp_store_release(&(p), (v)); 644 smp_store_release(&(p), (v)); \
689 }) 645 })
690 646
691 #define rcu_dereference(p) \ 647 #define rcu_dereference(p) \
692 ({ \ 648 ({ \
693 typeof(p) _________p1 = READ_O 649 typeof(p) _________p1 = READ_ONCE(p); \
694 (_________p1); \ 650 (_________p1); \
695 }) 651 })
696 652
697 653
698 The rcu_read_lock() and rcu_read_unlock() prim 654 The rcu_read_lock() and rcu_read_unlock() primitive read-acquire
699 and release a global reader-writer lock. The 655 and release a global reader-writer lock. The synchronize_rcu()
700 primitive write-acquires this same lock, then 656 primitive write-acquires this same lock, then releases it. This means
701 that once synchronize_rcu() exits, all RCU rea 657 that once synchronize_rcu() exits, all RCU read-side critical sections
702 that were in progress before synchronize_rcu() 658 that were in progress before synchronize_rcu() was called are guaranteed
703 to have completed -- there is no way that sync 659 to have completed -- there is no way that synchronize_rcu() would have
704 been able to write-acquire the lock otherwise. 660 been able to write-acquire the lock otherwise. The smp_mb__after_spinlock()
705 promotes synchronize_rcu() to a full memory ba 661 promotes synchronize_rcu() to a full memory barrier in compliance with
706 the "Memory-Barrier Guarantees" listed in: 662 the "Memory-Barrier Guarantees" listed in:
707 663
708 Design/Requirements/Requirements.rst !! 664 Documentation/RCU/Design/Requirements/Requirements.rst
709 665
710 It is possible to nest rcu_read_lock(), since 666 It is possible to nest rcu_read_lock(), since reader-writer locks may
711 be recursively acquired. Note also that rcu_r 667 be recursively acquired. Note also that rcu_read_lock() is immune
712 from deadlock (an important property of RCU). 668 from deadlock (an important property of RCU). The reason for this is
713 that the only thing that can block rcu_read_lo 669 that the only thing that can block rcu_read_lock() is a synchronize_rcu().
714 But synchronize_rcu() does not acquire any loc 670 But synchronize_rcu() does not acquire any locks while holding rcu_gp_mutex,
715 so there can be no deadlock cycle. 671 so there can be no deadlock cycle.
716 672
717 .. _quiz_1: 673 .. _quiz_1:
718 674
719 Quick Quiz #1: 675 Quick Quiz #1:
720 Why is this argument naive? H 676 Why is this argument naive? How could a deadlock
721 occur when using this algorith 677 occur when using this algorithm in a real-world Linux
722 kernel? How could this deadlo 678 kernel? How could this deadlock be avoided?
723 679
724 :ref:`Answers to Quick Quiz <9_whatisRCU>` !! 680 :ref:`Answers to Quick Quiz <8_whatisRCU>`
725 681
726 5B. "TOY" EXAMPLE #2: CLASSIC RCU 682 5B. "TOY" EXAMPLE #2: CLASSIC RCU
727 ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ 683 ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
728 This section presents a "toy" RCU implementati 684 This section presents a "toy" RCU implementation that is based on
729 "classic RCU". It is also short on performanc 685 "classic RCU". It is also short on performance (but only for updates) and
730 on features such as hotplug CPU and the abilit 686 on features such as hotplug CPU and the ability to run in CONFIG_PREEMPTION
731 kernels. The definitions of rcu_dereference() 687 kernels. The definitions of rcu_dereference() and rcu_assign_pointer()
732 are the same as those shown in the preceding s 688 are the same as those shown in the preceding section, so they are omitted.
733 :: 689 ::
734 690
735 void rcu_read_lock(void) { } 691 void rcu_read_lock(void) { }
736 692
737 void rcu_read_unlock(void) { } 693 void rcu_read_unlock(void) { }
738 694
739 void synchronize_rcu(void) 695 void synchronize_rcu(void)
740 { 696 {
741 int cpu; 697 int cpu;
742 698
743 for_each_possible_cpu(cpu) 699 for_each_possible_cpu(cpu)
744 run_on(cpu); 700 run_on(cpu);
745 } 701 }
746 702
747 Note that rcu_read_lock() and rcu_read_unlock( 703 Note that rcu_read_lock() and rcu_read_unlock() do absolutely nothing.
748 This is the great strength of classic RCU in a 704 This is the great strength of classic RCU in a non-preemptive kernel:
749 read-side overhead is precisely zero, at least 705 read-side overhead is precisely zero, at least on non-Alpha CPUs.
750 And there is absolutely no way that rcu_read_l 706 And there is absolutely no way that rcu_read_lock() can possibly
751 participate in a deadlock cycle! 707 participate in a deadlock cycle!
752 708
753 The implementation of synchronize_rcu() simply 709 The implementation of synchronize_rcu() simply schedules itself on each
754 CPU in turn. The run_on() primitive can be im 710 CPU in turn. The run_on() primitive can be implemented straightforwardly
755 in terms of the sched_setaffinity() primitive. 711 in terms of the sched_setaffinity() primitive. Of course, a somewhat less
756 "toy" implementation would restore the affinit 712 "toy" implementation would restore the affinity upon completion rather
757 than just leaving all tasks running on the las 713 than just leaving all tasks running on the last CPU, but when I said
758 "toy", I meant **toy**! 714 "toy", I meant **toy**!
