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Signed-off-by: Ian Campbell <ian.campbell@citrix.com>
author Keir Fraser <keir.fraser@citrix.com>
date Tue Apr 15 15:18:58 2008 +0100 (2008-04-15)
parents 831230e53067
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1 #
2 # Copyright (c) 2006 Steven Rostedt
3 # Licensed under the GNU Free Documentation License, Version 1.2
4 #
6 RT-mutex implementation design
7 ------------------------------
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
11 Documentation/rt-mutex.txt. Although this document does explain problems
12 that happen without this code, but that is in the concept to understand
13 what the code actually is doing.
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.
20 Unbounded Priority Inversion
21 ----------------------------
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.
33 The classic example of unbounded priority inversion is were you have three
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.
44 Here's a little ASCII art to show the problem.
46 grab lock L1 (owned by C)
47 |
48 A ---+
49 C preempted by B
50 |
51 C +----+
53 B +-------->
54 B now keeps A from running.
57 Priority Inheritance (PI)
58 -------------------------
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.
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.
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.
72 Terminology
73 -----------
75 Here I explain some terminology that is used in this document to help describe
76 the design that is used to implement PI.
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.
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.
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.
93 spin lock - Same as lock above.
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
100 the task is blocked on. It also has the plist node structures to
101 place the task in the waiter_list of a mutex as well as the
102 pi_list of a mutex owner task (described below).
104 waiter is sometimes used in reference to the task that is waiting
105 on a mutex. This is the same as waiter->task.
107 waiters - A list of processes that are blocked on a mutex.
109 top waiter - The highest priority process waiting on a specific mutex.
111 top pi waiter - The highest priority process waiting on one of the mutexes
112 that a specific process owns.
114 Note: task and process are used interchangeably in this document, mostly to
115 differentiate between two processes that are being described together.
118 PI chain
119 --------
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.
126 Example:
128 Process: A, B, C, D, E
129 Mutexes: L1, L2, L3, L4
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
140 The chain would be:
142 E->L4->D->L3->C->L2->B->L1->A
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.
147 The chain for F would be:
149 F->L5->B->L1->A
151 Since a process may own more than one mutex, but never be blocked on more than
152 one, the chains merge.
154 Here we show both chains:
156 E->L4->D->L3->C->L2-+
157 |
158 +->B->L1->A
159 |
160 F->L5-+
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.
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:
170 G->L2->B->L1->A
172 And once again, to show how this can grow I will show the merging chains
173 again.
175 E->L4->D->L3->C-+
176 +->L2-+
177 | |
178 G-+ +->B->L1->A
179 |
180 F->L5-+
183 Plist
184 -----
186 Before I go further and talk about how the PI chain is stored through lists
187 on both mutexes and processes, I'll explain the plist. This is similar to
188 the struct list_head functionality that is already in the kernel.
189 The implementation of plist is out of scope for this document, but it is
190 very important to understand what it does.
192 There are a few differences between plist and list, the most important one
193 being that plist is a priority sorted linked list. This means that the
194 priorities of the plist are sorted, such that it takes O(1) to retrieve the
195 highest priority item in the list. Obviously this is useful to store processes
196 based on their priorities.
198 Another difference, which is important for implementation, is that, unlike
199 list, the head of the list is a different element than the nodes of a list.
200 So the head of the list is declared as struct plist_head and nodes that will
201 be added to the list are declared as struct plist_node.
204 Mutex Waiter List
205 -----------------
207 Every mutex keeps track of all the waiters that are blocked on itself. The mutex
208 has a plist to store these waiters by priority. This list is protected by
209 a spin lock that is located in the struct of the mutex. This lock is called
210 wait_lock. Since the modification of the waiter list is never done in
211 interrupt context, the wait_lock can be taken without disabling interrupts.
214 Task PI List
215 ------------
217 To keep track of the PI chains, each process has its own PI list. This is
218 a list of all top waiters of the mutexes that are owned by the process.
