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Linux/Documentation/RCU/checklist.rst

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Diff markup

Differences between /Documentation/RCU/checklist.rst (Architecture mips) and /Documentation/RCU/checklist.rst (Architecture alpha)


  1 .. SPDX-License-Identifier: GPL-2.0                 1 .. SPDX-License-Identifier: GPL-2.0
  2                                                     2 
  3 ================================                    3 ================================
  4 Review Checklist for RCU Patches                    4 Review Checklist for RCU Patches
  5 ================================                    5 ================================
  6                                                     6 
  7                                                     7 
  8 This document contains a checklist for produci      8 This document contains a checklist for producing and reviewing patches
  9 that make use of RCU.  Violating any of the ru      9 that make use of RCU.  Violating any of the rules listed below will
 10 result in the same sorts of problems that leav     10 result in the same sorts of problems that leaving out a locking primitive
 11 would cause.  This list is based on experience     11 would cause.  This list is based on experiences reviewing such patches
 12 over a rather long period of time, but improve     12 over a rather long period of time, but improvements are always welcome!
 13                                                    13 
 14 0.      Is RCU being applied to a read-mostly      14 0.      Is RCU being applied to a read-mostly situation?  If the data
 15         structure is updated more than about 1     15         structure is updated more than about 10% of the time, then you
 16         should strongly consider some other ap     16         should strongly consider some other approach, unless detailed
 17         performance measurements show that RCU     17         performance measurements show that RCU is nonetheless the right
 18         tool for the job.  Yes, RCU does reduc     18         tool for the job.  Yes, RCU does reduce read-side overhead by
 19         increasing write-side overhead, which      19         increasing write-side overhead, which is exactly why normal uses
 20         of RCU will do much more reading than      20         of RCU will do much more reading than updating.
 21                                                    21 
 22         Another exception is where performance     22         Another exception is where performance is not an issue, and RCU
 23         provides a simpler implementation.  An     23         provides a simpler implementation.  An example of this situation
 24         is the dynamic NMI code in the Linux 2     24         is the dynamic NMI code in the Linux 2.6 kernel, at least on
 25         architectures where NMIs are rare.         25         architectures where NMIs are rare.
 26                                                    26 
 27         Yet another exception is where the low     27         Yet another exception is where the low real-time latency of RCU's
 28         read-side primitives is critically imp     28         read-side primitives is critically important.
 29                                                    29 
 30         One final exception is where RCU reade     30         One final exception is where RCU readers are used to prevent
 31         the ABA problem (https://en.wikipedia.     31         the ABA problem (https://en.wikipedia.org/wiki/ABA_problem)
 32         for lockless updates.  This does resul     32         for lockless updates.  This does result in the mildly
 33         counter-intuitive situation where rcu_     33         counter-intuitive situation where rcu_read_lock() and
 34         rcu_read_unlock() are used to protect      34         rcu_read_unlock() are used to protect updates, however, this
 35         approach can provide the same simplifi     35         approach can provide the same simplifications to certain types
 36         of lockless algorithms that garbage co     36         of lockless algorithms that garbage collectors do.
 37                                                    37 
 38 1.      Does the update code have proper mutua     38 1.      Does the update code have proper mutual exclusion?
 39                                                    39 
 40         RCU does allow *readers* to run (almos     40         RCU does allow *readers* to run (almost) naked, but *writers* must
 41         still use some sort of mutual exclusio     41         still use some sort of mutual exclusion, such as:
 42                                                    42 
 43         a.      locking,                           43         a.      locking,
 44         b.      atomic operations, or              44         b.      atomic operations, or
 45         c.      restricting updates to a singl     45         c.      restricting updates to a single task.
 46                                                    46 
 47         If you choose #b, be prepared to descr     47         If you choose #b, be prepared to describe how you have handled
 48         memory barriers on weakly ordered mach     48         memory barriers on weakly ordered machines (pretty much all of
 49         them -- even x86 allows later loads to     49         them -- even x86 allows later loads to be reordered to precede
 50         earlier stores), and be prepared to ex     50         earlier stores), and be prepared to explain why this added
 51         complexity is worthwhile.  If you choo     51         complexity is worthwhile.  If you choose #c, be prepared to
 52         explain how this single task does not      52         explain how this single task does not become a major bottleneck
 53         on large systems (for example, if the      53         on large systems (for example, if the task is updating information
 54         relating to itself that other tasks ca     54         relating to itself that other tasks can read, there by definition
 55         can be no bottleneck).  Note that the      55         can be no bottleneck).  Note that the definition of "large" has
 56         changed significantly:  Eight CPUs was     56         changed significantly:  Eight CPUs was "large" in the year 2000,
 57         but a hundred CPUs was unremarkable in     57         but a hundred CPUs was unremarkable in 2017.
 58                                                    58 
 59 2.      Do the RCU read-side critical sections     59 2.      Do the RCU read-side critical sections make proper use of
 60         rcu_read_lock() and friends?  These pr     60         rcu_read_lock() and friends?  These primitives are needed
 61         to prevent grace periods from ending p     61         to prevent grace periods from ending prematurely, which
 62         could result in data being unceremonio     62         could result in data being unceremoniously freed out from
 63         under your read-side code, which can g     63         under your read-side code, which can greatly increase the
 64         actuarial risk of your kernel.             64         actuarial risk of your kernel.
 65                                                    65 
 66         As a rough rule of thumb, any derefere     66         As a rough rule of thumb, any dereference of an RCU-protected
 67         pointer must be covered by rcu_read_lo     67         pointer must be covered by rcu_read_lock(), rcu_read_lock_bh(),
 68         rcu_read_lock_sched(), or by the appro     68         rcu_read_lock_sched(), or by the appropriate update-side lock.