759 715
760 So how the heck is this supposed to work??? 716 So how the heck is this supposed to work???
761 717
762 Remember that it is illegal to block while in 718 Remember that it is illegal to block while in an RCU read-side critical
763 section. Therefore, if a given CPU executes a 719 section. Therefore, if a given CPU executes a context switch, we know
764 that it must have completed all preceding RCU 720 that it must have completed all preceding RCU read-side critical sections.
765 Once **all** CPUs have executed a context swit 721 Once **all** CPUs have executed a context switch, then **all** preceding
766 RCU read-side critical sections will have comp 722 RCU read-side critical sections will have completed.
767 723
768 So, suppose that we remove a data item from it 724 So, suppose that we remove a data item from its structure and then invoke
769 synchronize_rcu(). Once synchronize_rcu() ret 725 synchronize_rcu(). Once synchronize_rcu() returns, we are guaranteed
770 that there are no RCU read-side critical secti 726 that there are no RCU read-side critical sections holding a reference
771 to that data item, so we can safely reclaim it 727 to that data item, so we can safely reclaim it.
772 728
773 .. _quiz_2: 729 .. _quiz_2:
774 730
775 Quick Quiz #2: 731 Quick Quiz #2:
776 Give an example where Classic 732 Give an example where Classic RCU's read-side
777 overhead is **negative**. 733 overhead is **negative**.
778 734
779 :ref:`Answers to Quick Quiz <9_whatisRCU>` !! 735 :ref:`Answers to Quick Quiz <8_whatisRCU>`
780 736
781 .. _quiz_3: 737 .. _quiz_3:
782 738
783 Quick Quiz #3: 739 Quick Quiz #3:
784 If it is illegal to block in a 740 If it is illegal to block in an RCU read-side
785 critical section, what the hec 741 critical section, what the heck do you do in
786 CONFIG_PREEMPT_RT, where norma 742 CONFIG_PREEMPT_RT, where normal spinlocks can block???
787 743
788 :ref:`Answers to Quick Quiz <9_whatisRCU>` !! 744 :ref:`Answers to Quick Quiz <8_whatisRCU>`
789 745
790 .. _6_whatisRCU: 746 .. _6_whatisRCU:
791 747
792 6. ANALOGY WITH READER-WRITER LOCKING 748 6. ANALOGY WITH READER-WRITER LOCKING
793 -------------------------------------- 749 --------------------------------------
794 750
795 Although RCU can be used in many different way 751 Although RCU can be used in many different ways, a very common use of
796 RCU is analogous to reader-writer locking. Th 752 RCU is analogous to reader-writer locking. The following unified
797 diff shows how closely related RCU and reader- 753 diff shows how closely related RCU and reader-writer locking can be.
798 :: 754 ::
799 755
800 @@ -5,5 +5,5 @@ struct el { 756 @@ -5,5 +5,5 @@ struct el {
801 int data; 757 int data;
802 /* Other data fields */ 758 /* Other data fields */
803 }; 759 };
804 -rwlock_t listmutex; 760 -rwlock_t listmutex;
805 +spinlock_t listmutex; 761 +spinlock_t listmutex;
806 struct el head; 762 struct el head;
807 763
808 @@ -13,15 +14,15 @@ 764 @@ -13,15 +14,15 @@
809 struct list_head *lp; 765 struct list_head *lp;
810 struct el *p; 766 struct el *p;
811 767
812 - read_lock(&listmutex); 768 - read_lock(&listmutex);
813 - list_for_each_entry(p, head, l 769 - list_for_each_entry(p, head, lp) {
814 + rcu_read_lock(); 770 + rcu_read_lock();
815 + list_for_each_entry_rcu(p, hea 771 + list_for_each_entry_rcu(p, head, lp) {
816 if (p->key == key) { 772 if (p->key == key) {
817 *result = p->d 773 *result = p->data;
818 - read_unlock(&l 774 - read_unlock(&listmutex);
819 + rcu_read_unloc 775 + rcu_read_unlock();
820 return 1; 776 return 1;
821 } 777 }
822 } 778 }
823 - read_unlock(&listmutex); 779 - read_unlock(&listmutex);
824 + rcu_read_unlock(); 780 + rcu_read_unlock();
825 return 0; 781 return 0;
826 } 782 }
827 783
828 @@ -29,15 +30,16 @@ 784 @@ -29,15 +30,16 @@
829 { 785 {
830 struct el *p; 786 struct el *p;
831 787
832 - write_lock(&listmutex); 788 - write_lock(&listmutex);
833 + spin_lock(&listmutex); 789 + spin_lock(&listmutex);
834 list_for_each_entry(p, head, l 790 list_for_each_entry(p, head, lp) {
835 if (p->key == key) { 791 if (p->key == key) {