219 Note that this list only holds the top waiters and not all waiters that are
220 blocked on mutexes owned by the process.
222 The top of the task's PI list is always the highest priority task that
223 is waiting on a mutex that is owned by the task. So if the task has
224 inherited a priority, it will always be the priority of the task that is
225 at the top of this list.
227 This list is stored in the task structure of a process as a plist called
228 pi_list. This list is protected by a spin lock also in the task structure,
229 called pi_lock. This lock may also be taken in interrupt context, so when
230 locking the pi_lock, interrupts must be disabled.
233 Depth of the PI Chain
234 ---------------------
236 The maximum depth of the PI chain is not dynamic, and could actually be
237 defined. But is very complex to figure it out, since it depends on all
238 the nesting of mutexes. Let's look at the example where we have 3 mutexes,
239 L1, L2, and L3, and four separate functions func1, func2, func3 and func4.
240 The following shows a locking order of L1->L2->L3, but may not actually
241 be directly nested that way.
243 void func1(void)
244 {
245 mutex_lock(L1);
247 /* do anything */
249 mutex_unlock(L1);
250 }
252 void func2(void)
253 {
254 mutex_lock(L1);
255 mutex_lock(L2);
257 /* do something */
259 mutex_unlock(L2);
260 mutex_unlock(L1);
261 }
263 void func3(void)
264 {
265 mutex_lock(L2);
266 mutex_lock(L3);
268 /* do something else */
270 mutex_unlock(L3);
271 mutex_unlock(L2);
272 }
274 void func4(void)
275 {
276 mutex_lock(L3);
278 /* do something again */
280 mutex_unlock(L3);
281 }
283 Now we add 4 processes that run each of these functions separately.
284 Processes A, B, C, and D which run functions func1, func2, func3 and func4
285 respectively, and such that D runs first and A last. With D being preempted
286 in func4 in the "do something again" area, we have a locking that follows:
288 D owns L3
289 C blocked on L3
290 C owns L2
291 B blocked on L2
292 B owns L1
293 A blocked on L1
295 And thus we have the chain A->L1->B->L2->C->L3->D.
297 This gives us a PI depth of 4 (four processes), but looking at any of the
298 functions individually, it seems as though they only have at most a locking
299 depth of two. So, although the locking depth is defined at compile time,
300 it still is very difficult to find the possibilities of that depth.
302 Now since mutexes can be defined by user-land applications, we don't want a DOS
303 type of application that nests large amounts of mutexes to create a large
304 PI chain, and have the code holding spin locks while looking at a large
305 amount of data. So to prevent this, the implementation not only implements
306 a maximum lock depth, but also only holds at most two different locks at a
307 time, as it walks the PI chain. More about this below.
310 Mutex owner and flags
311 ---------------------
313 The mutex structure contains a pointer to the owner of the mutex. If the
314 mutex is not owned, this owner is set to NULL. Since all architectures
315 have the task structure on at least a four byte alignment (and if this is
316 not true, the rtmutex.c code will be broken!), this allows for the two
317 least significant bits to be used as flags. This part is also described
318 in Documentation/rt-mutex.txt, but will also be briefly described here.
320 Bit 0 is used as the "Pending Owner" flag. This is described later.
321 Bit 1 is used as the "Has Waiters" flags. This is also described later
322 in more detail, but is set whenever there are waiters on a mutex.
325 cmpxchg Tricks
326 --------------
328 Some architectures implement an atomic cmpxchg (Compare and Exchange). This
329 is used (when applicable) to keep the fast path of grabbing and releasing
330 mutexes short.
332 cmpxchg is basically the following function performed atomically:
334 unsigned long _cmpxchg(unsigned long *A, unsigned long *B, unsigned long *C)
335 {
336 unsigned long T = *A;
337 if (*A == *B) {
338 *A = *C;
339 }
340 return T;
341 }
342 #define cmpxchg(a,b,c) _cmpxchg(&a,&b,&c)
344 This is really nice to have, since it allows you to only update a variable
345 if the variable is what you expect it to be. You know if it succeeded if
346 the return value (the old value of A) is equal to B.