 69         Explicit disabling of preemption (pree     69         Explicit disabling of preemption (preempt_disable(), for example)
 70         can serve as rcu_read_lock_sched(), bu     70         can serve as rcu_read_lock_sched(), but is less readable and
 71         prevents lockdep from detecting lockin     71         prevents lockdep from detecting locking issues.  Acquiring a
 72         spinlock also enters an RCU read-side      72         spinlock also enters an RCU read-side critical section.
 73                                                    73 
 74         Please note that you *cannot* rely on      74         Please note that you *cannot* rely on code known to be built
 75         only in non-preemptible kernels.  Such     75         only in non-preemptible kernels.  Such code can and will break,
 76         especially in kernels built with CONFI     76         especially in kernels built with CONFIG_PREEMPT_COUNT=y.
 77                                                    77 
 78         Letting RCU-protected pointers "leak"      78         Letting RCU-protected pointers "leak" out of an RCU read-side
 79         critical section is every bit as bad a     79         critical section is every bit as bad as letting them leak out
 80         from under a lock.  Unless, of course,     80         from under a lock.  Unless, of course, you have arranged some
 81         other means of protection, such as a l     81         other means of protection, such as a lock or a reference count
 82         *before* letting them out of the RCU r     82         *before* letting them out of the RCU read-side critical section.
 83                                                    83 
 84 3.      Does the update code tolerate concurre     84 3.      Does the update code tolerate concurrent accesses?
 85                                                    85 
 86         The whole point of RCU is to permit re     86         The whole point of RCU is to permit readers to run without
 87         any locks or atomic operations.  This      87         any locks or atomic operations.  This means that readers will
 88         be running while updates are in progre     88         be running while updates are in progress.  There are a number
 89         of ways to handle this concurrency, de     89         of ways to handle this concurrency, depending on the situation:
 90                                                    90 
 91         a.      Use the RCU variants of the li     91         a.      Use the RCU variants of the list and hlist update
 92                 primitives to add, remove, and     92                 primitives to add, remove, and replace elements on
 93                 an RCU-protected list.  Altern     93                 an RCU-protected list.  Alternatively, use the other
 94                 RCU-protected data structures      94                 RCU-protected data structures that have been added to
 95                 the Linux kernel.                  95                 the Linux kernel.
 96                                                    96 
 97                 This is almost always the best     97                 This is almost always the best approach.
 98                                                    98 
 99         b.      Proceed as in (a) above, but a     99         b.      Proceed as in (a) above, but also maintain per-element
100                 locks (that are acquired by bo    100                 locks (that are acquired by both readers and writers)
101                 that guard per-element state.     101                 that guard per-element state.  Fields that the readers
102                 refrain from accessing can be     102                 refrain from accessing can be guarded by some other lock
103                 acquired only by updaters, if     103                 acquired only by updaters, if desired.
104                                                   104 
105                 This also works quite well.       105                 This also works quite well.
106                                                   106 
107         c.      Make updates appear atomic to     107         c.      Make updates appear atomic to readers.  For example,
108                 pointer updates to properly al    108                 pointer updates to properly aligned fields will
109                 appear atomic, as will individ    109                 appear atomic, as will individual atomic primitives.
110                 Sequences of operations perfor    110                 Sequences of operations performed under a lock will *not*
111                 appear to be atomic to RCU rea    111                 appear to be atomic to RCU readers, nor will sequences
112                 of multiple atomic primitives.    112                 of multiple atomic primitives.  One alternative is to
113                 move multiple individual field    113                 move multiple individual fields to a separate structure,
114                 thus solving the multiple-fiel    114                 thus solving the multiple-field problem by imposing an
115                 additional level of indirectio    115                 additional level of indirection.
116                                                   116 
117                 This can work, but is starting    117                 This can work, but is starting to get a bit tricky.
118                                                   118 
119         d.      Carefully order the updates an    119         d.      Carefully order the updates and the reads so that readers
120                 see valid data at all phases o    120                 see valid data at all phases of the update.  This is often
121                 more difficult than it sounds,    121                 more difficult than it sounds, especially given modern
122                 CPUs' tendency to reorder memo    122                 CPUs' tendency to reorder memory references.  One must
123                 usually liberally sprinkle mem    123                 usually liberally sprinkle memory-ordering operations
124                 through the code, making it di    124                 through the code, making it difficult to understand and
125                 to test.  Where it works, it i    125                 to test.  Where it works, it is better to use things
126                 like smp_store_release() and s    126                 like smp_store_release() and smp_load_acquire(), but in
127                 some cases the smp_mb() full m    127                 some cases the smp_mb() full memory barrier is required.
128                                                   128 
129                 As noted earlier, it is usuall    129                 As noted earlier, it is usually better to group the
130                 changing data into a separate     130                 changing data into a separate structure, so that the
131                 change may be made to appear a    131                 change may be made to appear atomic by updating a pointer
132                 to reference a new structure c    132                 to reference a new structure containing updated values.
133                                                   133 
134 4.      Weakly ordered CPUs pose special chall    134 4.      Weakly ordered CPUs pose special challenges.  Almost all CPUs
135         are weakly ordered -- even x86 CPUs al    135         are weakly ordered -- even x86 CPUs allow later loads to be
136         reordered to precede earlier stores.      136         reordered to precede earlier stores.  RCU code must take all of
137         the following measures to prevent memo    137         the following measures to prevent memory-corruption problems:
138                                                   138 
139         a.      Readers must maintain proper o    139         a.      Readers must maintain proper ordering of their memory
140                 accesses.  The rcu_dereference    140                 accesses.  The rcu_dereference() primitive ensures that
141                 the CPU picks up the pointer b    141                 the CPU picks up the pointer before it picks up the data
142                 that the pointer points to.  T    142                 that the pointer points to.  This really is necessary
143                 on Alpha CPUs.                    143                 on Alpha CPUs.
