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1 | # |
2 | # Copyright (c) 2006 Steven Rostedt | |
3 | # Licensed under the GNU Free Documentation License, Version 1.2 | |
4 | # | |
5 | ||
6 | RT-mutex implementation design | |
7 | ------------------------------ | |
8 | ||
9 | This document tries to describe the design of the rtmutex.c implementation. | |
10 | It doesn't describe the reasons why rtmutex.c exists. For that please see | |
796cacdd | 11 | Documentation/locking/rt-mutex.txt. Although this document does explain problems |
a6537be9 SR |
12 | that happen without this code, but that is in the concept to understand |
13 | what the code actually is doing. | |
14 | ||
15 | The goal of this document is to help others understand the priority | |
16 | inheritance (PI) algorithm that is used, as well as reasons for the | |
17 | decisions that were made to implement PI in the manner that was done. | |
18 | ||
19 | ||
20 | Unbounded Priority Inversion | |
21 | ---------------------------- | |
22 | ||
23 | Priority inversion is when a lower priority process executes while a higher | |
24 | priority process wants to run. This happens for several reasons, and | |
25 | most of the time it can't be helped. Anytime a high priority process wants | |
26 | to use a resource that a lower priority process has (a mutex for example), | |
27 | the high priority process must wait until the lower priority process is done | |
28 | with the resource. This is a priority inversion. What we want to prevent | |
29 | is something called unbounded priority inversion. That is when the high | |
30 | priority process is prevented from running by a lower priority process for | |
31 | an undetermined amount of time. | |
32 | ||
c79a8d85 | 33 | The classic example of unbounded priority inversion is where you have three |
a6537be9 SR |
34 | processes, let's call them processes A, B, and C, where A is the highest |
35 | priority process, C is the lowest, and B is in between. A tries to grab a lock | |
36 | that C owns and must wait and lets C run to release the lock. But in the | |
37 | meantime, B executes, and since B is of a higher priority than C, it preempts C, | |
38 | but by doing so, it is in fact preempting A which is a higher priority process. | |
39 | Now there's no way of knowing how long A will be sleeping waiting for C | |
40 | to release the lock, because for all we know, B is a CPU hog and will | |
41 | never give C a chance to release the lock. This is called unbounded priority | |
42 | inversion. | |
43 | ||
44 | Here's a little ASCII art to show the problem. | |
45 | ||
46 | grab lock L1 (owned by C) | |
47 | | | |
48 | A ---+ | |
49 | C preempted by B | |
50 | | | |
51 | C +----+ | |
52 | ||
53 | B +--------> | |
54 | B now keeps A from running. | |
55 | ||
56 | ||
57 | Priority Inheritance (PI) | |
58 | ------------------------- | |
59 | ||
60 | There are several ways to solve this issue, but other ways are out of scope | |
61 | for this document. Here we only discuss PI. | |
62 | ||
63 | PI is where a process inherits the priority of another process if the other | |
64 | process blocks on a lock owned by the current process. To make this easier | |
65 | to understand, let's use the previous example, with processes A, B, and C again. | |
66 | ||
67 | This time, when A blocks on the lock owned by C, C would inherit the priority | |
68 | of A. So now if B becomes runnable, it would not preempt C, since C now has | |
69 | the high priority of A. As soon as C releases the lock, it loses its | |
70 | inherited priority, and A then can continue with the resource that C had. | |
71 | ||
72 | Terminology | |
73 | ----------- | |
74 | ||
75 | Here I explain some terminology that is used in this document to help describe | |
76 | the design that is used to implement PI. | |
77 | ||
78 | PI chain - The PI chain is an ordered series of locks and processes that cause | |