348 The macro rt_mutex_cmpxchg is used to try to lock and unlock mutexes. If
349 the architecture does not support CMPXCHG, then this macro is simply set
350 to fail every time. But if CMPXCHG is supported, then this will
351 help out extremely to keep the fast path short.
353 The use of rt_mutex_cmpxchg with the flags in the owner field help optimize
354 the system for architectures that support it. This will also be explained
355 later in this document.
358 Priority adjustments
359 --------------------
361 The implementation of the PI code in rtmutex.c has several places that a
362 process must adjust its priority. With the help of the pi_list of a
363 process this is rather easy to know what needs to be adjusted.
365 The functions implementing the task adjustments are rt_mutex_adjust_prio,
366 __rt_mutex_adjust_prio (same as the former, but expects the task pi_lock
367 to already be taken), rt_mutex_get_prio, and rt_mutex_setprio.
369 rt_mutex_getprio and rt_mutex_setprio are only used in __rt_mutex_adjust_prio.
371 rt_mutex_getprio returns the priority that the task should have. Either the
372 task's own normal priority, or if a process of a higher priority is waiting on
373 a mutex owned by the task, then that higher priority should be returned.
374 Since the pi_list of a task holds an order by priority list of all the top
375 waiters of all the mutexes that the task owns, rt_mutex_getprio simply needs
376 to compare the top pi waiter to its own normal priority, and return the higher
377 priority back.
379 (Note: if looking at the code, you will notice that the lower number of
380 prio is returned. This is because the prio field in the task structure
381 is an inverse order of the actual priority. So a "prio" of 5 is
382 of higher priority than a "prio" of 10.)
384 __rt_mutex_adjust_prio examines the result of rt_mutex_getprio, and if the
385 result does not equal the task's current priority, then rt_mutex_setprio
386 is called to adjust the priority of the task to the new priority.
387 Note that rt_mutex_setprio is defined in kernel/sched.c to implement the
388 actual change in priority.
390 It is interesting to note that __rt_mutex_adjust_prio can either increase
391 or decrease the priority of the task. In the case that a higher priority
392 process has just blocked on a mutex owned by the task, __rt_mutex_adjust_prio
393 would increase/boost the task's priority. But if a higher priority task
394 were for some reason to leave the mutex (timeout or signal), this same function
395 would decrease/unboost the priority of the task. That is because the pi_list
396 always contains the highest priority task that is waiting on a mutex owned
397 by the task, so we only need to compare the priority of that top pi waiter
398 to the normal priority of the given task.
401 High level overview of the PI chain walk
402 ----------------------------------------
404 The PI chain walk is implemented by the function rt_mutex_adjust_prio_chain.
406 The implementation has gone through several iterations, and has ended up
407 with what we believe is the best. It walks the PI chain by only grabbing
408 at most two locks at a time, and is very efficient.
410 The rt_mutex_adjust_prio_chain can be used either to boost or lower process
411 priorities.
413 rt_mutex_adjust_prio_chain is called with a task to be checked for PI
414 (de)boosting (the owner of a mutex that a process is blocking on), a flag to
415 check for deadlocking, the mutex that the task owns, and a pointer to a waiter
416 that is the process's waiter struct that is blocked on the mutex (although this
417 parameter may be NULL for deboosting).
419 For this explanation, I will not mention deadlock detection. This explanation
420 will try to stay at a high level.
422 When this function is called, there are no locks held. That also means
423 that the state of the owner and lock can change when entered into this function.