836 - list_del(&p->l 792 - list_del(&p->list);
837 - write_unlock(& 793 - write_unlock(&listmutex);
838 + list_del_rcu(& 794 + list_del_rcu(&p->list);
839 + spin_unlock(&l 795 + spin_unlock(&listmutex);
840 + synchronize_rc 796 + synchronize_rcu();
841 kfree(p); 797 kfree(p);
842 return 1; 798 return 1;
843 } 799 }
844 } 800 }
845 - write_unlock(&listmutex); 801 - write_unlock(&listmutex);
846 + spin_unlock(&listmutex); 802 + spin_unlock(&listmutex);
847 return 0; 803 return 0;
848 } 804 }
849 805
850 Or, for those who prefer a side-by-side listin 806 Or, for those who prefer a side-by-side listing::
851 807
852 1 struct el { 1 stru 808 1 struct el { 1 struct el {
853 2 struct list_head list; 2 st 809 2 struct list_head list; 2 struct list_head list;
854 3 long key; 3 lo 810 3 long key; 3 long key;
855 4 spinlock_t mutex; 4 sp 811 4 spinlock_t mutex; 4 spinlock_t mutex;
856 5 int data; 5 in 812 5 int data; 5 int data;
857 6 /* Other data fields */ 6 /* 813 6 /* Other data fields */ 6 /* Other data fields */
858 7 }; 7 }; 814 7 }; 7 };
859 8 rwlock_t listmutex; 8 spin 815 8 rwlock_t listmutex; 8 spinlock_t listmutex;
860 9 struct el head; 9 stru 816 9 struct el head; 9 struct el head;
861 817
862 :: 818 ::
863 819
864 1 int search(long key, int *result) 1 int 820 1 int search(long key, int *result) 1 int search(long key, int *result)
865 2 { 2 { 821 2 { 2 {
866 3 struct list_head *lp; 3 s 822 3 struct list_head *lp; 3 struct list_head *lp;
867 4 struct el *p; 4 s 823 4 struct el *p; 4 struct el *p;
868 5 5 824 5 5
869 6 read_lock(&listmutex); 6 r 825 6 read_lock(&listmutex); 6 rcu_read_lock();
870 7 list_for_each_entry(p, head, lp) { 7 l 826 7 list_for_each_entry(p, head, lp) { 7 list_for_each_entry_rcu(p, head, lp) {
871 8 if (p->key == key) { 8 827 8 if (p->key == key) { 8 if (p->key == key) {
872 9 *result = p->data; 9 828 9 *result = p->data; 9 *result = p->data;
873 10 read_unlock(&listmutex); 10 829 10 read_unlock(&listmutex); 10 rcu_read_unlock();
874 11 return 1; 11 830 11 return 1; 11 return 1;
875 12 } 12 831 12 } 12 }
876 13 } 13 } 832 13 } 13 }
877 14 read_unlock(&listmutex); 14 r 833 14 read_unlock(&listmutex); 14 rcu_read_unlock();
878 15 return 0; 15 r 834 15 return 0; 15 return 0;
879 16 } 16 } 835 16 } 16 }
880 836
881 :: 837 ::
882 838
883 1 int delete(long key) 1 int 839 1 int delete(long key) 1 int delete(long key)
884 2 { 2 { 840 2 { 2 {
885 3 struct el *p; 3 s 841 3 struct el *p; 3 struct el *p;
886 4 4 842 4 4
887 5 write_lock(&listmutex); 5 s 843 5 write_lock(&listmutex); 5 spin_lock(&listmutex);
888 6 list_for_each_entry(p, head, lp) { 6 l 844 6 list_for_each_entry(p, head, lp) { 6 list_for_each_entry(p, head, lp) {
889 7 if (p->key == key) { 7 845 7 if (p->key == key) { 7 if (p->key == key) {
890 8 list_del(&p->list); 8 846 8 list_del(&p->list); 8 list_del_rcu(&p->list);
891 9 write_unlock(&listmutex); 9 847 9 write_unlock(&listmutex); 9 spin_unlock(&listmutex);
892 10 848 10 synchronize_rcu();
893 10 kfree(p); 11 849 10 kfree(p); 11 kfree(p);
894 11 return 1; 12 850 11 return 1; 12 return 1;
895 12 } 13 851 12 } 13 }
896 13 } 14 } 852 13 } 14 }
897 14 write_unlock(&listmutex); 15 s 853 14 write_unlock(&listmutex); 15 spin_unlock(&listmutex);
898 15 return 0; 16 r 854 15 return 0; 16 return 0;
899 16 } 17 } 855 16 } 17 }
900 856
901 Either way, the differences are quite small. 857 Either way, the differences are quite small. Read-side locking moves
902 to rcu_read_lock() and rcu_read_unlock, update 858 to rcu_read_lock() and rcu_read_unlock, update-side locking moves from
903 a reader-writer lock to a simple spinlock, and 859 a reader-writer lock to a simple spinlock, and a synchronize_rcu()
904 precedes the kfree(). 860 precedes the kfree().
905 861
906 However, there is one potential catch: the rea 862 However, there is one potential catch: the read-side and update-side
907 critical sections can now run concurrently. I 863 critical sections can now run concurrently. In many cases, this will
908 not be a problem, but it is necessary to check 864 not be a problem, but it is necessary to check carefully regardless.