144                                                   144 
145                 The rcu_dereference() primitiv    145                 The rcu_dereference() primitive is also an excellent
146                 documentation aid, letting the    146                 documentation aid, letting the person reading the
147                 code know exactly which pointe    147                 code know exactly which pointers are protected by RCU.
148                 Please note that compilers can    148                 Please note that compilers can also reorder code, and
149                 they are becoming increasingly    149                 they are becoming increasingly aggressive about doing
150                 just that.  The rcu_dereferenc    150                 just that.  The rcu_dereference() primitive therefore also
151                 prevents destructive compiler     151                 prevents destructive compiler optimizations.  However,
152                 with a bit of devious creativi    152                 with a bit of devious creativity, it is possible to
153                 mishandle the return value fro    153                 mishandle the return value from rcu_dereference().
154                 Please see rcu_dereference.rst    154                 Please see rcu_dereference.rst for more information.
155                                                   155 
156                 The rcu_dereference() primitiv    156                 The rcu_dereference() primitive is used by the
157                 various "_rcu()" list-traversa    157                 various "_rcu()" list-traversal primitives, such
158                 as the list_for_each_entry_rcu    158                 as the list_for_each_entry_rcu().  Note that it is
159                 perfectly legal (if redundant)    159                 perfectly legal (if redundant) for update-side code to
160                 use rcu_dereference() and the     160                 use rcu_dereference() and the "_rcu()" list-traversal
161                 primitives.  This is particula    161                 primitives.  This is particularly useful in code that
162                 is common to readers and updat    162                 is common to readers and updaters.  However, lockdep
163                 will complain if you access rc    163                 will complain if you access rcu_dereference() outside
164                 of an RCU read-side critical s    164                 of an RCU read-side critical section.  See lockdep.rst
165                 to learn what to do about this    165                 to learn what to do about this.
166                                                   166 
167                 Of course, neither rcu_derefer    167                 Of course, neither rcu_dereference() nor the "_rcu()"
168                 list-traversal primitives can     168                 list-traversal primitives can substitute for a good
169                 concurrency design coordinatin    169                 concurrency design coordinating among multiple updaters.
170                                                   170 
171         b.      If the list macros are being u    171         b.      If the list macros are being used, the list_add_tail_rcu()
172                 and list_add_rcu() primitives     172                 and list_add_rcu() primitives must be used in order
173                 to prevent weakly ordered mach    173                 to prevent weakly ordered machines from misordering
174                 structure initialization and p    174                 structure initialization and pointer planting.
175                 Similarly, if the hlist macros    175                 Similarly, if the hlist macros are being used, the
176                 hlist_add_head_rcu() primitive    176                 hlist_add_head_rcu() primitive is required.
177                                                   177 
178         c.      If the list macros are being u    178         c.      If the list macros are being used, the list_del_rcu()
179                 primitive must be used to keep    179                 primitive must be used to keep list_del()'s pointer
180                 poisoning from inflicting toxi    180                 poisoning from inflicting toxic effects on concurrent
181                 readers.  Similarly, if the hl    181                 readers.  Similarly, if the hlist macros are being used,
182                 the hlist_del_rcu() primitive     182                 the hlist_del_rcu() primitive is required.
183                                                   183 
184                 The list_replace_rcu() and hli    184                 The list_replace_rcu() and hlist_replace_rcu() primitives
185                 may be used to replace an old     185                 may be used to replace an old structure with a new one
186                 in their respective types of R    186                 in their respective types of RCU-protected lists.
187                                                   187 
188         d.      Rules similar to (4b) and (4c)    188         d.      Rules similar to (4b) and (4c) apply to the "hlist_nulls"
189                 type of RCU-protected linked l    189                 type of RCU-protected linked lists.
190                                                   190 
191         e.      Updates must ensure that initi    191         e.      Updates must ensure that initialization of a given
192                 structure happens before point    192                 structure happens before pointers to that structure are
193                 publicized.  Use the rcu_assig    193                 publicized.  Use the rcu_assign_pointer() primitive
194                 when publicizing a pointer to     194                 when publicizing a pointer to a structure that can
195                 be traversed by an RCU read-si    195                 be traversed by an RCU read-side critical section.
196                                                   196 
197 5.      If any of call_rcu(), call_srcu(), cal    197 5.      If any of call_rcu(), call_srcu(), call_rcu_tasks(), or
198         call_rcu_tasks_trace() is used, the ca    198         call_rcu_tasks_trace() is used, the callback function may be
199         invoked from softirq context, and in a    199         invoked from softirq context, and in any case with bottom halves
200         disabled.  In particular, this callbac    200         disabled.  In particular, this callback function cannot block.
201         If you need the callback to block, run    201         If you need the callback to block, run that code in a workqueue
202         handler scheduled from the callback.      202         handler scheduled from the callback.  The queue_rcu_work()
203         function does this for you in the case    203         function does this for you in the case of call_rcu().
204                                                   204 
205 6.      Since synchronize_rcu() can block, it     205 6.      Since synchronize_rcu() can block, it cannot be called
206         from any sort of irq context.  The sam    206         from any sort of irq context.  The same rule applies
207         for synchronize_srcu(), synchronize_rc    207         for synchronize_srcu(), synchronize_rcu_expedited(),
208         synchronize_srcu_expedited(), synchron    208         synchronize_srcu_expedited(), synchronize_rcu_tasks(),
209         synchronize_rcu_tasks_rude(), and sync    209         synchronize_rcu_tasks_rude(), and synchronize_rcu_tasks_trace().
210                                                   210 
211         The expedited forms of these primitive    211         The expedited forms of these primitives have the same semantics
212         as the non-expedited forms, but expedi    212         as the non-expedited forms, but expediting is more CPU intensive.