425 Before this function is called, the task has already had rt_mutex_adjust_prio
426 performed on it. This means that the task is set to the priority that it
427 should be at, but the plist nodes of the task's waiter have not been updated
428 with the new priorities, and that this task may not be in the proper locations
429 in the pi_lists and wait_lists that the task is blocked on. This function
430 solves all that.
432 A loop is entered, where task is the owner to be checked for PI changes that
433 was passed by parameter (for the first iteration). The pi_lock of this task is
434 taken to prevent any more changes to the pi_list of the task. This also
435 prevents new tasks from completing the blocking on a mutex that is owned by this
436 task.
438 If the task is not blocked on a mutex then the loop is exited. We are at
439 the top of the PI chain.
441 A check is now done to see if the original waiter (the process that is blocked
442 on the current mutex) is the top pi waiter of the task. That is, is this
443 waiter on the top of the task's pi_list. If it is not, it either means that
444 there is another process higher in priority that is blocked on one of the
445 mutexes that the task owns, or that the waiter has just woken up via a signal
446 or timeout and has left the PI chain. In either case, the loop is exited, since
447 we don't need to do any more changes to the priority of the current task, or any
448 task that owns a mutex that this current task is waiting on. A priority chain
449 walk is only needed when a new top pi waiter is made to a task.
451 The next check sees if the task's waiter plist node has the priority equal to
452 the priority the task is set at. If they are equal, then we are done with
453 the loop. Remember that the function started with the priority of the
454 task adjusted, but the plist nodes that hold the task in other processes
455 pi_lists have not been adjusted.
457 Next, we look at the mutex that the task is blocked on. The mutex's wait_lock
458 is taken. This is done by a spin_trylock, because the locking order of the
459 pi_lock and wait_lock goes in the opposite direction. If we fail to grab the
460 lock, the pi_lock is released, and we restart the loop.
462 Now that we have both the pi_lock of the task as well as the wait_lock of
463 the mutex the task is blocked on, we update the task's waiter's plist node
464 that is located on the mutex's wait_list.
466 Now we release the pi_lock of the task.
468 Next the owner of the mutex has its pi_lock taken, so we can update the
469 task's entry in the owner's pi_list. If the task is the highest priority
470 process on the mutex's wait_list, then we remove the previous top waiter
471 from the owner's pi_list, and replace it with the task.
473 Note: It is possible that the task was the current top waiter on the mutex,
474 in which case the task is not yet on the pi_list of the waiter. This
475 is OK, since plist_del does nothing if the plist node is not on any
476 list.
478 If the task was not the top waiter of the mutex, but it was before we
479 did the priority updates, that means we are deboosting/lowering the
480 task. In this case, the task is removed from the pi_list of the owner,
481 and the new top waiter is added.
483 Lastly, we unlock both the pi_lock of the task, as well as the mutex's
484 wait_lock, and continue the loop again. On the next iteration of the
485 loop, the previous owner of the mutex will be the task that will be
486 processed.
488 Note: One might think that the owner of this mutex might have changed
489 since we just grab the mutex's wait_lock. And one could be right.
490 The important thing to remember is that the owner could not have
491 become the task that is being processed in the PI chain, since
492 we have taken that task's pi_lock at the beginning of the loop.
493 So as long as there is an owner of this mutex that is not the same
494 process as the tasked being worked on, we are OK.
496 Looking closely at the code, one might be confused. The check for the
497 end of the PI chain is when the task isn't blocked on anything or the
498 task's waiter structure "task" element is NULL. This check is
499 protected only by the task's pi_lock. But the code to unlock the mutex
500 sets the task's waiter structure "task" element to NULL with only
501 the protection of the mutex's wait_lock, which was not taken yet.
502 Isn't this a race condition if the task becomes the new owner?
504 The answer is No! The trick is the spin_trylock of the mutex's
505 wait_lock. If we fail that lock, we release the pi_lock of the
506 task and continue the loop, doing the end of PI chain check again.