909 For example, if multiple independent list upda 865 For example, if multiple independent list updates must be seen as
910 a single atomic update, converting to RCU will 866 a single atomic update, converting to RCU will require special care.
911 867
912 Also, the presence of synchronize_rcu() means 868 Also, the presence of synchronize_rcu() means that the RCU version of
913 delete() can now block. If this is a problem, 869 delete() can now block. If this is a problem, there is a callback-based
914 mechanism that never blocks, namely call_rcu() 870 mechanism that never blocks, namely call_rcu() or kfree_rcu(), that can
915 be used in place of synchronize_rcu(). 871 be used in place of synchronize_rcu().
916 872
917 .. _7_whatisRCU: 873 .. _7_whatisRCU:
918 874
919 7. ANALOGY WITH REFERENCE COUNTING !! 875 7. FULL LIST OF RCU APIs
920 ----------------------------------- <<
921 <<
922 The reader-writer analogy (illustrated by the <<
923 always the best way to think about using RCU. <<
924 considers RCU an effective reference count on <<
925 protected by RCU. <<
926 <<
927 A reference count typically does not prevent t <<
928 values from changing, but does prevent changes <<
929 gross change of type that happens when that ob <<
930 re-allocated for some other purpose. Once a t <<
931 object is obtained, some other mechanism is ne <<
932 access to the data in the object. This could <<
933 but with RCU the typical approach is to perfor <<
934 operations such as smp_load_acquire(), to perf <<
935 read-modify-write operations, and to provide t <<
936 RCU provides a number of support functions tha <<
937 operations and ordering, such as the list_for_ <<
938 used in the previous section. <<
939 <<
940 A more focused view of the reference counting <<
941 between rcu_read_lock() and rcu_read_unlock(), <<
942 rcu_dereference() on a pointer marked as ``__r <<
943 though a reference-count on that object has be <<
944 This prevents the object from changing type. <<
945 will depend on normal expectations of objects <<
946 typically includes that spinlocks can still be <<
947 reference counters can be safely manipulated, <<
948 can be safely dereferenced. <<
949 <<
950 Some operations that one might expect to see o <<
951 which an RCU reference is held include: <<
952 <<
953 - Copying out data that is guaranteed to be s <<
954 - Using kref_get_unless_zero() or similar to <<
955 reference. This may fail of course. <<
956 - Acquiring a spinlock in the object, and che <<
957 is the expected object and if so, manipulat <<
958 <<
959 The understanding that RCU provides a referenc <<
960 change of type is particularly visible with ob <<
961 slab cache marked ``SLAB_TYPESAFE_BY_RCU``. R <<
962 reference to an object from such a cache that <<
963 and the memory reallocated to a completely dif <<
964 the same type. In this case RCU doesn't even <<
965 object from changing, only its type. So the o <<
966 one expected, but it will be one where it is s <<
967 (and then potentially acquiring a spinlock), a <<
968 to check whether the identity matches expectat <<
969 to simply acquire the spinlock without first t <<
970 unfortunately any spinlock in a ``SLAB_TYPESAF <<
971 initialized after each and every call to kmem_ <<
972 reference-free spinlock acquisition completely <<
973 using ``SLAB_TYPESAFE_BY_RCU``, make proper us <<
974 (Those willing to initialize their locks in a <<
975 may also use locking, including cache-friendly <<
976 <<
977 With traditional reference counting -- such as <<
978 kref library in Linux -- there is typically co <<
979 reference to an object is dropped. With kref, <<
980 passed to kref_put(). When RCU is being used, <<
981 must not be run until all ``__rcu`` pointers r <<
982 been updated, and then a grace period has pass <<
983 globally visible pointer to the object must be <<
984 potential counted reference, and the finalizat <<
985 using call_rcu() only after all those pointers <<
986 <<
987 To see how to choose between these two analogi <<
988 reader-writer lock and RCU as a reference coun <<
989 to reflect on the scale of the thing being pro <<
990 lock analogy looks at larger multi-part object <<
991 and shows how RCU can facilitate concurrency w <<
992 to, and removed from, the list. The reference <<
993 the individual objects and looks at how they c <<
994 within whatever whole they are a part of. <<
995 <<
996 .. _8_whatisRCU: <<
997 <<
998 8. FULL LIST OF RCU APIs <<
999 ------------------------- 876 -------------------------
1000 877
1001 The RCU APIs are documented in docbook-format 878 The RCU APIs are documented in docbook-format header comments in the
1002 Linux-kernel source code, but it helps to hav 879 Linux-kernel source code, but it helps to have a full list of the
1003 APIs, since there does not appear to be a way 880 APIs, since there does not appear to be a way to categorize them
1004 in docbook. Here is the list, by category. 881 in docbook. Here is the list, by category.