213         Use of the expedited primitives should    213         Use of the expedited primitives should be restricted to rare
214         configuration-change operations that w    214         configuration-change operations that would not normally be
215         undertaken while a real-time workload     215         undertaken while a real-time workload is running.  Note that
216         IPI-sensitive real-time workloads can     216         IPI-sensitive real-time workloads can use the rcupdate.rcu_normal
217         kernel boot parameter to completely di    217         kernel boot parameter to completely disable expedited grace
218         periods, though this might have perfor    218         periods, though this might have performance implications.
219                                                   219 
220         In particular, if you find yourself in    220         In particular, if you find yourself invoking one of the expedited
221         primitives repeatedly in a loop, pleas    221         primitives repeatedly in a loop, please do everyone a favor:
222         Restructure your code so that it batch    222         Restructure your code so that it batches the updates, allowing
223         a single non-expedited primitive to co    223         a single non-expedited primitive to cover the entire batch.
224         This will very likely be faster than t    224         This will very likely be faster than the loop containing the
225         expedited primitive, and will be much     225         expedited primitive, and will be much much easier on the rest
226         of the system, especially to real-time    226         of the system, especially to real-time workloads running on the
227         rest of the system.  Alternatively, in    227         rest of the system.  Alternatively, instead use asynchronous
228         primitives such as call_rcu().            228         primitives such as call_rcu().
229                                                   229 
230 7.      As of v4.20, a given kernel implements    230 7.      As of v4.20, a given kernel implements only one RCU flavor, which
231         is RCU-sched for PREEMPTION=n and RCU-    231         is RCU-sched for PREEMPTION=n and RCU-preempt for PREEMPTION=y.
232         If the updater uses call_rcu() or sync    232         If the updater uses call_rcu() or synchronize_rcu(), then
233         the corresponding readers may use:  (1    233         the corresponding readers may use:  (1) rcu_read_lock() and
234         rcu_read_unlock(), (2) any pair of pri    234         rcu_read_unlock(), (2) any pair of primitives that disables
235         and re-enables softirq, for example, r    235         and re-enables softirq, for example, rcu_read_lock_bh() and
236         rcu_read_unlock_bh(), or (3) any pair     236         rcu_read_unlock_bh(), or (3) any pair of primitives that disables
237         and re-enables preemption, for example    237         and re-enables preemption, for example, rcu_read_lock_sched() and
238         rcu_read_unlock_sched().  If the updat    238         rcu_read_unlock_sched().  If the updater uses synchronize_srcu()
239         or call_srcu(), then the corresponding    239         or call_srcu(), then the corresponding readers must use
240         srcu_read_lock() and srcu_read_unlock(    240         srcu_read_lock() and srcu_read_unlock(), and with the same
241         srcu_struct.  The rules for the expedi    241         srcu_struct.  The rules for the expedited RCU grace-period-wait
242         primitives are the same as for their n    242         primitives are the same as for their non-expedited counterparts.
243                                                   243 
244         Similarly, it is necessary to correctl    244         Similarly, it is necessary to correctly use the RCU Tasks flavors:
245                                                   245 
246         a.      If the updater uses synchroniz    246         a.      If the updater uses synchronize_rcu_tasks() or
247                 call_rcu_tasks(), then the rea    247                 call_rcu_tasks(), then the readers must refrain from
248                 executing voluntary context sw    248                 executing voluntary context switches, that is, from
249                 blocking.                         249                 blocking.
250                                                   250 
251         b.      If the updater uses call_rcu_t    251         b.      If the updater uses call_rcu_tasks_trace()
252                 or synchronize_rcu_tasks_trace    252                 or synchronize_rcu_tasks_trace(), then the
253                 corresponding readers must use    253                 corresponding readers must use rcu_read_lock_trace()
254                 and rcu_read_unlock_trace().      254                 and rcu_read_unlock_trace().
255                                                   255 
256         c.      If an updater uses synchronize    256         c.      If an updater uses synchronize_rcu_tasks_rude(),
257                 then the corresponding readers    257                 then the corresponding readers must use anything that
258                 disables preemption, for examp    258                 disables preemption, for example, preempt_disable()
259                 and preempt_enable().             259                 and preempt_enable().
260                                                   260 
261         Mixing things up will result in confus    261         Mixing things up will result in confusion and broken kernels, and
262         has even resulted in an exploitable se    262         has even resulted in an exploitable security issue.  Therefore,
263         when using non-obvious pairs of primit    263         when using non-obvious pairs of primitives, commenting is
264         of course a must.  One example of non-    264         of course a must.  One example of non-obvious pairing is
265         the XDP feature in networking, which c    265         the XDP feature in networking, which calls BPF programs from
266         network-driver NAPI (softirq) context.    266         network-driver NAPI (softirq) context.  BPF relies heavily on RCU
267         protection for its data structures, bu    267         protection for its data structures, but because the BPF program
268         invocation happens entirely within a s    268         invocation happens entirely within a single local_bh_disable()
269         section in a NAPI poll cycle, this usa    269         section in a NAPI poll cycle, this usage is safe.  The reason
270         that this usage is safe is that reader    270         that this usage is safe is that readers can use anything that
271         disables BH when updaters use call_rcu    271         disables BH when updaters use call_rcu() or synchronize_rcu().
272                                                   272 
273 8.      Although synchronize_rcu() is slower t    273 8.      Although synchronize_rcu() is slower than is call_rcu(),
274         it usually results in simpler code.  S    274         it usually results in simpler code.  So, unless update
275         performance is critically important, t    275         performance is critically important, the updaters cannot block,
276         or the latency of synchronize_rcu() is    276         or the latency of synchronize_rcu() is visible from userspace,
277         synchronize_rcu() should be used in pr    277         synchronize_rcu() should be used in preference to call_rcu().