508 In the code to release the lock, the wait_lock of the mutex is held
509 the entire time, and it is not let go when we grab the pi_lock of the
510 new owner of the mutex. So if the switch of a new owner were to happen
511 after the check for end of the PI chain and the grabbing of the
512 wait_lock, the unlocking code would spin on the new owner's pi_lock
513 but never give up the wait_lock. So the PI chain loop is guaranteed to
514 fail the spin_trylock on the wait_lock, release the pi_lock, and
515 try again.
517 If you don't quite understand the above, that's OK. You don't have to,
518 unless you really want to make a proof out of it ;)
521 Pending Owners and Lock stealing
522 --------------------------------
524 One of the flags in the owner field of the mutex structure is "Pending Owner".
525 What this means is that an owner was chosen by the process releasing the
526 mutex, but that owner has yet to wake up and actually take the mutex.
528 Why is this important? Why can't we just give the mutex to another process
529 and be done with it?
531 The PI code is to help with real-time processes, and to let the highest
532 priority process run as long as possible with little latencies and delays.
533 If a high priority process owns a mutex that a lower priority process is
534 blocked on, when the mutex is released it would be given to the lower priority
535 process. What if the higher priority process wants to take that mutex again.
536 The high priority process would fail to take that mutex that it just gave up
537 and it would need to boost the lower priority process to run with full
538 latency of that critical section (since the low priority process just entered
539 it).
541 There's no reason a high priority process that gives up a mutex should be
542 penalized if it tries to take that mutex again. If the new owner of the
543 mutex has not woken up yet, there's no reason that the higher priority process
544 could not take that mutex away.
546 To solve this, we introduced Pending Ownership and Lock Stealing. When a
547 new process is given a mutex that it was blocked on, it is only given
548 pending ownership. This means that it's the new owner, unless a higher
549 priority process comes in and tries to grab that mutex. If a higher priority
550 process does come along and wants that mutex, we let the higher priority
551 process "steal" the mutex from the pending owner (only if it is still pending)
552 and continue with the mutex.
555 Taking of a mutex (The walk through)
556 ------------------------------------
558 OK, now let's take a look at the detailed walk through of what happens when
559 taking a mutex.
561 The first thing that is tried is the fast taking of the mutex. This is
562 done when we have CMPXCHG enabled (otherwise the fast taking automatically
563 fails). Only when the owner field of the mutex is NULL can the lock be
564 taken with the CMPXCHG and nothing else needs to be done.
566 If there is contention on the lock, whether it is owned or pending owner
567 we go about the slow path (rt_mutex_slowlock).
569 The slow path function is where the task's waiter structure is created on
570 the stack. This is because the waiter structure is only needed for the
571 scope of this function. The waiter structure holds the nodes to store
572 the task on the wait_list of the mutex, and if need be, the pi_list of
573 the owner.
575 The wait_lock of the mutex is taken since the slow path of unlocking the
576 mutex also takes this lock.
578 We then call try_to_take_rt_mutex. This is where the architecture that
579 does not implement CMPXCHG would always grab the lock (if there's no
580 contention).
582 try_to_take_rt_mutex is used every time the task tries to grab a mutex in the
583 slow path. The first thing that is done here is an atomic setting of
584 the "Has Waiters" flag of the mutex's owner field. Yes, this could really
585 be false, because if the the mutex has no owner, there are no waiters and
586 the current task also won't have any waiters. But we don't have the lock
587 yet, so we assume we are going to be a waiter. The reason for this is to
588 play nice for those architectures that do have CMPXCHG. By setting this flag
589 now, the owner of the mutex can't release the mutex without going into the
590 slow unlock path, and it would then need to grab the wait_lock, which this
591 code currently holds. So setting the "Has Waiters" flag forces the owner
592 to synchronize with this code.
594 Now that we know that we can't have any races with the owner releasing the
595 mutex, we check to see if we can take the ownership. This is done if the
596 mutex doesn't have a owner, or if we can steal the mutex from a pending
597 owner. Let's look at the situations we have here.