79 | processes to inherit priorities from a previous process that is | |
80 | blocked on one of its locks. This is described in more detail | |
81 | later in this document. | |
82 | ||
83 | mutex - In this document, to differentiate from locks that implement | |
84 | PI and spin locks that are used in the PI code, from now on | |
85 | the PI locks will be called a mutex. | |
86 | ||
87 | lock - In this document from now on, I will use the term lock when | |
88 | referring to spin locks that are used to protect parts of the PI | |
89 | algorithm. These locks disable preemption for UP (when | |
90 | CONFIG_PREEMPT is enabled) and on SMP prevents multiple CPUs from | |
91 | entering critical sections simultaneously. | |
92 | ||
93 | spin lock - Same as lock above. | |
94 | ||
95 | waiter - A waiter is a struct that is stored on the stack of a blocked | |
96 | process. Since the scope of the waiter is within the code for | |
97 | a process being blocked on the mutex, it is fine to allocate | |
98 | the waiter on the process's stack (local variable). This | |
99 | structure holds a pointer to the task, as well as the mutex that | |
f1824df1 AS |
100 | the task is blocked on. It also has rbtree node structures to |
101 | place the task in the waiters rbtree of a mutex as well as the | |
102 | pi_waiters rbtree of a mutex owner task (described below). | |
a6537be9 SR |
103 | |
104 | waiter is sometimes used in reference to the task that is waiting | |
105 | on a mutex. This is the same as waiter->task. | |
106 | ||
107 | waiters - A list of processes that are blocked on a mutex. | |
108 | ||
109 | top waiter - The highest priority process waiting on a specific mutex. | |
110 | ||
111 | top pi waiter - The highest priority process waiting on one of the mutexes | |
112 | that a specific process owns. | |
113 | ||
114 | Note: task and process are used interchangeably in this document, mostly to | |
115 | differentiate between two processes that are being described together. | |
116 | ||
117 | ||
118 | PI chain | |
119 | -------- | |
120 | ||
121 | The PI chain is a list of processes and mutexes that may cause priority | |
122 | inheritance to take place. Multiple chains may converge, but a chain | |
123 | would never diverge, since a process can't be blocked on more than one | |
124 | mutex at a time. | |
125 | ||
126 | Example: | |
127 | ||
128 | Process: A, B, C, D, E | |
129 | Mutexes: L1, L2, L3, L4 | |
130 | ||
131 | A owns: L1 | |
132 | B blocked on L1 | |
133 | B owns L2 | |
134 | C blocked on L2 | |
135 | C owns L3 | |
136 | D blocked on L3 | |
137 | D owns L4 | |
138 | E blocked on L4 | |
139 | ||
140 | The chain would be: | |
141 | ||
142 | E->L4->D->L3->C->L2->B->L1->A | |
143 | ||
144 | To show where two chains merge, we could add another process F and | |
145 | another mutex L5 where B owns L5 and F is blocked on mutex L5. | |
146 | ||
147 | The chain for F would be: | |
148 | ||
149 | F->L5->B->L1->A | |
150 | ||
151 | Since a process may own more than one mutex, but never be blocked on more than | |
152 | one, the chains merge. | |
153 | ||
154 | Here we show both chains: | |
155 | ||
156 | E->L4->D->L3->C->L2-+ | |
157 | | | |
158 | +->B->L1->A | |
159 | | | |
160 | F->L5-+ | |
161 | ||
162 | For PI to work, the processes at the right end of these chains (or we may | |
163 | also call it the Top of the chain) must be equal to or higher in priority | |
164 | than the processes to the left or below in the chain. | |
165 | ||
166 | Also since a mutex may have more than one process blocked on it, we can | |
167 | have multiple chains merge at mutexes. If we add another process G that is | |
168 | blocked on mutex L2: | |
169 | ||
170 | G->L2->B->L1->A | |
171 | ||
172 | And once again, to show how this can grow I will show the merging chains | |
173 | again. | |
174 | ||
175 | E->L4->D->L3->C-+ | |