278         Furthermore, kfree_rcu() and kvfree_rc    278         Furthermore, kfree_rcu() and kvfree_rcu() usually result
279         in even simpler code than does synchro    279         in even simpler code than does synchronize_rcu() without
280         synchronize_rcu()'s multi-millisecond     280         synchronize_rcu()'s multi-millisecond latency.  So please take
281         advantage of kfree_rcu()'s and kvfree_    281         advantage of kfree_rcu()'s and kvfree_rcu()'s "fire and forget"
282         memory-freeing capabilities where it a    282         memory-freeing capabilities where it applies.
283                                                   283 
284         An especially important property of th    284         An especially important property of the synchronize_rcu()
285         primitive is that it automatically sel    285         primitive is that it automatically self-limits: if grace periods
286         are delayed for whatever reason, then     286         are delayed for whatever reason, then the synchronize_rcu()
287         primitive will correspondingly delay u    287         primitive will correspondingly delay updates.  In contrast,
288         code using call_rcu() should explicitl    288         code using call_rcu() should explicitly limit update rate in
289         cases where grace periods are delayed,    289         cases where grace periods are delayed, as failing to do so can
290         result in excessive realtime latencies    290         result in excessive realtime latencies or even OOM conditions.
291                                                   291 
292         Ways of gaining this self-limiting pro    292         Ways of gaining this self-limiting property when using call_rcu(),
293         kfree_rcu(), or kvfree_rcu() include:     293         kfree_rcu(), or kvfree_rcu() include:
294                                                   294 
295         a.      Keeping a count of the number     295         a.      Keeping a count of the number of data-structure elements
296                 used by the RCU-protected data    296                 used by the RCU-protected data structure, including
297                 those waiting for a grace peri    297                 those waiting for a grace period to elapse.  Enforce a
298                 limit on this number, stalling    298                 limit on this number, stalling updates as needed to allow
299                 previously deferred frees to c    299                 previously deferred frees to complete.  Alternatively,
300                 limit only the number awaiting    300                 limit only the number awaiting deferred free rather than
301                 the total number of elements.     301                 the total number of elements.
302                                                   302 
303                 One way to stall the updates i    303                 One way to stall the updates is to acquire the update-side
304                 mutex.  (Don't try this with a    304                 mutex.  (Don't try this with a spinlock -- other CPUs
305                 spinning on the lock could pre    305                 spinning on the lock could prevent the grace period
306                 from ever ending.)  Another wa    306                 from ever ending.)  Another way to stall the updates
307                 is for the updates to use a wr    307                 is for the updates to use a wrapper function around
308                 the memory allocator, so that     308                 the memory allocator, so that this wrapper function
309                 simulates OOM when there is to    309                 simulates OOM when there is too much memory awaiting an
310                 RCU grace period.  There are o    310                 RCU grace period.  There are of course many other
311                 variations on this theme.         311                 variations on this theme.
312                                                   312 
313         b.      Limiting update rate.  For exa    313         b.      Limiting update rate.  For example, if updates occur only
314                 once per hour, then no explici    314                 once per hour, then no explicit rate limiting is
315                 required, unless your system i    315                 required, unless your system is already badly broken.
316                 Older versions of the dcache s    316                 Older versions of the dcache subsystem take this approach,
317                 guarding updates with a global    317                 guarding updates with a global lock, limiting their rate.
318                                                   318 
319         c.      Trusted update -- if updates c    319         c.      Trusted update -- if updates can only be done manually by
320                 superuser or some other truste    320                 superuser or some other trusted user, then it might not
321                 be necessary to automatically     321                 be necessary to automatically limit them.  The theory
322                 here is that superuser already    322                 here is that superuser already has lots of ways to crash
323                 the machine.                      323                 the machine.
324                                                   324 
325         d.      Periodically invoke rcu_barrie    325         d.      Periodically invoke rcu_barrier(), permitting a limited
326                 number of updates per grace pe    326                 number of updates per grace period.
327                                                   327 
328         The same cautions apply to call_srcu()    328         The same cautions apply to call_srcu(), call_rcu_tasks(), and
329         call_rcu_tasks_trace().  This is why t    329         call_rcu_tasks_trace().  This is why there is an srcu_barrier(),
330         rcu_barrier_tasks(), and rcu_barrier_t    330         rcu_barrier_tasks(), and rcu_barrier_tasks_trace(), respectively.
331                                                   331 
332         Note that although these primitives do    332         Note that although these primitives do take action to avoid
333         memory exhaustion when any given CPU h    333         memory exhaustion when any given CPU has too many callbacks,
334         a determined user or administrator can    334         a determined user or administrator can still exhaust memory.
335         This is especially the case if a syste    335         This is especially the case if a system with a large number of
336         CPUs has been configured to offload al    336         CPUs has been configured to offload all of its RCU callbacks onto
337         a single CPU, or if the system has rel    337         a single CPU, or if the system has relatively little free memory.
338                                                   338 
339 9.      All RCU list-traversal primitives, whi    339 9.      All RCU list-traversal primitives, which include
340         rcu_dereference(), list_for_each_entry    340         rcu_dereference(), list_for_each_entry_rcu(), and
341         list_for_each_safe_rcu(), must be eith    341         list_for_each_safe_rcu(), must be either within an RCU read-side
342         critical section or must be protected     342         critical section or must be protected by appropriate update-side
343         locks.  RCU read-side critical section    343         locks.  RCU read-side critical sections are delimited by
344         rcu_read_lock() and rcu_read_unlock(),    344         rcu_read_lock() and rcu_read_unlock(), or by similar primitives
345         such as rcu_read_lock_bh() and rcu_rea    345         such as rcu_read_lock_bh() and rcu_read_unlock_bh(), in which
346         case the matching rcu_dereference() pr    346         case the matching rcu_dereference() primitive must be used in
347         order to keep lockdep happy, in this c    347         order to keep lockdep happy, in this case, rcu_dereference_bh().
