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Based on kernel version 3.13. Page generated on 2014-01-20 22:04 EST.

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
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.
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	
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.
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
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).
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	
182	
183	Plist
184	-----
185	
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.
191	
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.
197	
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.
202	
203	
204	Mutex Waiter List
205	-----------------
206	
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.
212	
213	
214	Task PI List
215	------------
216	
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.
221	
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.
226	
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.
231	
232	
233	Depth of the PI Chain
234	---------------------
235	
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.
242	
243	void func1(void)
244	{
245		mutex_lock(L1);
246	
247		/* do anything */
248	
249		mutex_unlock(L1);
250	}
251	
252	void func2(void)
253	{
254		mutex_lock(L1);
255		mutex_lock(L2);
256	
257		/* do something */
258	
259		mutex_unlock(L2);
260		mutex_unlock(L1);
261	}
262	
263	void func3(void)
264	{
265		mutex_lock(L2);
266		mutex_lock(L3);
267	
268		/* do something else */
269	
270		mutex_unlock(L3);
271		mutex_unlock(L2);
272	}
273	
274	void func4(void)
275	{
276		mutex_lock(L3);
277	
278		/* do something again */
279	
280		mutex_unlock(L3);
281	}
282	
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:
287	
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
294	
295	And thus we have the chain A->L1->B->L2->C->L3->D.
296	
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.
301	
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.
308	
309	
310	Mutex owner and flags
311	---------------------
312	
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.
319	
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.
323	
324	
325	cmpxchg Tricks
326	--------------
327	
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.
331	
332	cmpxchg is basically the following function performed atomically:
333	
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)
343	
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.
347	
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.
352	
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.
356	
357	
358	Priority adjustments
359	--------------------
360	
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.
364	
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_getprio, and rt_mutex_setprio.
368	
369	rt_mutex_getprio and rt_mutex_setprio are only used in __rt_mutex_adjust_prio.
370	
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.
378	
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.)
383	
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/core.c to implement the
388	actual change in priority.
389	
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.
399	
400	
401	High level overview of the PI chain walk
402	----------------------------------------
403	
404	The PI chain walk is implemented by the function rt_mutex_adjust_prio_chain.
405	
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.
409	
410	The rt_mutex_adjust_prio_chain can be used either to boost or lower process
411	priorities.
412	
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).
418	
419	For this explanation, I will not mention deadlock detection. This explanation
420	will try to stay at a high level.
421	
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.
424	
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.
431	
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.
437	
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.
440	
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.
450	
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.
456	
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.
461	
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.
465	
466	Now we release the pi_lock of the task.
467	
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.
472	
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.
477	
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.
482	
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.
487	
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.
495	
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?
503	
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.
507	
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.
516	
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 ;)
519	
520	
521	Pending Owners and Lock stealing
522	--------------------------------
523	
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.
527	
528	Why is this important?  Why can't we just give the mutex to another process
529	and be done with it?
530	
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).
540	
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.
545	
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.
553	
554	
555	Taking of a mutex (The walk through)
556	------------------------------------
557	
558	OK, now let's take a look at the detailed walk through of what happens when
559	taking a mutex.
560	
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.
565	
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).
568	
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.
574	
575	The wait_lock of the mutex is taken since the slow path of unlocking the
576	mutex also takes this lock.
577	
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).
581	
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 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.
593	
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.
598	
599	  1) Has owner that is pending
600	  ----------------------------
601	
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.
609	
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.
612	
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.
618	
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.
626	
627	  2) No owner
628	  -----------
629	
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.
634	
635	  3) Failed to grab ownership
636	  ---------------------------
637	
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.
641	
642	We'll continue on the failed case.
643	
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.
646	
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.
649	
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.
654	
655	If the mutex is TASK_INTERRUPTIBLE a check for signals and timeout is done
656	here.
657	
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 its mutex stolen.  If the "task"
661	field is NULL then we need to set up the accounting for it.
662	
663	Task blocks on mutex
664	--------------------
665	
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.
670	
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.
678	
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.
682	
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).
685	
686	Waking up in the loop
687	---------------------
688	
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
693	
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.
698	
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.
704	
705	Failed to get mutex on Timeout or Signal
706	----------------------------------------
707	
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.
713	
714	
715	Unlocking the Mutex
716	-------------------
717	
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.
724	
725	If the owner field has the "Has Waiters" bit set (or CMPXCHG is not available),
726	the slow unlock path is taken.
727	
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.
730	
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 then the mutex
739	owner field is set to NULL, the wait_lock is released and nothing more is
740	needed.
741	
742	If there are waiters, then we need to wake one up and give that waiter
743	pending ownership.
744	
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.
752	
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.
757	
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.
761	
762	Finally we unlock the pi_lock of the pending owner and wake it up.
763	
764	
765	Contact
766	-------
767	
768	For updates on this document, please email Steven Rostedt <rostedt@goodmis.org>
769	
770	
771	Credits
772	-------
773	
774	Author:  Steven Rostedt <rostedt@goodmis.org>
775	
776	Reviewers:  Ingo Molnar, Thomas Gleixner, Thomas Duetsch, and Randy Dunlap
777	
778	Updates
779	-------
780	
781	This document was originally written for 2.6.17-rc3-mm1
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