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-#
-# Copyright (c) 2006 Steven Rostedt
-# Licensed under the GNU Free Documentation License, Version 1.2
-#
-
-RT-mutex implementation design
-------------------------------
-
-This document tries to describe the design of the rtmutex.c implementation.
-It doesn't describe the reasons why rtmutex.c exists. For that please see
-Documentation/rt-mutex.txt. Although this document does explain problems
-that happen without this code, but that is in the concept to understand
-what the code actually is doing.
-
-The goal of this document is to help others understand the priority
-inheritance (PI) algorithm that is used, as well as reasons for the
-decisions that were made to implement PI in the manner that was done.
-
-
-Unbounded Priority Inversion
-----------------------------
-
-Priority inversion is when a lower priority process executes while a higher
-priority process wants to run. This happens for several reasons, and
-most of the time it can't be helped. Anytime a high priority process wants
-to use a resource that a lower priority process has (a mutex for example),
-the high priority process must wait until the lower priority process is done
-with the resource. This is a priority inversion. What we want to prevent
-is something called unbounded priority inversion. That is when the high
-priority process is prevented from running by a lower priority process for
-an undetermined amount of time.
-
-The classic example of unbounded priority inversion is were you have three
-processes, let's call them processes A, B, and C, where A is the highest
-priority process, C is the lowest, and B is in between. A tries to grab a lock
-that C owns and must wait and lets C run to release the lock. But in the
-meantime, B executes, and since B is of a higher priority than C, it preempts C,
-but by doing so, it is in fact preempting A which is a higher priority process.
-Now there's no way of knowing how long A will be sleeping waiting for C
-to release the lock, because for all we know, B is a CPU hog and will
-never give C a chance to release the lock. This is called unbounded priority
-inversion.
-
-Here's a little ASCII art to show the problem.
-
- grab lock L1 (owned by C)
- |
-A ---+
- C preempted by B
- |
-C +----+
-
-B +-------->
- B now keeps A from running.
-
-
-Priority Inheritance (PI)
--------------------------
-
-There are several ways to solve this issue, but other ways are out of scope
-for this document. Here we only discuss PI.
-
-PI is where a process inherits the priority of another process if the other
-process blocks on a lock owned by the current process. To make this easier
-to understand, let's use the previous example, with processes A, B, and C again.
-
-This time, when A blocks on the lock owned by C, C would inherit the priority
-of A. So now if B becomes runnable, it would not preempt C, since C now has
-the high priority of A. As soon as C releases the lock, it loses its
-inherited priority, and A then can continue with the resource that C had.
-
-Terminology
------------
-
-Here I explain some terminology that is used in this document to help describe
-the design that is used to implement PI.
-
-PI chain - The PI chain is an ordered series of locks and processes that cause
- processes to inherit priorities from a previous process that is
- blocked on one of its locks. This is described in more detail
- later in this document.
-
-mutex - In this document, to differentiate from locks that implement
- PI and spin locks that are used in the PI code, from now on
- the PI locks will be called a mutex.
-
-lock - In this document from now on, I will use the term lock when
- referring to spin locks that are used to protect parts of the PI
- algorithm. These locks disable preemption for UP (when
- CONFIG_PREEMPT is enabled) and on SMP prevents multiple CPUs from
- entering critical sections simultaneously.
-
-spin lock - Same as lock above.
-
-waiter - A waiter is a struct that is stored on the stack of a blocked
- process. Since the scope of the waiter is within the code for
- a process being blocked on the mutex, it is fine to allocate
- the waiter on the process's stack (local variable). This
- structure holds a pointer to the task, as well as the mutex that
- the task is blocked on. It also has the plist node structures to
- place the task in the waiter_list of a mutex as well as the
- pi_list of a mutex owner task (described below).
-
- waiter is sometimes used in reference to the task that is waiting
- on a mutex. This is the same as waiter->task.
-
-waiters - A list of processes that are blocked on a mutex.
-
-top waiter - The highest priority process waiting on a specific mutex.
-
-top pi waiter - The highest priority process waiting on one of the mutexes
- that a specific process owns.
-
-Note: task and process are used interchangeably in this document, mostly to
- differentiate between two processes that are being described together.