176 | +->L2-+ | |
177 | | | | |
178 | G-+ +->B->L1->A | |
179 | | | |
180 | F->L5-+ | |
181 | ||
f1824df1 AS |
182 | If process G has the highest priority in the chain, then all the tasks up |
183 | the chain (A and B in this example), must have their priorities increased | |
184 | to that of G. | |
a6537be9 | 185 | |
f1824df1 | 186 | Mutex Waiters Tree |
a6537be9 SR |
187 | ----------------- |
188 | ||
f1824df1 AS |
189 | Every mutex keeps track of all the waiters that are blocked on itself. The |
190 | mutex has a rbtree to store these waiters by priority. This tree is protected | |
191 | by a spin lock that is located in the struct of the mutex. This lock is called | |
192 | wait_lock. | |
a6537be9 SR |
193 | |
194 | ||
f1824df1 | 195 | Task PI Tree |
a6537be9 SR |
196 | ------------ |
197 | ||
f1824df1 AS |
198 | To keep track of the PI chains, each process has its own PI rbtree. This is |
199 | a tree of all top waiters of the mutexes that are owned by the process. | |
200 | Note that this tree only holds the top waiters and not all waiters that are | |
a6537be9 SR |
201 | blocked on mutexes owned by the process. |
202 | ||
f1824df1 | 203 | The top of the task's PI tree is always the highest priority task that |
a6537be9 SR |
204 | is waiting on a mutex that is owned by the task. So if the task has |
205 | inherited a priority, it will always be the priority of the task that is | |
f1824df1 | 206 | at the top of this tree. |
a6537be9 | 207 | |
f1824df1 AS |
208 | This tree is stored in the task structure of a process as a rbtree called |
209 | pi_waiters. It is protected by a spin lock also in the task structure, | |
a6537be9 SR |
210 | called pi_lock. This lock may also be taken in interrupt context, so when |
211 | locking the pi_lock, interrupts must be disabled. | |
212 | ||
213 | ||
214 | Depth of the PI Chain | |
215 | --------------------- | |
216 | ||
217 | The maximum depth of the PI chain is not dynamic, and could actually be | |
218 | defined. But is very complex to figure it out, since it depends on all | |
219 | the nesting of mutexes. Let's look at the example where we have 3 mutexes, | |
220 | L1, L2, and L3, and four separate functions func1, func2, func3 and func4. | |
221 | The following shows a locking order of L1->L2->L3, but may not actually | |
222 | be directly nested that way. | |
223 | ||
224 | void func1(void) | |
225 | { | |
226 | mutex_lock(L1); | |
227 | ||
228 | /* do anything */ | |
229 | ||
230 | mutex_unlock(L1); | |
231 | } | |
232 | ||
233 | void func2(void) | |
234 | { | |
235 | mutex_lock(L1); | |
236 | mutex_lock(L2); | |
237 | ||
238 | /* do something */ | |
239 | ||
240 | mutex_unlock(L2); | |
241 | mutex_unlock(L1); | |
242 | } | |
243 | ||
244 | void func3(void) | |
245 | { | |
246 | mutex_lock(L2); | |
247 | mutex_lock(L3); | |
248 | ||
249 | /* do something else */ | |
250 | ||
251 | mutex_unlock(L3); | |
252 | mutex_unlock(L2); | |
253 | } | |
254 | ||
255 | void func4(void) | |
256 | { | |
257 | mutex_lock(L3); | |
258 | ||
259 | /* do something again */ | |
260 | ||
261 | mutex_unlock(L3); | |
262 | } | |
263 | ||
264 | Now we add 4 processes that run each of these functions separately. | |
265 | Processes A, B, C, and D which run functions func1, func2, func3 and func4 | |
266 | respectively, and such that D runs first and A last. With D being preempted | |
267 | in func4 in the "do something again" area, we have a locking that follows: | |
268 | ||
269 | D owns L3 | |
270 | C blocked on L3 | |
271 | C owns L2 | |
272 | B blocked on L2 | |
273 | B owns L1 | |
274 | A blocked on L1 | |
275 | ||
276 | And thus we have the chain A->L1->B->L2->C->L3->D. | |
277 | ||
278 | This gives us a PI depth of 4 (four processes), but looking at any of the | |
279 | functions individually, it seems as though they only have at most a locking | |
280 | depth of two. So, although the locking depth is defined at compile time, | |