1005 882
1006 RCU list traversal:: 883 RCU list traversal::
1007 884
1008 list_entry_rcu 885 list_entry_rcu
1009 list_entry_lockless 886 list_entry_lockless
1010 list_first_entry_rcu 887 list_first_entry_rcu
1011 list_next_rcu 888 list_next_rcu
1012 list_for_each_entry_rcu 889 list_for_each_entry_rcu
1013 list_for_each_entry_continue_rcu 890 list_for_each_entry_continue_rcu
1014 list_for_each_entry_from_rcu 891 list_for_each_entry_from_rcu
1015 list_first_or_null_rcu 892 list_first_or_null_rcu
1016 list_next_or_null_rcu 893 list_next_or_null_rcu
1017 hlist_first_rcu 894 hlist_first_rcu
1018 hlist_next_rcu 895 hlist_next_rcu
1019 hlist_pprev_rcu 896 hlist_pprev_rcu
1020 hlist_for_each_entry_rcu 897 hlist_for_each_entry_rcu
1021 hlist_for_each_entry_rcu_bh 898 hlist_for_each_entry_rcu_bh
1022 hlist_for_each_entry_from_rcu 899 hlist_for_each_entry_from_rcu
1023 hlist_for_each_entry_continue_rcu 900 hlist_for_each_entry_continue_rcu
1024 hlist_for_each_entry_continue_rcu_bh 901 hlist_for_each_entry_continue_rcu_bh
1025 hlist_nulls_first_rcu 902 hlist_nulls_first_rcu
1026 hlist_nulls_for_each_entry_rcu 903 hlist_nulls_for_each_entry_rcu
1027 hlist_bl_first_rcu 904 hlist_bl_first_rcu
1028 hlist_bl_for_each_entry_rcu 905 hlist_bl_for_each_entry_rcu
1029 906
1030 RCU pointer/list update:: 907 RCU pointer/list update::
1031 908
1032 rcu_assign_pointer 909 rcu_assign_pointer
1033 list_add_rcu 910 list_add_rcu
1034 list_add_tail_rcu 911 list_add_tail_rcu
1035 list_del_rcu 912 list_del_rcu
1036 list_replace_rcu 913 list_replace_rcu
1037 hlist_add_behind_rcu 914 hlist_add_behind_rcu
1038 hlist_add_before_rcu 915 hlist_add_before_rcu
1039 hlist_add_head_rcu 916 hlist_add_head_rcu
1040 hlist_add_tail_rcu 917 hlist_add_tail_rcu
1041 hlist_del_rcu 918 hlist_del_rcu
1042 hlist_del_init_rcu 919 hlist_del_init_rcu
1043 hlist_replace_rcu 920 hlist_replace_rcu
1044 list_splice_init_rcu 921 list_splice_init_rcu
1045 list_splice_tail_init_rcu 922 list_splice_tail_init_rcu
1046 hlist_nulls_del_init_rcu 923 hlist_nulls_del_init_rcu
1047 hlist_nulls_del_rcu 924 hlist_nulls_del_rcu
1048 hlist_nulls_add_head_rcu 925 hlist_nulls_add_head_rcu
1049 hlist_bl_add_head_rcu 926 hlist_bl_add_head_rcu
1050 hlist_bl_del_init_rcu 927 hlist_bl_del_init_rcu
1051 hlist_bl_del_rcu 928 hlist_bl_del_rcu
1052 hlist_bl_set_first_rcu 929 hlist_bl_set_first_rcu
1053 930
1054 RCU:: 931 RCU::
1055 932
1056 Critical sections Grace period 933 Critical sections Grace period Barrier
1057 934
1058 rcu_read_lock synchronize_n 935 rcu_read_lock synchronize_net rcu_barrier
1059 rcu_read_unlock synchronize_r 936 rcu_read_unlock synchronize_rcu
1060 rcu_dereference synchronize_r 937 rcu_dereference synchronize_rcu_expedited
1061 rcu_read_lock_held call_rcu 938 rcu_read_lock_held call_rcu
1062 rcu_dereference_check kfree_rcu 939 rcu_dereference_check kfree_rcu
1063 rcu_dereference_protected 940 rcu_dereference_protected
1064 941
1065 bh:: 942 bh::
1066 943
1067 Critical sections Grace period 944 Critical sections Grace period Barrier
1068 945
1069 rcu_read_lock_bh call_rcu 946 rcu_read_lock_bh call_rcu rcu_barrier
1070 rcu_read_unlock_bh synchronize_r 947 rcu_read_unlock_bh synchronize_rcu
1071 [local_bh_disable] synchronize_r 948 [local_bh_disable] synchronize_rcu_expedited
1072 [and friends] 949 [and friends]
1073 rcu_dereference_bh 950 rcu_dereference_bh
1074 rcu_dereference_bh_check 951 rcu_dereference_bh_check
1075 rcu_dereference_bh_protected 952 rcu_dereference_bh_protected
1076 rcu_read_lock_bh_held 953 rcu_read_lock_bh_held
1077 954
1078 sched:: 955 sched::
1079 956
1080 Critical sections Grace period 957 Critical sections Grace period Barrier
1081 958
1082 rcu_read_lock_sched call_rcu 959 rcu_read_lock_sched call_rcu rcu_barrier
1083 rcu_read_unlock_sched synchronize_r 960 rcu_read_unlock_sched synchronize_rcu
1084 [preempt_disable] synchronize_r 961 [preempt_disable] synchronize_rcu_expedited