-
-
-PI chain
---------
-
-The PI chain is a list of processes and mutexes that may cause priority
-inheritance to take place. Multiple chains may converge, but a chain
-would never diverge, since a process can't be blocked on more than one
-mutex at a time.
-
-Example:
-
- Process: A, B, C, D, E
- Mutexes: L1, L2, L3, L4
-
- A owns: L1
- B blocked on L1
- B owns L2
- C blocked on L2
- C owns L3
- D blocked on L3
- D owns L4
- E blocked on L4
-
-The chain would be:
-
- E->L4->D->L3->C->L2->B->L1->A
-
-To show where two chains merge, we could add another process F and
-another mutex L5 where B owns L5 and F is blocked on mutex L5.
-
-The chain for F would be:
-
- F->L5->B->L1->A
-
-Since a process may own more than one mutex, but never be blocked on more than
-one, the chains merge.
-
-Here we show both chains:
-
- E->L4->D->L3->C->L2-+
- |
- +->B->L1->A
- |
- F->L5-+
-
-For PI to work, the processes at the right end of these chains (or we may
-also call it the Top of the chain) must be equal to or higher in priority
-than the processes to the left or below in the chain.
-
-Also since a mutex may have more than one process blocked on it, we can
-have multiple chains merge at mutexes. If we add another process G that is
-blocked on mutex L2:
-
- G->L2->B->L1->A
-
-And once again, to show how this can grow I will show the merging chains
-again.
-
- E->L4->D->L3->C-+
- +->L2-+
- | |
- G-+ +->B->L1->A
- |
- F->L5-+
-
-
-Plist
------
-
-Before I go further and talk about how the PI chain is stored through lists
-on both mutexes and processes, I'll explain the plist. This is similar to
-the struct list_head functionality that is already in the kernel.
-The implementation of plist is out of scope for this document, but it is
-very important to understand what it does.
-
-There are a few differences between plist and list, the most important one
-being that plist is a priority sorted linked list. This means that the
-priorities of the plist are sorted, such that it takes O(1) to retrieve the
-highest priority item in the list. Obviously this is useful to store processes
-based on their priorities.
-
-Another difference, which is important for implementation, is that, unlike
-list, the head of the list is a different element than the nodes of a list.
-So the head of the list is declared as struct plist_head and nodes that will
-be added to the list are declared as struct plist_node.
-
-
-Mutex Waiter List
------------------
-
-Every mutex keeps track of all the waiters that are blocked on itself. The mutex
-has a plist to store these waiters by priority. This list is protected by
-a spin lock that is located in the struct of the mutex. This lock is called
-wait_lock. Since the modification of the waiter list is never done in
-interrupt context, the wait_lock can be taken without disabling interrupts.
-
-
-Task PI List
-------------
-
-To keep track of the PI chains, each process has its own PI list. This is
-a list of all top waiters of the mutexes that are owned by the process.
-Note that this list only holds the top waiters and not all waiters that are
-blocked on mutexes owned by the process.
-
-The top of the task's PI list is always the highest priority task that
-is waiting on a mutex that is owned by the task. So if the task has
-inherited a priority, it will always be the priority of the task that is
-at the top of this list.
-
-This list is stored in the task structure of a process as a plist called
-pi_list. This list is protected by a spin lock also in the task structure,
-called pi_lock. This lock may also be taken in interrupt context, so when
-locking the pi_lock, interrupts must be disabled.
-
-
-Depth of the PI Chain
----------------------
-
-The maximum depth of the PI chain is not dynamic, and could actually be
-defined. But is very complex to figure it out, since it depends on all
-the nesting of mutexes. Let's look at the example where we have 3 mutexes,
-L1, L2, and L3, and four separate functions func1, func2, func3 and func4.
-The following shows a locking order of L1->L2->L3, but may not actually
-be directly nested that way.
-
-void func1(void)
-{
- mutex_lock(L1);
-
- /* do anything */
-
- mutex_unlock(L1);
-}
-
-void func2(void)
-{
- mutex_lock(L1);
- mutex_lock(L2);
-
- /* do something */
-
- mutex_unlock(L2);
- mutex_unlock(L1);
-}
-
-void func3(void)
-{
- mutex_lock(L2);
- mutex_lock(L3);
-
- /* do something else */
-
- mutex_unlock(L3);
- mutex_unlock(L2);
-}
-
-void func4(void)
-{
- mutex_lock(L3);
-
- /* do something again */
-
- mutex_unlock(L3);
-}
-
-Now we add 4 processes that run each of these functions separately.
