1 ==============================
2 RT-mutex implementation design
3 ==============================
4
5 Copyright (c) 2006 Steven Rostedt
6
7 Licensed under the GNU Free Documentation License, Version 1.2
8
9
10 This document tries to describe the design of the rtmutex.c implementation.
11 It doesn't describe the reasons why rtmutex.c exists. For that please see
12 Documentation/locking/rt-mutex.rst. Although this document does explain problems
13 that happen without this code, but that is in the concept to understand
14 what the code actually is doing.
15
16 The goal of this document is to help others understand the priority
17 inheritance (PI) algorithm that is used, as well as reasons for the
18 decisions that were made to implement PI in the manner that was done.
19
20
21 Unbounded Priority Inversion
22 ----------------------------
23
24 Priority inversion is when a lower priority process executes while a higher
25 priority process wants to run. This happens for several reasons, and
26 most of the time it can't be helped. Anytime a high priority process wants
27 to use a resource that a lower priority process has (a mutex for example),
28 the high priority process must wait until the lower priority process is done
29 with the resource. This is a priority inversion. What we want to prevent
30 is something called unbounded priority inversion. That is when the high
31 priority process is prevented from running by a lower priority process for
32 an undetermined amount of time.
33
34 The classic example of unbounded priority inversion is where you have three
35 processes, let's call them processes A, B, and C, where A is the highest
36 priority process, C is the lowest, and B is in between. A tries to grab a lock
37 that C owns and must wait and lets C run to release the lock. But in the
38 meantime, B executes, and since B is of a higher priority than C, it preempts C,
39 but by doing so, it is in fact preempting A which is a higher priority process.
40 Now there's no way of knowing how long A will be sleeping waiting for C
41 to release the lock, because for all we know, B is a CPU hog and will
42 never give C a chance to release the lock. This is called unbounded priority
43 inversion.
44
45 Here's a little ASCII art to show the problem::
46
47 grab lock L1 (owned by C)
48 |
49 A ---+
50 C preempted by B
51 |
52 C +----+
53
54 B +-------->
55 B now keeps A from running.
56
57
58 Priority Inheritance (PI)
59 -------------------------
60
61 There are several ways to solve this issue, but other ways are out of scope
62 for this document. Here we only discuss PI.
63
64 PI is where a process inherits the priority of another process if the other
65 process blocks on a lock owned by the current process. To make this easier
66 to understand, let's use the previous example, with processes A, B, and C again.
67
68 This time, when A blocks on the lock owned by C, C would inherit the priority
69 of A. So now if B becomes runnable, it would not preempt C, since C now has
70 the high priority of A. As soon as C releases the lock, it loses its
71 inherited priority, and A then can continue with the resource that C had.
72
73 Terminology
74 -----------
75
76 Here I explain some terminology that is used in this document to help describe
77 the design that is used to implement PI.
78
79 PI chain
80 - The PI chain is an ordered series of locks and processes that cause
81 processes to inherit priorities from a previous process that is
82 blocked on one of its locks. This is described in more detail
83 later in this document.
84
85 mutex
86 - In this document, to differentiate from locks that implement
87 PI and spin locks that are used in the PI code, from now on
88 the PI locks will be called a mutex.
89
90 lock
91 - In this document from now on, I will use the term lock when
92 referring to spin locks that are used to protect parts of the PI
93 algorithm. These locks disable preemption for UP (when
94 CONFIG_PREEMPT is enabled) and on SMP prevents multiple CPUs from
95 entering critical sections simultaneously.
96
97 spin lock
98 - Same as lock above.
99
100 waiter
101 - A waiter is a struct that is stored on the stack of a blocked
102 process. Since the scope of the waiter is within the code for
103 a process being blocked on the mutex, it is fine to allocate
104 the waiter on the process's stack (local variable). This
105 structure holds a pointer to the task, as well as the mutex that
106 the task is blocked on. It also has rbtree node structures to
107 place the task in the waiters rbtree of a mutex as well as the
108 pi_waiters rbtree of a mutex owner task (described below).
109
110 waiter is sometimes used in reference to the task that is waiting
111 on a mutex. This is the same as waiter->task.
112
113 waiters
114 - A list of processes that are blocked on a mutex.
115
116 top waiter
117 - The highest priority process waiting on a specific mutex.