1085 [and friends] 962 [and friends]
1086 rcu_read_lock_sched_notrace 963 rcu_read_lock_sched_notrace
1087 rcu_read_unlock_sched_notrace 964 rcu_read_unlock_sched_notrace
1088 rcu_dereference_sched 965 rcu_dereference_sched
1089 rcu_dereference_sched_check 966 rcu_dereference_sched_check
1090 rcu_dereference_sched_protected 967 rcu_dereference_sched_protected
1091 rcu_read_lock_sched_held 968 rcu_read_lock_sched_held
1092 969
1093 970
1094 RCU-Tasks:: <<
1095 <<
1096 Critical sections Grace period <<
1097 <<
1098 N/A call_rcu_task <<
1099 synchronize_r <<
1100 <<
1101 <<
1102 RCU-Tasks-Rude:: <<
1103 <<
1104 Critical sections Grace period <<
1105 <<
1106 N/A call_rcu_task <<
1107 synchronize_r <<
1108 <<
1109 <<
1110 RCU-Tasks-Trace:: <<
1111 <<
1112 Critical sections Grace period <<
1113 <<
1114 rcu_read_lock_trace call_rcu_task <<
1115 rcu_read_unlock_trace synchronize_r <<
1116 <<
1117 <<
1118 SRCU:: 971 SRCU::
1119 972
1120 Critical sections Grace period 973 Critical sections Grace period Barrier
1121 974
1122 srcu_read_lock call_srcu 975 srcu_read_lock call_srcu srcu_barrier
1123 srcu_read_unlock synchronize_s 976 srcu_read_unlock synchronize_srcu
1124 srcu_dereference synchronize_s 977 srcu_dereference synchronize_srcu_expedited
1125 srcu_dereference_check 978 srcu_dereference_check
1126 srcu_read_lock_held 979 srcu_read_lock_held
1127 980
1128 SRCU: Initialization/cleanup:: 981 SRCU: Initialization/cleanup::
1129 982
1130 DEFINE_SRCU 983 DEFINE_SRCU
1131 DEFINE_STATIC_SRCU 984 DEFINE_STATIC_SRCU
1132 init_srcu_struct 985 init_srcu_struct
1133 cleanup_srcu_struct 986 cleanup_srcu_struct
1134 987
1135 All: lockdep-checked RCU utility APIs:: !! 988 All: lockdep-checked RCU-protected pointer access::
1136 989
>> 990 rcu_access_pointer
>> 991 rcu_dereference_raw
1137 RCU_LOCKDEP_WARN 992 RCU_LOCKDEP_WARN
1138 rcu_sleep_check 993 rcu_sleep_check
1139 !! 994 RCU_NONIDLE
1140 All: Unchecked RCU-protected pointer access:: <<
1141 <<
1142 rcu_dereference_raw <<
1143 <<
1144 All: Unchecked RCU-protected pointer access w <<
1145 <<
1146 rcu_access_pointer <<
1147 995
1148 See the comment headers in the source code (o 996 See the comment headers in the source code (or the docbook generated
1149 from them) for more information. 997 from them) for more information.
1150 998
1151 However, given that there are no fewer than f 999 However, given that there are no fewer than four families of RCU APIs
1152 in the Linux kernel, how do you choose which 1000 in the Linux kernel, how do you choose which one to use? The following
1153 list can be helpful: 1001 list can be helpful:
1154 1002
1155 a. Will readers need to block? If so, y 1003 a. Will readers need to block? If so, you need SRCU.
1156 1004
1157 b. Will readers need to block and are yo !! 1005 b. What about the -rt patchset? If readers would need to block
1158 example, ftrace or BPF? If so, you n !! 1006 in an non-rt kernel, you need SRCU. If readers would block
1159 RCU-tasks-rude, and/or RCU-tasks-trac !! 1007 in a -rt kernel, but not in a non-rt kernel, SRCU is not
1160 !! 1008 necessary. (The -rt patchset turns spinlocks into sleeplocks,
1161 c. What about the -rt patchset? If read !! 1009 hence this distinction.)
1162 an non-rt kernel, you need SRCU. If <<
1163 acquiring spinlocks in a -rt kernel, <<
1164 SRCU is not necessary. (The -rt patc <<
1165 sleeplocks, hence this distinction.) <<
1166 1010
1167 d. Do you need to treat NMI handlers, ha !! 1011 c. Do you need to treat NMI handlers, hardirq handlers,
1168 and code segments with preemption dis 1012 and code segments with preemption disabled (whether
1169 via preempt_disable(), local_irq_save 1013 via preempt_disable(), local_irq_save(), local_bh_disable(),
1170 or some other mechanism) as if they w 1014 or some other mechanism) as if they were explicit RCU readers?