-Processes A, B, C, and D which run functions func1, func2, func3 and func4
-respectively, and such that D runs first and A last. With D being preempted
-in func4 in the "do something again" area, we have a locking that follows:
-
-D owns L3
- C blocked on L3
- C owns L2
- B blocked on L2
- B owns L1
- A blocked on L1
-
-And thus we have the chain A->L1->B->L2->C->L3->D.
-
-This gives us a PI depth of 4 (four processes), but looking at any of the
-functions individually, it seems as though they only have at most a locking
-depth of two. So, although the locking depth is defined at compile time,
-it still is very difficult to find the possibilities of that depth.
-
-Now since mutexes can be defined by user-land applications, we don't want a DOS
-type of application that nests large amounts of mutexes to create a large
-PI chain, and have the code holding spin locks while looking at a large
-amount of data. So to prevent this, the implementation not only implements
-a maximum lock depth, but also only holds at most two different locks at a
-time, as it walks the PI chain. More about this below.
-
-
-Mutex owner and flags
----------------------
-
-The mutex structure contains a pointer to the owner of the mutex. If the
-mutex is not owned, this owner is set to NULL. Since all architectures
-have the task structure on at least a four byte alignment (and if this is
-not true, the rtmutex.c code will be broken!), this allows for the two
-least significant bits to be used as flags. This part is also described
-in Documentation/rt-mutex.txt, but will also be briefly described here.
-
-Bit 0 is used as the "Pending Owner" flag. This is described later.
-Bit 1 is used as the "Has Waiters" flags. This is also described later
- in more detail, but is set whenever there are waiters on a mutex.
-
-
-cmpxchg Tricks
---------------
-
-Some architectures implement an atomic cmpxchg (Compare and Exchange). This
-is used (when applicable) to keep the fast path of grabbing and releasing
-mutexes short.
-
-cmpxchg is basically the following function performed atomically:
-
-unsigned long _cmpxchg(unsigned long *A, unsigned long *B, unsigned long *C)
-{
- unsigned long T = *A;
- if (*A == *B) {
- *A = *C;
- }
- return T;
-}
-#define cmpxchg(a,b,c) _cmpxchg(&a,&b,&c)
-
-This is really nice to have, since it allows you to only update a variable
-if the variable is what you expect it to be. You know if it succeeded if
-the return value (the old value of A) is equal to B.
-
-The macro rt_mutex_cmpxchg is used to try to lock and unlock mutexes. If
-the architecture does not support CMPXCHG, then this macro is simply set
-to fail every time. But if CMPXCHG is supported, then this will
-help out extremely to keep the fast path short.
-
-The use of rt_mutex_cmpxchg with the flags in the owner field help optimize
-the system for architectures that support it. This will also be explained
-later in this document.
-
-
-Priority adjustments
---------------------
-
-The implementation of the PI code in rtmutex.c has several places that a
-process must adjust its priority. With the help of the pi_list of a
-process this is rather easy to know what needs to be adjusted.
-
-The functions implementing the task adjustments are rt_mutex_adjust_prio,
-__rt_mutex_adjust_prio (same as the former, but expects the task pi_lock
-to already be taken), rt_mutex_getprio, and rt_mutex_setprio.
-
-rt_mutex_getprio and rt_mutex_setprio are only used in __rt_mutex_adjust_prio.
-
-rt_mutex_getprio returns the priority that the task should have. Either the
-task's own normal priority, or if a process of a higher priority is waiting on
-a mutex owned by the task, then that higher priority should be returned.
-Since the pi_list of a task holds an order by priority list of all the top
-waiters of all the mutexes that the task owns, rt_mutex_getprio simply needs
-to compare the top pi waiter to its own normal priority, and return the higher
-priority back.
-
-(Note: if looking at the code, you will notice that the lower number of
- prio is returned. This is because the prio field in the task structure
- is an inverse order of the actual priority. So a "prio" of 5 is
- of higher priority than a "prio" of 10.)