348                                                   348 
349         The reason that it is permissible to u    349         The reason that it is permissible to use RCU list-traversal
350         primitives when the update-side lock i    350         primitives when the update-side lock is held is that doing so
351         can be quite helpful in reducing code     351         can be quite helpful in reducing code bloat when common code is
352         shared between readers and updaters.      352         shared between readers and updaters.  Additional primitives
353         are provided for this case, as discuss    353         are provided for this case, as discussed in lockdep.rst.
354                                                   354 
355         One exception to this rule is when dat    355         One exception to this rule is when data is only ever added to
356         the linked data structure, and is neve    356         the linked data structure, and is never removed during any
357         time that readers might be accessing t    357         time that readers might be accessing that structure.  In such
358         cases, READ_ONCE() may be used in plac    358         cases, READ_ONCE() may be used in place of rcu_dereference()
359         and the read-side markers (rcu_read_lo    359         and the read-side markers (rcu_read_lock() and rcu_read_unlock(),
360         for example) may be omitted.              360         for example) may be omitted.
361                                                   361 
362 10.     Conversely, if you are in an RCU read-    362 10.     Conversely, if you are in an RCU read-side critical section,
363         and you don't hold the appropriate upd    363         and you don't hold the appropriate update-side lock, you *must*
364         use the "_rcu()" variants of the list     364         use the "_rcu()" variants of the list macros.  Failing to do so
365         will break Alpha, cause aggressive com    365         will break Alpha, cause aggressive compilers to generate bad code,
366         and confuse people trying to understan    366         and confuse people trying to understand your code.
367                                                   367 
368 11.     Any lock acquired by an RCU callback m    368 11.     Any lock acquired by an RCU callback must be acquired elsewhere
369         with softirq disabled, e.g., via spin_    369         with softirq disabled, e.g., via spin_lock_bh().  Failing to
370         disable softirq on a given acquisition    370         disable softirq on a given acquisition of that lock will result
371         in deadlock as soon as the RCU softirq    371         in deadlock as soon as the RCU softirq handler happens to run
372         your RCU callback while interrupting t    372         your RCU callback while interrupting that acquisition's critical
373         section.                                  373         section.
374                                                   374 
375 12.     RCU callbacks can be and are executed     375 12.     RCU callbacks can be and are executed in parallel.  In many cases,
376         the callback code simply wrappers arou    376         the callback code simply wrappers around kfree(), so that this
377         is not an issue (or, more accurately,     377         is not an issue (or, more accurately, to the extent that it is
378         an issue, the memory-allocator locking    378         an issue, the memory-allocator locking handles it).  However,
379         if the callbacks do manipulate a share    379         if the callbacks do manipulate a shared data structure, they
380         must use whatever locking or other syn    380         must use whatever locking or other synchronization is required
381         to safely access and/or modify that da    381         to safely access and/or modify that data structure.
382                                                   382 
383         Do not assume that RCU callbacks will     383         Do not assume that RCU callbacks will be executed on the same
384         CPU that executed the corresponding ca    384         CPU that executed the corresponding call_rcu(), call_srcu(),
385         call_rcu_tasks(), or call_rcu_tasks_tr    385         call_rcu_tasks(), or call_rcu_tasks_trace().  For example, if
386         a given CPU goes offline while having     386         a given CPU goes offline while having an RCU callback pending,
387         then that RCU callback will execute on    387         then that RCU callback will execute on some surviving CPU.
388         (If this was not the case, a self-spaw    388         (If this was not the case, a self-spawning RCU callback would
389         prevent the victim CPU from ever going    389         prevent the victim CPU from ever going offline.)  Furthermore,
390         CPUs designated by rcu_nocbs= might we    390         CPUs designated by rcu_nocbs= might well *always* have their
391         RCU callbacks executed on some other C    391         RCU callbacks executed on some other CPUs, in fact, for some
392         real-time workloads, this is the whole    392         real-time workloads, this is the whole point of using the
393         rcu_nocbs= kernel boot parameter.         393         rcu_nocbs= kernel boot parameter.
394                                                   394 
395         In addition, do not assume that callba    395         In addition, do not assume that callbacks queued in a given order
396         will be invoked in that order, even if    396         will be invoked in that order, even if they all are queued on the
397         same CPU.  Furthermore, do not assume     397         same CPU.  Furthermore, do not assume that same-CPU callbacks will
398         be invoked serially.  For example, in     398         be invoked serially.  For example, in recent kernels, CPUs can be
399         switched between offloaded and de-offl    399         switched between offloaded and de-offloaded callback invocation,
400         and while a given CPU is undergoing su    400         and while a given CPU is undergoing such a switch, its callbacks
401         might be concurrently invoked by that     401         might be concurrently invoked by that CPU's softirq handler and
402         that CPU's rcuo kthread.  At such time    402         that CPU's rcuo kthread.  At such times, that CPU's callbacks
403         might be executed both concurrently an    403         might be executed both concurrently and out of order.
404                                                   404 
405 13.     Unlike most flavors of RCU, it *is* pe    405 13.     Unlike most flavors of RCU, it *is* permissible to block in an
406         SRCU read-side critical section (demar    406         SRCU read-side critical section (demarked by srcu_read_lock()
407         and srcu_read_unlock()), hence the "SR    407         and srcu_read_unlock()), hence the "SRCU": "sleepable RCU".