118
119 top pi waiter
120 - The highest priority process waiting on one of the mutexes
121 that a specific process owns.
122
123 Note:
124 task and process are used interchangeably in this document, mostly to
125 differentiate between two processes that are being described together.
126
127
128 PI chain
129 --------
130
131 The PI chain is a list of processes and mutexes that may cause priority
132 inheritance to take place. Multiple chains may converge, but a chain
133 would never diverge, since a process can't be blocked on more than one
134 mutex at a time.
135
136 Example::
137
138 Process: A, B, C, D, E
139 Mutexes: L1, L2, L3, L4
140
141 A owns: L1
142 B blocked on L1
143 B owns L2
144 C blocked on L2
145 C owns L3
146 D blocked on L3
147 D owns L4
148 E blocked on L4
149
150 The chain would be::
151
152 E->L4->D->L3->C->L2->B->L1->A
153
154 To show where two chains merge, we could add another process F and
155 another mutex L5 where B owns L5 and F is blocked on mutex L5.
156
157 The chain for F would be::
158
159 F->L5->B->L1->A
160
161 Since a process may own more than one mutex, but never be blocked on more than
162 one, the chains merge.
163
164 Here we show both chains::
165
166 E->L4->D->L3->C->L2-+
167 |
168 +->B->L1->A
169 |
170 F->L5-+
171
172 For PI to work, the processes at the right end of these chains (or we may
173 also call it the Top of the chain) must be equal to or higher in priority
174 than the processes to the left or below in the chain.
175
176 Also since a mutex may have more than one process blocked on it, we can
177 have multiple chains merge at mutexes. If we add another process G that is
178 blocked on mutex L2::
179
180 G->L2->B->L1->A
181
182 And once again, to show how this can grow I will show the merging chains
183 again::
184
185 E->L4->D->L3->C-+
186 +->L2-+
187 | |
188 G-+ +->B->L1->A
189 |
190 F->L5-+
191
192 If process G has the highest priority in the chain, then all the tasks up
193 the chain (A and B in this example), must have their priorities increased
194 to that of G.
195
196 Mutex Waiters Tree
197 ------------------
198
199 Every mutex keeps track of all the waiters that are blocked on itself. The
200 mutex has a rbtree to store these waiters by priority. This tree is protected
201 by a spin lock that is located in the struct of the mutex. This lock is called
202 wait_lock.
203
204
205 Task PI Tree
206 ------------
207
208 To keep track of the PI chains, each process has its own PI rbtree. This is
209 a tree of all top waiters of the mutexes that are owned by the process.
210 Note that this tree only holds the top waiters and not all waiters that are
211 blocked on mutexes owned by the process.
212
213 The top of the task's PI tree is always the highest priority task that
214 is waiting on a mutex that is owned by the task. So if the task has
215 inherited a priority, it will always be the priority of the task that is
216 at the top of this tree.
217
218 This tree is stored in the task structure of a process as a rbtree called
219 pi_waiters. It is protected by a spin lock also in the task structure,
220 called pi_lock. This lock may also be taken in interrupt context, so when
221 locking the pi_lock, interrupts must be disabled.
222
223
224 Depth of the PI Chain
225 ---------------------
226
227 The maximum depth of the PI chain is not dynamic, and could actually be
228 defined. But is very complex to figure it out, since it depends on all
229 the nesting of mutexes. Let's look at the example where we have 3 mutexes,
230 L1, L2, and L3, and four separate functions func1, func2, func3 and func4.
231 The following shows a locking order of L1->L2->L3, but may not actually
232 be directly nested that way::
233
234 void func1(void)
235 {
236 mutex_lock(L1);
237
238 /* do anything */
239
240 mutex_unlock(L1);
241 }
242
243 void func2(void)
244 {
245 mutex_lock(L1);
246 mutex_lock(L2);
247
248 /* do something */
249
250 mutex_unlock(L2);
251 mutex_unlock(L1);
252 }
253
254 void func3(void)
255 {
256 mutex_lock(L2);
257 mutex_lock(L3);
258
259 /* do something else */
260
261 mutex_unlock(L3);
262 mutex_unlock(L2);
263 }
264
265 void func4(void)
266 {
267 mutex_lock(L3);
268
269 /* do something again */
270
271 mutex_unlock(L3);