1171 If so, RCU-sched readers are the only !! 1015 If so, RCU-sched is the only choice that will work for you.
1172 for you, but since about v4.20 you us <<
1173 update primitives. <<
1174 <<
1175 e. Do you need RCU grace periods to comp <<
1176 softirq monopolization of one or more <<
1177 is your code subject to network-based <<
1178 If so, you should disable softirq acr <<
1179 example, by using rcu_read_lock_bh(). <<
1180 use can use the vanilla RCU update pr <<
1181 1016
1182 f. Is your workload too update-intensive !! 1017 d. Do you need RCU grace periods to complete even in the face
>> 1018 of softirq monopolization of one or more of the CPUs? For
>> 1019 example, is your code subject to network-based denial-of-service
>> 1020 attacks? If so, you should disable softirq across your readers,
>> 1021 for example, by using rcu_read_lock_bh().
>> 1022
>> 1023 e. Is your workload too update-intensive for normal use of
1183 RCU, but inappropriate for other sync 1024 RCU, but inappropriate for other synchronization mechanisms?
1184 If so, consider SLAB_TYPESAFE_BY_RCU 1025 If so, consider SLAB_TYPESAFE_BY_RCU (which was originally
1185 named SLAB_DESTROY_BY_RCU). But plea 1026 named SLAB_DESTROY_BY_RCU). But please be careful!
1186 1027
1187 g. Do you need read-side critical sectio !! 1028 f. Do you need read-side critical sections that are respected
1188 on CPUs that are deep in the idle loo !! 1029 even though they are in the middle of the idle loop, during
1189 from user-mode execution, or on an of !! 1030 user-mode execution, or on an offlined CPU? If so, SRCU is the
1190 and RCU Tasks Trace are the only choi !! 1031 only choice that will work for you.
1191 with SRCU being strongly preferred in <<
1192 1032
1193 h. Otherwise, use RCU. !! 1033 g. Otherwise, use RCU.
1194 1034
1195 Of course, this all assumes that you have det 1035 Of course, this all assumes that you have determined that RCU is in fact
1196 the right tool for your job. 1036 the right tool for your job.
1197 1037
1198 .. _9_whatisRCU: !! 1038 .. _8_whatisRCU:
1199 1039
1200 9. ANSWERS TO QUICK QUIZZES !! 1040 8. ANSWERS TO QUICK QUIZZES
1201 ---------------------------- 1041 ----------------------------
1202 1042
1203 Quick Quiz #1: 1043 Quick Quiz #1:
1204 Why is this argument naive? 1044 Why is this argument naive? How could a deadlock
1205 occur when using this algorit 1045 occur when using this algorithm in a real-world Linux
1206 kernel? [Referring to the lo 1046 kernel? [Referring to the lock-based "toy" RCU
1207 algorithm.] 1047 algorithm.]
1208 1048
1209 Answer: 1049 Answer:
1210 Consider the following sequen 1050 Consider the following sequence of events:
1211 1051
1212 1. CPU 0 acquires some u 1052 1. CPU 0 acquires some unrelated lock, call it
1213 "problematic_lock", d 1053 "problematic_lock", disabling irq via
1214 spin_lock_irqsave(). 1054 spin_lock_irqsave().
1215 1055
1216 2. CPU 1 enters synchron 1056 2. CPU 1 enters synchronize_rcu(), write-acquiring
1217 rcu_gp_mutex. 1057 rcu_gp_mutex.
1218 1058
1219 3. CPU 0 enters rcu_read 1059 3. CPU 0 enters rcu_read_lock(), but must wait
1220 because CPU 1 holds r 1060 because CPU 1 holds rcu_gp_mutex.
1221 1061
1222 4. CPU 1 is interrupted, 1062 4. CPU 1 is interrupted, and the irq handler
1223 attempts to acquire p 1063 attempts to acquire problematic_lock.
1224 1064
1225 The system is now deadlocked. 1065 The system is now deadlocked.
1226 1066
1227 One way to avoid this deadloc 1067 One way to avoid this deadlock is to use an approach like
1228 that of CONFIG_PREEMPT_RT, wh 1068 that of CONFIG_PREEMPT_RT, where all normal spinlocks
1229 become blocking locks, and al 1069 become blocking locks, and all irq handlers execute in
1230 the context of special tasks. 1070 the context of special tasks. In this case, in step 4
1231 above, the irq handler would 1071 above, the irq handler would block, allowing CPU 1 to
1232 release rcu_gp_mutex, avoidin 1072 release rcu_gp_mutex, avoiding the deadlock.
1233 1073
1234 Even in the absence of deadlo 1074 Even in the absence of deadlock, this RCU implementation
1235 allows latency to "bleed" fro 1075 allows latency to "bleed" from readers to other
1236 readers through synchronize_r 1076 readers through synchronize_rcu(). To see this,
1237 consider task A in an RCU rea 1077 consider task A in an RCU read-side critical section
1238 (thus read-holding rcu_gp_mut 1078 (thus read-holding rcu_gp_mutex), task B blocked
1239 attempting to write-acquire r 1079 attempting to write-acquire rcu_gp_mutex, and
1240 task C blocked in rcu_read_lo 1080 task C blocked in rcu_read_lock() attempting to
1241 read_acquire rcu_gp_mutex. T 1081 read_acquire rcu_gp_mutex. Task A's RCU read-side
1242 latency is holding up task C, 1082 latency is holding up task C, albeit indirectly via
1243 task B. 1083 task B.