281 | it still is very difficult to find the possibilities of that depth. | |
282 | ||
283 | Now since mutexes can be defined by user-land applications, we don't want a DOS | |
284 | type of application that nests large amounts of mutexes to create a large | |
285 | PI chain, and have the code holding spin locks while looking at a large | |
286 | amount of data. So to prevent this, the implementation not only implements | |
287 | a maximum lock depth, but also only holds at most two different locks at a | |
288 | time, as it walks the PI chain. More about this below. | |
289 | ||
290 | ||
291 | Mutex owner and flags | |
292 | --------------------- | |
293 | ||
294 | The mutex structure contains a pointer to the owner of the mutex. If the | |
295 | mutex is not owned, this owner is set to NULL. Since all architectures | |
f1824df1 AS |
296 | have the task structure on at least a two byte alignment (and if this is |
297 | not true, the rtmutex.c code will be broken!), this allows for the least | |
298 | significant bit to be used as a flag. Bit 0 is used as the "Has Waiters" | |
299 | flag. It's set whenever there are waiters on a mutex. | |
a6537be9 | 300 | |
f1824df1 | 301 | See Documentation/locking/rt-mutex.txt for further details. |
a6537be9 SR |
302 | |
303 | cmpxchg Tricks | |
304 | -------------- | |
305 | ||
306 | Some architectures implement an atomic cmpxchg (Compare and Exchange). This | |
307 | is used (when applicable) to keep the fast path of grabbing and releasing | |
308 | mutexes short. | |
309 | ||
310 | cmpxchg is basically the following function performed atomically: | |
311 | ||
312 | unsigned long _cmpxchg(unsigned long *A, unsigned long *B, unsigned long *C) | |
313 | { | |
9ba0bdfd JA |
314 | unsigned long T = *A; |
315 | if (*A == *B) { | |
316 | *A = *C; | |
317 | } | |
318 | return T; | |
a6537be9 SR |
319 | } |
320 | #define cmpxchg(a,b,c) _cmpxchg(&a,&b,&c) | |
321 | ||
322 | This is really nice to have, since it allows you to only update a variable | |
323 | if the variable is what you expect it to be. You know if it succeeded if | |
324 | the return value (the old value of A) is equal to B. | |
325 | ||
326 | The macro rt_mutex_cmpxchg is used to try to lock and unlock mutexes. If | |
327 | the architecture does not support CMPXCHG, then this macro is simply set | |
328 | to fail every time. But if CMPXCHG is supported, then this will | |
329 | help out extremely to keep the fast path short. | |
330 | ||
331 | The use of rt_mutex_cmpxchg with the flags in the owner field help optimize | |
332 | the system for architectures that support it. This will also be explained | |
333 | later in this document. | |
334 | ||
335 | ||
336 | Priority adjustments | |
337 | -------------------- | |
338 | ||
339 | The implementation of the PI code in rtmutex.c has several places that a | |
f1824df1 | 340 | process must adjust its priority. With the help of the pi_waiters of a |
a6537be9 SR |
341 | process this is rather easy to know what needs to be adjusted. |
342 | ||
f1824df1 AS |
343 | The functions implementing the task adjustments are rt_mutex_adjust_prio |
344 | and rt_mutex_setprio. rt_mutex_setprio is only used in rt_mutex_adjust_prio. | |
a6537be9 | 345 | |
f1824df1 AS |
346 | rt_mutex_adjust_prio examines the priority of the task, and the highest |
347 | priority process that is waiting any of mutexes owned by the task. Since | |
348 | the pi_waiters of a task holds an order by priority of all the top waiters | |
349 | of all the mutexes that the task owns, we simply need to compare the top | |
350 | pi waiter to its own normal/deadline priority and take the higher one. | |
351 | Then rt_mutex_setprio is called to adjust the priority of the task to the | |
352 | new priority. Note that rt_mutex_setprio is defined in kernel/sched/core.c | |
353 | to implement the actual change in priority. | |