272 }
273
274 Now we add 4 processes that run each of these functions separately.
275 Processes A, B, C, and D which run functions func1, func2, func3 and func4
276 respectively, and such that D runs first and A last. With D being preempted
277 in func4 in the "do something again" area, we have a locking that follows::
278
279 D owns L3
280 C blocked on L3
281 C owns L2
282 B blocked on L2
283 B owns L1
284 A blocked on L1
285
286 And thus we have the chain A->L1->B->L2->C->L3->D.
287
288 This gives us a PI depth of 4 (four processes), but looking at any of the
289 functions individually, it seems as though they only have at most a locking
290 depth of two. So, although the locking depth is defined at compile time,
291 it still is very difficult to find the possibilities of that depth.
292
293 Now since mutexes can be defined by user-land applications, we don't want a DOS
294 type of application that nests large amounts of mutexes to create a large
295 PI chain, and have the code holding spin locks while looking at a large
296 amount of data. So to prevent this, the implementation not only implements
297 a maximum lock depth, but also only holds at most two different locks at a
298 time, as it walks the PI chain. More about this below.
299
300
301 Mutex owner and flags
302 ---------------------
303
304 The mutex structure contains a pointer to the owner of the mutex. If the
305 mutex is not owned, this owner is set to NULL. Since all architectures
306 have the task structure on at least a two byte alignment (and if this is
307 not true, the rtmutex.c code will be broken!), this allows for the least
308 significant bit to be used as a flag. Bit 0 is used as the "Has Waiters"
309 flag. It's set whenever there are waiters on a mutex.
310
311 See Documentation/locking/rt-mutex.rst for further details.
312
313 cmpxchg Tricks
314 --------------
315
316 Some architectures implement an atomic cmpxchg (Compare and Exchange). This
317 is used (when applicable) to keep the fast path of grabbing and releasing
318 mutexes short.
319
320 cmpxchg is basically the following function performed atomically::
321
322 unsigned long _cmpxchg(unsigned long *A, unsigned long *B, unsigned long *C)
323 {
324 unsigned long T = *A;
325 if (*A == *B) {
326 *A = *C;
327 }
328 return T;
329 }
330 #define cmpxchg(a,b,c) _cmpxchg(&a,&b,&c)
331
332 This is really nice to have, since it allows you to only update a variable
333 if the variable is what you expect it to be. You know if it succeeded if
334 the return value (the old value of A) is equal to B.
335
336 The macro rt_mutex_cmpxchg is used to try to lock and unlock mutexes. If
337 the architecture does not support CMPXCHG, then this macro is simply set
338 to fail every time. But if CMPXCHG is supported, then this will
339 help out extremely to keep the fast path short.
340
341 The use of rt_mutex_cmpxchg with the flags in the owner field help optimize
342 the system for architectures that support it. This will also be explained
343 later in this document.
344
345
346 Priority adjustments
347 --------------------
348
349 The implementation of the PI code in rtmutex.c has several places that a
350 process must adjust its priority. With the help of the pi_waiters of a
351 process this is rather easy to know what needs to be adjusted.
352
353 The functions implementing the task adjustments are rt_mutex_adjust_prio
354 and rt_mutex_setprio. rt_mutex_setprio is only used in rt_mutex_adjust_prio.
355
356 rt_mutex_adjust_prio examines the priority of the task, and the highest
357 priority process that is waiting any of mutexes owned by the task. Since
358 the pi_waiters of a task holds an order by priority of all the top waiters
359 of all the mutexes that the task owns, we simply need to compare the top
360 pi waiter to its own normal/deadline priority and take the higher one.
361 Then rt_mutex_setprio is called to adjust the priority of the task to the
362 new priority. Note that rt_mutex_setprio is defined in kernel/sched/core.c
363 to implement the actual change in priority.
364
365 Note:
366 For the "prio" field in task_struct, the lower the number, the
367 higher the priority. A "prio" of 5 is of higher priority than a
368 "prio" of 10.
369
370 It is interesting to note that rt_mutex_adjust_prio can either increase
371 or decrease the priority of the task. In the case that a higher priority
372 process has just blocked on a mutex owned by the task, rt_mutex_adjust_prio
373 would increase/boost the task's priority. But if a higher priority task
374 were for some reason to leave the mutex (timeout or signal), this same function
375 would decrease/unboost the priority of the task. That is because the pi_waiters
376 always contains the highest priority task that is waiting on a mutex owned
377 by the task, so we only need to compare the priority of that top pi waiter
378 to the normal priority of the given task.