-
-__rt_mutex_adjust_prio examines the result of rt_mutex_getprio, and if the
-result does not equal the task's current priority, then rt_mutex_setprio
-is called to adjust the priority of the task to the new priority.
-Note that rt_mutex_setprio is defined in kernel/sched.c to implement the
-actual change in priority.
-
-It is interesting to note that __rt_mutex_adjust_prio can either increase
-or decrease the priority of the task. In the case that a higher priority
-process has just blocked on a mutex owned by the task, __rt_mutex_adjust_prio
-would increase/boost the task's priority. But if a higher priority task
-were for some reason to leave the mutex (timeout or signal), this same function
-would decrease/unboost the priority of the task. That is because the pi_list
-always contains the highest priority task that is waiting on a mutex owned
-by the task, so we only need to compare the priority of that top pi waiter
-to the normal priority of the given task.
-
-
-High level overview of the PI chain walk
-----------------------------------------
-
-The PI chain walk is implemented by the function rt_mutex_adjust_prio_chain.
-
-The implementation has gone through several iterations, and has ended up
-with what we believe is the best. It walks the PI chain by only grabbing
-at most two locks at a time, and is very efficient.
-
-The rt_mutex_adjust_prio_chain can be used either to boost or lower process
-priorities.
-
-rt_mutex_adjust_prio_chain is called with a task to be checked for PI
-(de)boosting (the owner of a mutex that a process is blocking on), a flag to
-check for deadlocking, the mutex that the task owns, and a pointer to a waiter
-that is the process's waiter struct that is blocked on the mutex (although this
-parameter may be NULL for deboosting).
-
-For this explanation, I will not mention deadlock detection. This explanation
-will try to stay at a high level.
-
-When this function is called, there are no locks held. That also means
-that the state of the owner and lock can change when entered into this function.
-
-Before this function is called, the task has already had rt_mutex_adjust_prio
-performed on it. This means that the task is set to the priority that it
-should be at, but the plist nodes of the task's waiter have not been updated
-with the new priorities, and that this task may not be in the proper locations
-in the pi_lists and wait_lists that the task is blocked on. This function
-solves all that.
-
-A loop is entered, where task is the owner to be checked for PI changes that
-was passed by parameter (for the first iteration). The pi_lock of this task is
-taken to prevent any more changes to the pi_list of the task. This also
-prevents new tasks from completing the blocking on a mutex that is owned by this
-task.
-
-If the task is not blocked on a mutex then the loop is exited. We are at
-the top of the PI chain.
-
-A check is now done to see if the original waiter (the process that is blocked
-on the current mutex) is the top pi waiter of the task. That is, is this
-waiter on the top of the task's pi_list. If it is not, it either means that
-there is another process higher in priority that is blocked on one of the
-mutexes that the task owns, or that the waiter has just woken up via a signal
-or timeout and has left the PI chain. In either case, the loop is exited, since
-we don't need to do any more changes to the priority of the current task, or any
-task that owns a mutex that this current task is waiting on. A priority chain
-walk is only needed when a new top pi waiter is made to a task.
-
-The next check sees if the task's waiter plist node has the priority equal to
-the priority the task is set at. If they are equal, then we are done with
-the loop. Remember that the function started with the priority of the
-task adjusted, but the plist nodes that hold the task in other processes
-pi_lists have not been adjusted.
-
-Next, we look at the mutex that the task is blocked on. The mutex's wait_lock
-is taken. This is done by a spin_trylock, because the locking order of the
-pi_lock and wait_lock goes in the opposite direction. If we fail to grab the
-lock, the pi_lock is released, and we restart the loop.
-
-Now that we have both the pi_lock of the task as well as the wait_lock of
-the mutex the task is blocked on, we update the task's waiter's plist node
-that is located on the mutex's wait_list.
-
-Now we release the pi_lock of the task.
-
-Next the owner of the mutex has its pi_lock taken, so we can update the
-task's entry in the owner's pi_list. If the task is the highest priority
-process on the mutex's wait_list, then we remove the previous top waiter
-from the owner's pi_list, and replace it with the task.