408         Please note that if you don't need to     408         Please note that if you don't need to sleep in read-side critical
409         sections, you should be using RCU rath    409         sections, you should be using RCU rather than SRCU, because RCU
410         is almost always faster and easier to     410         is almost always faster and easier to use than is SRCU.
411                                                   411 
412         Also unlike other forms of RCU, explic    412         Also unlike other forms of RCU, explicit initialization and
413         cleanup is required either at build ti    413         cleanup is required either at build time via DEFINE_SRCU()
414         or DEFINE_STATIC_SRCU() or at runtime     414         or DEFINE_STATIC_SRCU() or at runtime via init_srcu_struct()
415         and cleanup_srcu_struct().  These last    415         and cleanup_srcu_struct().  These last two are passed a
416         "struct srcu_struct" that defines the     416         "struct srcu_struct" that defines the scope of a given
417         SRCU domain.  Once initialized, the sr    417         SRCU domain.  Once initialized, the srcu_struct is passed
418         to srcu_read_lock(), srcu_read_unlock(    418         to srcu_read_lock(), srcu_read_unlock() synchronize_srcu(),
419         synchronize_srcu_expedited(), and call    419         synchronize_srcu_expedited(), and call_srcu().  A given
420         synchronize_srcu() waits only for SRCU    420         synchronize_srcu() waits only for SRCU read-side critical
421         sections governed by srcu_read_lock()     421         sections governed by srcu_read_lock() and srcu_read_unlock()
422         calls that have been passed the same s    422         calls that have been passed the same srcu_struct.  This property
423         is what makes sleeping read-side criti    423         is what makes sleeping read-side critical sections tolerable --
424         a given subsystem delays only its own     424         a given subsystem delays only its own updates, not those of other
425         subsystems using SRCU.  Therefore, SRC    425         subsystems using SRCU.  Therefore, SRCU is less prone to OOM the
426         system than RCU would be if RCU's read    426         system than RCU would be if RCU's read-side critical sections
427         were permitted to sleep.                  427         were permitted to sleep.
428                                                   428 
429         The ability to sleep in read-side crit    429         The ability to sleep in read-side critical sections does not
430         come for free.  First, corresponding s    430         come for free.  First, corresponding srcu_read_lock() and
431         srcu_read_unlock() calls must be passe    431         srcu_read_unlock() calls must be passed the same srcu_struct.
432         Second, grace-period-detection overhea    432         Second, grace-period-detection overhead is amortized only
433         over those updates sharing a given src    433         over those updates sharing a given srcu_struct, rather than
434         being globally amortized as they are f    434         being globally amortized as they are for other forms of RCU.
435         Therefore, SRCU should be used in pref    435         Therefore, SRCU should be used in preference to rw_semaphore
436         only in extremely read-intensive situa    436         only in extremely read-intensive situations, or in situations
437         requiring SRCU's read-side deadlock im    437         requiring SRCU's read-side deadlock immunity or low read-side
438         realtime latency.  You should also con    438         realtime latency.  You should also consider percpu_rw_semaphore
439         when you need lightweight readers.        439         when you need lightweight readers.
440                                                   440 
441         SRCU's expedited primitive (synchroniz    441         SRCU's expedited primitive (synchronize_srcu_expedited())
442         never sends IPIs to other CPUs, so it     442         never sends IPIs to other CPUs, so it is easier on
443         real-time workloads than is synchroniz    443         real-time workloads than is synchronize_rcu_expedited().
444                                                   444 
445         It is also permissible to sleep in RCU    445         It is also permissible to sleep in RCU Tasks Trace read-side
446         critical section, which are delimited     446         critical section, which are delimited by rcu_read_lock_trace() and
447         rcu_read_unlock_trace().  However, thi    447         rcu_read_unlock_trace().  However, this is a specialized flavor
448         of RCU, and you should not use it with    448         of RCU, and you should not use it without first checking with
449         its current users.  In most cases, you    449         its current users.  In most cases, you should instead use SRCU.
450                                                   450 
451         Note that rcu_assign_pointer() relates    451         Note that rcu_assign_pointer() relates to SRCU just as it does to
452         other forms of RCU, but instead of rcu    452         other forms of RCU, but instead of rcu_dereference() you should
453         use srcu_dereference() in order to avo    453         use srcu_dereference() in order to avoid lockdep splats.
454                                                   454 
455 14.     The whole point of call_rcu(), synchro    455 14.     The whole point of call_rcu(), synchronize_rcu(), and friends
456         is to wait until all pre-existing read    456         is to wait until all pre-existing readers have finished before
457         carrying out some otherwise-destructiv    457         carrying out some otherwise-destructive operation.  It is
458         therefore critically important to *fir    458         therefore critically important to *first* remove any path
459         that readers can follow that could be     459         that readers can follow that could be affected by the
460         destructive operation, and *only then*    460         destructive operation, and *only then* invoke call_rcu(),
461         synchronize_rcu(), or friends.            461         synchronize_rcu(), or friends.
462                                                   462 
463         Because these primitives only wait for    463         Because these primitives only wait for pre-existing readers, it
464         is the caller's responsibility to guar    464         is the caller's responsibility to guarantee that any subsequent
465         readers will execute safely.              465         readers will execute safely.
466                                                   466 
467 15.     The various RCU read-side primitives d    467 15.     The various RCU read-side primitives do *not* necessarily contain
468         memory barriers.  You should therefore    468         memory barriers.  You should therefore plan for the CPU
469         and the compiler to freely reorder cod    469         and the compiler to freely reorder code into and out of RCU
470         read-side critical sections.  It is th    470         read-side critical sections.  It is the responsibility of the
471         RCU update-side primitives to deal wit    471         RCU update-side primitives to deal with this.
472                                                   472 
473         For SRCU readers, you can use smp_mb__    473         For SRCU readers, you can use smp_mb__after_srcu_read_unlock()
474         immediately after an srcu_read_unlock(    474         immediately after an srcu_read_unlock() to get a full barrier.