379
380
381 High level overview of the PI chain walk
382 ----------------------------------------
383
384 The PI chain walk is implemented by the function rt_mutex_adjust_prio_chain.
385
386 The implementation has gone through several iterations, and has ended up
387 with what we believe is the best. It walks the PI chain by only grabbing
388 at most two locks at a time, and is very efficient.
389
390 The rt_mutex_adjust_prio_chain can be used either to boost or lower process
391 priorities.
392
393 rt_mutex_adjust_prio_chain is called with a task to be checked for PI
394 (de)boosting (the owner of a mutex that a process is blocking on), a flag to
395 check for deadlocking, the mutex that the task owns, a pointer to a waiter
396 that is the process's waiter struct that is blocked on the mutex (although this
397 parameter may be NULL for deboosting), a pointer to the mutex on which the task
398 is blocked, and a top_task as the top waiter of the mutex.
399
400 For this explanation, I will not mention deadlock detection. This explanation
401 will try to stay at a high level.
402
403 When this function is called, there are no locks held. That also means
404 that the state of the owner and lock can change when entered into this function.
405
406 Before this function is called, the task has already had rt_mutex_adjust_prio
407 performed on it. This means that the task is set to the priority that it
408 should be at, but the rbtree nodes of the task's waiter have not been updated
409 with the new priorities, and this task may not be in the proper locations
410 in the pi_waiters and waiters trees that the task is blocked on. This function
411 solves all that.
412
413 The main operation of this function is summarized by Thomas Gleixner in
414 rtmutex.c. See the 'Chain walk basics and protection scope' comment for further
415 details.
416
417 Taking of a mutex (The walk through)
418 ------------------------------------
419
420 OK, now let's take a look at the detailed walk through of what happens when
421 taking a mutex.
422
423 The first thing that is tried is the fast taking of the mutex. This is
424 done when we have CMPXCHG enabled (otherwise the fast taking automatically
425 fails). Only when the owner field of the mutex is NULL can the lock be
426 taken with the CMPXCHG and nothing else needs to be done.
427
428 If there is contention on the lock, we go about the slow path
429 (rt_mutex_slowlock).
430
431 The slow path function is where the task's waiter structure is created on
432 the stack. This is because the waiter structure is only needed for the
433 scope of this function. The waiter structure holds the nodes to store
434 the task on the waiters tree of the mutex, and if need be, the pi_waiters
435 tree of the owner.
436
437 The wait_lock of the mutex is taken since the slow path of unlocking the
438 mutex also takes this lock.
439
440 We then call try_to_take_rt_mutex. This is where the architecture that
441 does not implement CMPXCHG would always grab the lock (if there's no
442 contention).
443
444 try_to_take_rt_mutex is used every time the task tries to grab a mutex in the
445 slow path. The first thing that is done here is an atomic setting of
446 the "Has Waiters" flag of the mutex's owner field. By setting this flag
447 now, the current owner of the mutex being contended for can't release the mutex
448 without going into the slow unlock path, and it would then need to grab the
449 wait_lock, which this code currently holds. So setting the "Has Waiters" flag
450 forces the current owner to synchronize with this code.
451
452 The lock is taken if the following are true:
453
454 1) The lock has no owner
455 2) The current task is the highest priority against all other
456 waiters of the lock
457
458 If the task succeeds to acquire the lock, then the task is set as the
459 owner of the lock, and if the lock still has waiters, the top_waiter
460 (highest priority task waiting on the lock) is added to this task's
461 pi_waiters tree.
462
463 If the lock is not taken by try_to_take_rt_mutex(), then the
464 task_blocks_on_rt_mutex() function is called. This will add the task to
465 the lock's waiter tree and propagate the pi chain of the lock as well
466 as the lock's owner's pi_waiters tree. This is described in the next
467 section.
468
469 Task blocks on mutex
470 --------------------
471
472 The accounting of a mutex and process is done with the waiter structure of
473 the process. The "task" field is set to the process, and the "lock" field
474 to the mutex. The rbtree node of waiter are initialized to the processes
475 current priority.