a6537be9 | 354 | |
f1824df1 AS |
355 | (Note: For the "prio" field in task_struct, the lower the number, the |
356 | higher the priority. A "prio" of 5 is of higher priority than a | |
357 | "prio" of 10.) | |
a6537be9 | 358 | |
f1824df1 | 359 | It is interesting to note that rt_mutex_adjust_prio can either increase |
a6537be9 | 360 | or decrease the priority of the task. In the case that a higher priority |
f1824df1 | 361 | process has just blocked on a mutex owned by the task, rt_mutex_adjust_prio |
a6537be9 SR |
362 | would increase/boost the task's priority. But if a higher priority task |
363 | were for some reason to leave the mutex (timeout or signal), this same function | |
f1824df1 | 364 | would decrease/unboost the priority of the task. That is because the pi_waiters |
a6537be9 SR |
365 | always contains the highest priority task that is waiting on a mutex owned |
366 | by the task, so we only need to compare the priority of that top pi waiter | |
367 | to the normal priority of the given task. | |
368 | ||
369 | ||
370 | High level overview of the PI chain walk | |
371 | ---------------------------------------- | |
372 | ||
373 | The PI chain walk is implemented by the function rt_mutex_adjust_prio_chain. | |
374 | ||
375 | The implementation has gone through several iterations, and has ended up | |
376 | with what we believe is the best. It walks the PI chain by only grabbing | |
377 | at most two locks at a time, and is very efficient. | |
378 | ||
379 | The rt_mutex_adjust_prio_chain can be used either to boost or lower process | |
380 | priorities. | |
381 | ||
382 | rt_mutex_adjust_prio_chain is called with a task to be checked for PI | |
383 | (de)boosting (the owner of a mutex that a process is blocking on), a flag to | |
f1824df1 | 384 | check for deadlocking, the mutex that the task owns, a pointer to a waiter |
a6537be9 | 385 | that is the process's waiter struct that is blocked on the mutex (although this |
f1824df1 AS |
386 | parameter may be NULL for deboosting), a pointer to the mutex on which the task |
387 | is blocked, and a top_task as the top waiter of the mutex. | |
a6537be9 SR |
388 | |
389 | For this explanation, I will not mention deadlock detection. This explanation | |
390 | will try to stay at a high level. | |
391 | ||
392 | When this function is called, there are no locks held. That also means | |
393 | that the state of the owner and lock can change when entered into this function. | |
394 | ||
395 | Before this function is called, the task has already had rt_mutex_adjust_prio | |
396 | performed on it. This means that the task is set to the priority that it | |
f1824df1 AS |
397 | should be at, but the rbtree nodes of the task's waiter have not been updated |
398 | with the new priorities, and this task may not be in the proper locations | |
399 | in the pi_waiters and waiters trees that the task is blocked on. This function | |
a6537be9 SR |
400 | solves all that. |
401 | ||
f1824df1 AS |
402 | The main operation of this function is summarized by Thomas Gleixner in |
403 | rtmutex.c. See the 'Chain walk basics and protection scope' comment for further | |
404 | details. | |
a6537be9 SR |
405 | |
406 | Taking of a mutex (The walk through) | |
407 | ------------------------------------ | |
408 | ||
409 | OK, now let's take a look at the detailed walk through of what happens when | |
410 | taking a mutex. | |
411 | ||
412 | The first thing that is tried is the fast taking of the mutex. This is | |
413 | done when we have CMPXCHG enabled (otherwise the fast taking automatically | |
414 | fails). Only when the owner field of the mutex is NULL can the lock be | |
415 | taken with the CMPXCHG and nothing else needs to be done. | |
416 | ||
f1824df1 AS |
417 | If there is contention on the lock, we go about the slow path |
418 | (rt_mutex_slowlock). | |
a6537be9 SR |
419 | |