599 1) Has owner that is pending
600 ----------------------------
602 The mutex has a owner, but it hasn't woken up and the mutex flag
603 "Pending Owner" is set. The first check is to see if the owner isn't the
604 current task. This is because this function is also used for the pending
605 owner to grab the mutex. When a pending owner wakes up, it checks to see
606 if it can take the mutex, and this is done if the owner is already set to
607 itself. If so, we succeed and leave the function, clearing the "Pending
608 Owner" bit.
610 If the pending owner is not current, we check to see if the current priority is
611 higher than the pending owner. If not, we fail the function and return.
613 There's also something special about a pending owner. That is a pending owner
614 is never blocked on a mutex. So there is no PI chain to worry about. It also
615 means that if the mutex doesn't have any waiters, there's no accounting needed
616 to update the pending owner's pi_list, since we only worry about processes
617 blocked on the current mutex.
619 If there are waiters on this mutex, and we just stole the ownership, we need
620 to take the top waiter, remove it from the pi_list of the pending owner, and
621 add it to the current pi_list. Note that at this moment, the pending owner
622 is no longer on the list of waiters. This is fine, since the pending owner
623 would add itself back when it realizes that it had the ownership stolen
624 from itself. When the pending owner tries to grab the mutex, it will fail
625 in try_to_take_rt_mutex if the owner field points to another process.
627 2) No owner
628 -----------
630 If there is no owner (or we successfully stole the lock), we set the owner
631 of the mutex to current, and set the flag of "Has Waiters" if the current
632 mutex actually has waiters, or we clear the flag if it doesn't. See, it was
633 OK that we set that flag early, since now it is cleared.
635 3) Failed to grab ownership
636 ---------------------------
638 The most interesting case is when we fail to take ownership. This means that
639 there exists an owner, or there's a pending owner with equal or higher
640 priority than the current task.
642 We'll continue on the failed case.
644 If the mutex has a timeout, we set up a timer to go off to break us out
645 of this mutex if we failed to get it after a specified amount of time.
647 Now we enter a loop that will continue to try to take ownership of the mutex, or
648 fail from a timeout or signal.
650 Once again we try to take the mutex. This will usually fail the first time
651 in the loop, since it had just failed to get the mutex. But the second time
652 in the loop, this would likely succeed, since the task would likely be
653 the pending owner.
655 If the mutex is TASK_INTERRUPTIBLE a check for signals and timeout is done
656 here.
658 The waiter structure has a "task" field that points to the task that is blocked
659 on the mutex. This field can be NULL the first time it goes through the loop
660 or if the task is a pending owner and had it's mutex stolen. If the "task"
661 field is NULL then we need to set up the accounting for it.
663 Task blocks on mutex
664 --------------------
666 The accounting of a mutex and process is done with the waiter structure of
667 the process. The "task" field is set to the process, and the "lock" field
668 to the mutex. The plist nodes are initialized to the processes current
669 priority.
671 Since the wait_lock was taken at the entry of the slow lock, we can safely
672 add the waiter to the wait_list. If the current process is the highest
673 priority process currently waiting on this mutex, then we remove the
674 previous top waiter process (if it exists) from the pi_list of the owner,
675 and add the current process to that list. Since the pi_list of the owner
676 has changed, we call rt_mutex_adjust_prio on the owner to see if the owner
677 should adjust its priority accordingly.
679 If the owner is also blocked on a lock, and had its pi_list changed
680 (or deadlock checking is on), we unlock the wait_lock of the mutex and go ahead
681 and run rt_mutex_adjust_prio_chain on the owner, as described earlier.
683 Now all locks are released, and if the current process is still blocked on a
684 mutex (waiter "task" field is not NULL), then we go to sleep (call schedule).
686 Waking up in the loop
687 ---------------------
689 The schedule can then wake up for a few reasons.
690 1) we were given pending ownership of the mutex.