476
477 Since the wait_lock was taken at the entry of the slow lock, we can safely
478 add the waiter to the task waiter tree. If the current process is the
479 highest priority process currently waiting on this mutex, then we remove the
480 previous top waiter process (if it exists) from the pi_waiters of the owner,
481 and add the current process to that tree. Since the pi_waiter of the owner
482 has changed, we call rt_mutex_adjust_prio on the owner to see if the owner
483 should adjust its priority accordingly.
484
485 If the owner is also blocked on a lock, and had its pi_waiters changed
486 (or deadlock checking is on), we unlock the wait_lock of the mutex and go ahead
487 and run rt_mutex_adjust_prio_chain on the owner, as described earlier.
488
489 Now all locks are released, and if the current process is still blocked on a
490 mutex (waiter "task" field is not NULL), then we go to sleep (call schedule).
491
492 Waking up in the loop
493 ---------------------
494
495 The task can then wake up for a couple of reasons:
496 1) The previous lock owner released the lock, and the task now is top_waiter
497 2) we received a signal or timeout
498
499 In both cases, the task will try again to acquire the lock. If it
500 does, then it will take itself off the waiters tree and set itself back
501 to the TASK_RUNNING state.
502
503 In first case, if the lock was acquired by another task before this task
504 could get the lock, then it will go back to sleep and wait to be woken again.
505
506 The second case is only applicable for tasks that are grabbing a mutex
507 that can wake up before getting the lock, either due to a signal or
508 a timeout (i.e. rt_mutex_timed_futex_lock()). When woken, it will try to
509 take the lock again, if it succeeds, then the task will return with the
510 lock held, otherwise it will return with -EINTR if the task was woken
511 by a signal, or -ETIMEDOUT if it timed out.
512
513
514 Unlocking the Mutex
515 -------------------
516
517 The unlocking of a mutex also has a fast path for those architectures with
518 CMPXCHG. Since the taking of a mutex on contention always sets the
519 "Has Waiters" flag of the mutex's owner, we use this to know if we need to
520 take the slow path when unlocking the mutex. If the mutex doesn't have any
521 waiters, the owner field of the mutex would equal the current process and
522 the mutex can be unlocked by just replacing the owner field with NULL.
523
524 If the owner field has the "Has Waiters" bit set (or CMPXCHG is not available),
525 the slow unlock path is taken.
526
527 The first thing done in the slow unlock path is to take the wait_lock of the
528 mutex. This synchronizes the locking and unlocking of the mutex.
529
530 A check is made to see if the mutex has waiters or not. On architectures that
531 do not have CMPXCHG, this is the location that the owner of the mutex will
532 determine if a waiter needs to be awoken or not. On architectures that
533 do have CMPXCHG, that check is done in the fast path, but it is still needed
534 in the slow path too. If a waiter of a mutex woke up because of a signal
535 or timeout between the time the owner failed the fast path CMPXCHG check and
536 the grabbing of the wait_lock, the mutex may not have any waiters, thus the
537 owner still needs to make this check. If there are no waiters then the mutex
538 owner field is set to NULL, the wait_lock is released and nothing more is
539 needed.
540
541 If there are waiters, then we need to wake one up.
542
543 On the wake up code, the pi_lock of the current owner is taken. The top
544 waiter of the lock is found and removed from the waiters tree of the mutex
545 as well as the pi_waiters tree of the current owner. The "Has Waiters" bit is
546 marked to prevent lower priority tasks from stealing the lock.
547
548 Finally we unlock the pi_lock of the pending owner and wake it up.
549
550
551 Contact
552 -------
553
554 For updates on this document, please email Steven Rostedt <rostedt@goodmis.org>
555
556
557 Credits
558 -------
559
560 Author: Steven Rostedt <rostedt@goodmis.org>
561
562 Updated: Alex Shi <alex.shi@linaro.org> - 7/6/2017
563
564 Original Reviewers:
565 Ingo Molnar, Thomas Gleixner, Thomas Duetsch, and
566 Randy Dunlap
567
568 Update (7/6/2017) Reviewers: Steven Rostedt and Sebastian Siewior
569
570 Updates
571 -------
572
573 This document was originally written for 2.6.17-rc3-mm1
574 was updated on 4.12
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