420 | The slow path function is where the task's waiter structure is created on | |
421 | the stack. This is because the waiter structure is only needed for the | |
422 | scope of this function. The waiter structure holds the nodes to store | |
f1824df1 AS |
423 | the task on the waiters tree of the mutex, and if need be, the pi_waiters |
424 | tree of the owner. | |
a6537be9 SR |
425 | |
426 | The wait_lock of the mutex is taken since the slow path of unlocking the | |
427 | mutex also takes this lock. | |
428 | ||
429 | We then call try_to_take_rt_mutex. This is where the architecture that | |
430 | does not implement CMPXCHG would always grab the lock (if there's no | |
431 | contention). | |
432 | ||
433 | try_to_take_rt_mutex is used every time the task tries to grab a mutex in the | |
434 | slow path. The first thing that is done here is an atomic setting of | |
f1824df1 AS |
435 | the "Has Waiters" flag of the mutex's owner field. By setting this flag |
436 | now, the current owner of the mutex being contended for can't release the mutex | |
437 | without going into the slow unlock path, and it would then need to grab the | |
438 | wait_lock, which this code currently holds. So setting the "Has Waiters" flag | |
439 | forces the current owner to synchronize with this code. | |
440 | ||
441 | The lock is taken if the following are true: | |
442 | 1) The lock has no owner | |
443 | 2) The current task is the highest priority against all other | |
444 | waiters of the lock | |
445 | ||
446 | If the task succeeds to acquire the lock, then the task is set as the | |
447 | owner of the lock, and if the lock still has waiters, the top_waiter | |
448 | (highest priority task waiting on the lock) is added to this task's | |
449 | pi_waiters tree. | |
450 | ||
451 | If the lock is not taken by try_to_take_rt_mutex(), then the | |
452 | task_blocks_on_rt_mutex() function is called. This will add the task to | |
453 | the lock's waiter tree and propagate the pi chain of the lock as well | |
454 | as the lock's owner's pi_waiters tree. This is described in the next | |
455 | section. | |
a6537be9 SR |
456 | |
457 | Task blocks on mutex | |
458 | -------------------- | |
459 | ||
460 | The accounting of a mutex and process is done with the waiter structure of | |
461 | the process. The "task" field is set to the process, and the "lock" field | |
f1824df1 AS |
462 | to the mutex. The rbtree node of waiter are initialized to the processes |
463 | current priority. | |
a6537be9 SR |
464 | |
465 | Since the wait_lock was taken at the entry of the slow lock, we can safely | |
f1824df1 AS |
466 | add the waiter to the task waiter tree. If the current process is the |
467 | highest priority process currently waiting on this mutex, then we remove the | |
468 | previous top waiter process (if it exists) from the pi_waiters of the owner, | |
469 | and add the current process to that tree. Since the pi_waiter of the owner | |
a6537be9 SR |
470 | has changed, we call rt_mutex_adjust_prio on the owner to see if the owner |
471 | should adjust its priority accordingly. | |
472 | ||
f1824df1 | 473 | If the owner is also blocked on a lock, and had its pi_waiters changed |
a6537be9 SR |
474 | (or deadlock checking is on), we unlock the wait_lock of the mutex and go ahead |
475 | and run rt_mutex_adjust_prio_chain on the owner, as described earlier. | |
476 | ||
477 | Now all locks are released, and if the current process is still blocked on a | |
478 | mutex (waiter "task" field is not NULL), then we go to sleep (call schedule). | |
479 | ||
480 | Waking up in the loop | |
481 | --------------------- | |
482 | ||
f1824df1 AS |
483 | The task can then wake up for a couple of reasons: |
484 | 1) The previous lock owner released the lock, and the task now is top_waiter | |
485 | 2) we received a signal or timeout | |
a6537be9 | 486 | |
f1824df1 AS |