475                                                   475 
476 16.     Use CONFIG_PROVE_LOCKING, CONFIG_DEBUG    476 16.     Use CONFIG_PROVE_LOCKING, CONFIG_DEBUG_OBJECTS_RCU_HEAD, and the
477         __rcu sparse checks to validate your R    477         __rcu sparse checks to validate your RCU code.  These can help
478         find problems as follows:                 478         find problems as follows:
479                                                   479 
480         CONFIG_PROVE_LOCKING:                     480         CONFIG_PROVE_LOCKING:
481                 check that accesses to RCU-pro    481                 check that accesses to RCU-protected data structures
482                 are carried out under the prop    482                 are carried out under the proper RCU read-side critical
483                 section, while holding the rig    483                 section, while holding the right combination of locks,
484                 or whatever other conditions a    484                 or whatever other conditions are appropriate.
485                                                   485 
486         CONFIG_DEBUG_OBJECTS_RCU_HEAD:            486         CONFIG_DEBUG_OBJECTS_RCU_HEAD:
487                 check that you don't pass the     487                 check that you don't pass the same object to call_rcu()
488                 (or friends) before an RCU gra    488                 (or friends) before an RCU grace period has elapsed
489                 since the last time that you p    489                 since the last time that you passed that same object to
490                 call_rcu() (or friends).          490                 call_rcu() (or friends).
491                                                   491 
492         CONFIG_RCU_STRICT_GRACE_PERIOD:           492         CONFIG_RCU_STRICT_GRACE_PERIOD:
493                 combine with KASAN to check fo    493                 combine with KASAN to check for pointers leaked out
494                 of RCU read-side critical sect    494                 of RCU read-side critical sections.  This Kconfig
495                 option is tough on both perfor    495                 option is tough on both performance and scalability,
496                 and so is limited to four-CPU     496                 and so is limited to four-CPU systems.
497                                                   497 
498         __rcu sparse checks:                      498         __rcu sparse checks:
499                 tag the pointer to the RCU-pro    499                 tag the pointer to the RCU-protected data structure
500                 with __rcu, and sparse will wa    500                 with __rcu, and sparse will warn you if you access that
501                 pointer without the services o    501                 pointer without the services of one of the variants
502                 of rcu_dereference().             502                 of rcu_dereference().
503                                                   503 
504         These debugging aids can help you find    504         These debugging aids can help you find problems that are
505         otherwise extremely difficult to spot.    505         otherwise extremely difficult to spot.
506                                                   506 
507 17.     If you pass a callback function define    507 17.     If you pass a callback function defined within a module
508         to one of call_rcu(), call_srcu(), cal    508         to one of call_rcu(), call_srcu(), call_rcu_tasks(), or
509         call_rcu_tasks_trace(), then it is nec    509         call_rcu_tasks_trace(), then it is necessary to wait for all
510         pending callbacks to be invoked before    510         pending callbacks to be invoked before unloading that module.
511         Note that it is absolutely *not* suffi    511         Note that it is absolutely *not* sufficient to wait for a grace
512         period!  For example, synchronize_rcu(    512         period!  For example, synchronize_rcu() implementation is *not*
513         guaranteed to wait for callbacks regis    513         guaranteed to wait for callbacks registered on other CPUs via
514         call_rcu().  Or even on the current CP    514         call_rcu().  Or even on the current CPU if that CPU recently
515         went offline and came back online.        515         went offline and came back online.
516                                                   516 
517         You instead need to use one of the bar    517         You instead need to use one of the barrier functions:
518                                                   518 
519         -       call_rcu() -> rcu_barrier()       519         -       call_rcu() -> rcu_barrier()
520         -       call_srcu() -> srcu_barrier()     520         -       call_srcu() -> srcu_barrier()
521         -       call_rcu_tasks() -> rcu_barrie    521         -       call_rcu_tasks() -> rcu_barrier_tasks()
522         -       call_rcu_tasks_trace() -> rcu_    522         -       call_rcu_tasks_trace() -> rcu_barrier_tasks_trace()
523                                                   523 
524         However, these barrier functions are a    524         However, these barrier functions are absolutely *not* guaranteed
525         to wait for a grace period.  For examp    525         to wait for a grace period.  For example, if there are no
526         call_rcu() callbacks queued anywhere i    526         call_rcu() callbacks queued anywhere in the system, rcu_barrier()
527         can and will return immediately.          527         can and will return immediately.
528                                                   528 
529         So if you need to wait for both a grac    529         So if you need to wait for both a grace period and for all
530         pre-existing callbacks, you will need     530         pre-existing callbacks, you will need to invoke both functions,
531         with the pair depending on the flavor     531         with the pair depending on the flavor of RCU:
532                                                   532 
533         -       Either synchronize_rcu() or sy    533         -       Either synchronize_rcu() or synchronize_rcu_expedited(),
534                 together with rcu_barrier()       534                 together with rcu_barrier()
535         -       Either synchronize_srcu() or s    535         -       Either synchronize_srcu() or synchronize_srcu_expedited(),
536                 together with and srcu_barrier    536                 together with and srcu_barrier()
537         -       synchronize_rcu_tasks() and rc    537         -       synchronize_rcu_tasks() and rcu_barrier_tasks()
538         -       synchronize_tasks_trace() and     538         -       synchronize_tasks_trace() and rcu_barrier_tasks_trace()
539                                                   539 
540         If necessary, you can use something li    540         If necessary, you can use something like workqueues to execute
541         the requisite pair of functions concur    541         the requisite pair of functions concurrently.
542                                                   542 
543         See rcubarrier.rst for more informatio    543         See rcubarrier.rst for more information.
                                                      

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