1244 1084
1245 Realtime RCU implementations 1085 Realtime RCU implementations therefore use a counter-based
1246 approach where tasks in RCU r 1086 approach where tasks in RCU read-side critical sections
1247 cannot be blocked by tasks ex 1087 cannot be blocked by tasks executing synchronize_rcu().
1248 1088
1249 :ref:`Back to Quick Quiz #1 <quiz_1>` 1089 :ref:`Back to Quick Quiz #1 <quiz_1>`
1250 1090
1251 Quick Quiz #2: 1091 Quick Quiz #2:
1252 Give an example where Classic 1092 Give an example where Classic RCU's read-side
1253 overhead is **negative**. 1093 overhead is **negative**.
1254 1094
1255 Answer: 1095 Answer:
1256 Imagine a single-CPU system w 1096 Imagine a single-CPU system with a non-CONFIG_PREEMPTION
1257 kernel where a routing table 1097 kernel where a routing table is used by process-context
1258 code, but can be updated by i 1098 code, but can be updated by irq-context code (for example,
1259 by an "ICMP REDIRECT" packet) 1099 by an "ICMP REDIRECT" packet). The usual way of handling
1260 this would be to have the pro 1100 this would be to have the process-context code disable
1261 interrupts while searching th 1101 interrupts while searching the routing table. Use of
1262 RCU allows such interrupt-dis 1102 RCU allows such interrupt-disabling to be dispensed with.
1263 Thus, without RCU, you pay th 1103 Thus, without RCU, you pay the cost of disabling interrupts,
1264 and with RCU you don't. 1104 and with RCU you don't.
1265 1105
1266 One can argue that the overhe 1106 One can argue that the overhead of RCU in this
1267 case is negative with respect 1107 case is negative with respect to the single-CPU
1268 interrupt-disabling approach. 1108 interrupt-disabling approach. Others might argue that
1269 the overhead of RCU is merely 1109 the overhead of RCU is merely zero, and that replacing
1270 the positive overhead of the 1110 the positive overhead of the interrupt-disabling scheme
1271 with the zero-overhead RCU sc 1111 with the zero-overhead RCU scheme does not constitute
1272 negative overhead. 1112 negative overhead.
1273 1113
1274 In real life, of course, thin 1114 In real life, of course, things are more complex. But
1275 even the theoretical possibil 1115 even the theoretical possibility of negative overhead for
1276 a synchronization primitive i 1116 a synchronization primitive is a bit unexpected. ;-)
1277 1117
1278 :ref:`Back to Quick Quiz #2 <quiz_2>` 1118 :ref:`Back to Quick Quiz #2 <quiz_2>`
1279 1119
1280 Quick Quiz #3: 1120 Quick Quiz #3:
1281 If it is illegal to block in 1121 If it is illegal to block in an RCU read-side
1282 critical section, what the he 1122 critical section, what the heck do you do in
1283 CONFIG_PREEMPT_RT, where norm 1123 CONFIG_PREEMPT_RT, where normal spinlocks can block???
1284 1124
1285 Answer: 1125 Answer:
1286 Just as CONFIG_PREEMPT_RT per 1126 Just as CONFIG_PREEMPT_RT permits preemption of spinlock
1287 critical sections, it permits 1127 critical sections, it permits preemption of RCU
1288 read-side critical sections. 1128 read-side critical sections. It also permits
1289 spinlocks blocking while in R 1129 spinlocks blocking while in RCU read-side critical
1290 sections. 1130 sections.
1291 1131
1292 Why the apparent inconsistenc 1132 Why the apparent inconsistency? Because it is
1293 possible to use priority boos 1133 possible to use priority boosting to keep the RCU
1294 grace periods short if need b 1134 grace periods short if need be (for example, if running
1295 short of memory). In contras 1135 short of memory). In contrast, if blocking waiting
1296 for (say) network reception, 1136 for (say) network reception, there is no way to know
1297 what should be boosted. Espe 1137 what should be boosted. Especially given that the
1298 process we need to boost migh 1138 process we need to boost might well be a human being
1299 who just went out for a pizza 1139 who just went out for a pizza or something. And although
1300 a computer-operated cattle pr 1140 a computer-operated cattle prod might arouse serious
1301 interest, it might also provo 1141 interest, it might also provoke serious objections.
1302 Besides, how does the compute 1142 Besides, how does the computer know what pizza parlor
1303 the human being went to??? 1143 the human being went to???
1304 1144
1305 :ref:`Back to Quick Quiz #3 <quiz_3>` 1145 :ref:`Back to Quick Quiz #3 <quiz_3>`
1306 1146
1307 ACKNOWLEDGEMENTS 1147 ACKNOWLEDGEMENTS
1308 1148
1309 My thanks to the people who helped make this 1149 My thanks to the people who helped make this human-readable, including
1310 Jon Walpole, Josh Triplett, Serge Hallyn, Suz 1150 Jon Walpole, Josh Triplett, Serge Hallyn, Suzanne Wood, and Alan Stern.
1311 1151
1312 1152
1313 For more information, see http://www.rdrop.co 1153 For more information, see http://www.rdrop.com/users/paulmck/RCU.
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