691 2) we received a signal and was TASK_INTERRUPTIBLE
692 3) we had a timeout and was TASK_INTERRUPTIBLE
694 In any of these cases, we continue the loop and once again try to grab the
695 ownership of the mutex. If we succeed, we exit the loop, otherwise we continue
696 and on signal and timeout, will exit the loop, or if we had the mutex stolen
697 we just simply add ourselves back on the lists and go back to sleep.
699 Note: For various reasons, because of timeout and signals, the steal mutex
700 algorithm needs to be careful. This is because the current process is
701 still on the wait_list. And because of dynamic changing of priorities,
702 especially on SCHED_OTHER tasks, the current process can be the
703 highest priority task on the wait_list.
705 Failed to get mutex on Timeout or Signal
706 ----------------------------------------
708 If a timeout or signal occurred, the waiter's "task" field would not be
709 NULL and the task needs to be taken off the wait_list of the mutex and perhaps
710 pi_list of the owner. If this process was a high priority process, then
711 the rt_mutex_adjust_prio_chain needs to be executed again on the owner,
712 but this time it will be lowering the priorities.
715 Unlocking the Mutex
716 -------------------
718 The unlocking of a mutex also has a fast path for those architectures with
719 CMPXCHG. Since the taking of a mutex on contention always sets the
720 "Has Waiters" flag of the mutex's owner, we use this to know if we need to
721 take the slow path when unlocking the mutex. If the mutex doesn't have any
722 waiters, the owner field of the mutex would equal the current process and
723 the mutex can be unlocked by just replacing the owner field with NULL.
725 If the owner field has the "Has Waiters" bit set (or CMPXCHG is not available),
726 the slow unlock path is taken.
728 The first thing done in the slow unlock path is to take the wait_lock of the
729 mutex. This synchronizes the locking and unlocking of the mutex.
731 A check is made to see if the mutex has waiters or not. On architectures that
732 do not have CMPXCHG, this is the location that the owner of the mutex will
733 determine if a waiter needs to be awoken or not. On architectures that
734 do have CMPXCHG, that check is done in the fast path, but it is still needed
735 in the slow path too. If a waiter of a mutex woke up because of a signal
736 or timeout between the time the owner failed the fast path CMPXCHG check and
737 the grabbing of the wait_lock, the mutex may not have any waiters, thus the
738 owner still needs to make this check. If there are no waiters than the mutex
739 owner field is set to NULL, the wait_lock is released and nothing more is
740 needed.
742 If there are waiters, then we need to wake one up and give that waiter
743 pending ownership.
745 On the wake up code, the pi_lock of the current owner is taken. The top
746 waiter of the lock is found and removed from the wait_list of the mutex
747 as well as the pi_list of the current owner. The task field of the new
748 pending owner's waiter structure is set to NULL, and the owner field of the
749 mutex is set to the new owner with the "Pending Owner" bit set, as well
750 as the "Has Waiters" bit if there still are other processes blocked on the
751 mutex.
753 The pi_lock of the previous owner is released, and the new pending owner's
754 pi_lock is taken. Remember that this is the trick to prevent the race
755 condition in rt_mutex_adjust_prio_chain from adding itself as a waiter
756 on the mutex.
758 We now clear the "pi_blocked_on" field of the new pending owner, and if
759 the mutex still has waiters pending, we add the new top waiter to the pi_list
760 of the pending owner.
762 Finally we unlock the pi_lock of the pending owner and wake it up.
765 Contact
766 -------
768 For updates on this document, please email Steven Rostedt <rostedt@goodmis.org>
771 Credits
772 -------
774 Author: Steven Rostedt <rostedt@goodmis.org>
776 Reviewers: Ingo Molnar, Thomas Gleixner, Thomas Duetsch, and Randy Dunlap
778 Updates
779 -------
781 This document was originally written for 2.6.17-rc3-mm1