487 | In both cases, the task will try again to acquire the lock. If it |
488 | does, then it will take itself off the waiters tree and set itself back | |
489 | to the TASK_RUNNING state. | |
a6537be9 | 490 | |
f1824df1 AS |
491 | In first case, if the lock was acquired by another task before this task |
492 | could get the lock, then it will go back to sleep and wait to be woken again. | |
a6537be9 | 493 | |
f1824df1 AS |
494 | The second case is only applicable for tasks that are grabbing a mutex |
495 | that can wake up before getting the lock, either due to a signal or | |
496 | a timeout (i.e. rt_mutex_timed_futex_lock()). When woken, it will try to | |
497 | take the lock again, if it succeeds, then the task will return with the | |
498 | lock held, otherwise it will return with -EINTR if the task was woken | |
499 | by a signal, or -ETIMEDOUT if it timed out. | |
a6537be9 SR |
500 | |
501 | ||
502 | Unlocking the Mutex | |
503 | ------------------- | |
504 | ||
505 | The unlocking of a mutex also has a fast path for those architectures with | |
506 | CMPXCHG. Since the taking of a mutex on contention always sets the | |
507 | "Has Waiters" flag of the mutex's owner, we use this to know if we need to | |
508 | take the slow path when unlocking the mutex. If the mutex doesn't have any | |
509 | waiters, the owner field of the mutex would equal the current process and | |
510 | the mutex can be unlocked by just replacing the owner field with NULL. | |
511 | ||
512 | If the owner field has the "Has Waiters" bit set (or CMPXCHG is not available), | |
513 | the slow unlock path is taken. | |
514 | ||
515 | The first thing done in the slow unlock path is to take the wait_lock of the | |
516 | mutex. This synchronizes the locking and unlocking of the mutex. | |
517 | ||
518 | A check is made to see if the mutex has waiters or not. On architectures that | |
519 | do not have CMPXCHG, this is the location that the owner of the mutex will | |
520 | determine if a waiter needs to be awoken or not. On architectures that | |
521 | do have CMPXCHG, that check is done in the fast path, but it is still needed | |
522 | in the slow path too. If a waiter of a mutex woke up because of a signal | |
523 | or timeout between the time the owner failed the fast path CMPXCHG check and | |
524 | the grabbing of the wait_lock, the mutex may not have any waiters, thus the | |
9ba0bdfd | 525 | owner still needs to make this check. If there are no waiters then the mutex |
a6537be9 SR |
526 | owner field is set to NULL, the wait_lock is released and nothing more is |
527 | needed. | |
528 | ||
f1824df1 | 529 | If there are waiters, then we need to wake one up. |
a6537be9 SR |
530 | |
531 | On the wake up code, the pi_lock of the current owner is taken. The top | |
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532 | waiter of the lock is found and removed from the waiters tree of the mutex |
533 | as well as the pi_waiters tree of the current owner. The "Has Waiters" bit is | |
534 | marked to prevent lower priority tasks from stealing the lock. | |
a6537be9 SR |
535 | |
536 | Finally we unlock the pi_lock of the pending owner and wake it up. | |
537 | ||
538 | ||
539 | Contact | |
540 | ------- | |
541 | ||
542 | For updates on this document, please email Steven Rostedt <rostedt@goodmis.org> | |
543 | ||
544 | ||
545 | Credits | |
546 | ------- | |
547 | ||
548 | Author: Steven Rostedt <rostedt@goodmis.org> | |
f1824df1 | 549 | Updated: Alex Shi <alex.shi@linaro.org> - 7/6/2017 |
a6537be9 | 550 | |
f1824df1 AS |
551 | Original Reviewers: Ingo Molnar, Thomas Gleixner, Thomas Duetsch, and |
552 | Randy Dunlap | |
553 | Update (7/6/2017) Reviewers: Steven Rostedt and Sebastian Siewior | |
a6537be9 SR |
554 | |
555 | Updates | |
556 | ------- | |
557 | ||
558 | This document was originally written for 2.6.17-rc3-mm1 | |
f1824df1 | 559 | was updated on 4.12 |