-
-Note: It is possible that the task was the current top waiter on the mutex,
- in which case the task is not yet on the pi_list of the waiter. This
- is OK, since plist_del does nothing if the plist node is not on any
- list.
-
-If the task was not the top waiter of the mutex, but it was before we
-did the priority updates, that means we are deboosting/lowering the
-task. In this case, the task is removed from the pi_list of the owner,
-and the new top waiter is added.
-
-Lastly, we unlock both the pi_lock of the task, as well as the mutex's
-wait_lock, and continue the loop again. On the next iteration of the
-loop, the previous owner of the mutex will be the task that will be
-processed.
-
-Note: One might think that the owner of this mutex might have changed
- since we just grab the mutex's wait_lock. And one could be right.
- The important thing to remember is that the owner could not have
- become the task that is being processed in the PI chain, since
- we have taken that task's pi_lock at the beginning of the loop.
- So as long as there is an owner of this mutex that is not the same
- process as the tasked being worked on, we are OK.
-
- Looking closely at the code, one might be confused. The check for the
- end of the PI chain is when the task isn't blocked on anything or the
- task's waiter structure "task" element is NULL. This check is
- protected only by the task's pi_lock. But the code to unlock the mutex
- sets the task's waiter structure "task" element to NULL with only
- the protection of the mutex's wait_lock, which was not taken yet.
- Isn't this a race condition if the task becomes the new owner?
-
- The answer is No! The trick is the spin_trylock of the mutex's
- wait_lock. If we fail that lock, we release the pi_lock of the
- task and continue the loop, doing the end of PI chain check again.
-
- In the code to release the lock, the wait_lock of the mutex is held
- the entire time, and it is not let go when we grab the pi_lock of the
- new owner of the mutex. So if the switch of a new owner were to happen
- after the check for end of the PI chain and the grabbing of the
- wait_lock, the unlocking code would spin on the new owner's pi_lock
- but never give up the wait_lock. So the PI chain loop is guaranteed to
- fail the spin_trylock on the wait_lock, release the pi_lock, and
- try again.
-
- If you don't quite understand the above, that's OK. You don't have to,
- unless you really want to make a proof out of it ;)
-
-
-Pending Owners and Lock stealing
---------------------------------
-
-One of the flags in the owner field of the mutex structure is "Pending Owner".
-What this means is that an owner was chosen by the process releasing the
-mutex, but that owner has yet to wake up and actually take the mutex.
-
-Why is this important? Why can't we just give the mutex to another process
-and be done with it?
-
-The PI code is to help with real-time processes, and to let the highest
-priority process run as long as possible with little latencies and delays.
-If a high priority process owns a mutex that a lower priority process is
-blocked on, when the mutex is released it would be given to the lower priority
-process. What if the higher priority process wants to take that mutex again.
-The high priority process would fail to take that mutex that it just gave up
-and it would need to boost the lower priority process to run with full
-latency of that critical section (since the low priority process just entered
-it).
-
-There's no reason a high priority process that gives up a mutex should be
-penalized if it tries to take that mutex again. If the new owner of the
-mutex has not woken up yet, there's no reason that the higher priority process
-could not take that mutex away.
-
-To solve this, we introduced Pending Ownership and Lock Stealing. When a
-new process is given a mutex that it was blocked on, it is only given
-pending ownership. This means that it's the new owner, unless a higher
-priority process comes in and tries to grab that mutex. If a higher priority
-process does come along and wants that mutex, we let the higher priority
-process "steal" the mutex from the pending owner (only if it is still pending)
-and continue with the mutex.
-
-
-Taking of a mutex (The walk through)
-------------------------------------
-
-OK, now let's take a look at the detailed walk through of what happens when
-taking a mutex.
-
-The first thing that is tried is the fast taking of the mutex. This is
-done when we have CMPXCHG enabled (otherwise the fast taking automatically
-fails). Only when the owner field of the mutex is NULL can the lock be
-taken with the CMPXCHG and nothing else needs to be done.
-
-If there is contention on the lock, whether it is owned or pending owner
-we go about the slow path (rt_mutex_slowlock).
-
-The slow path function is where the task's waiter structure is created on
-the stack. This is because the waiter structure is only needed for the
-scope of this function. The waiter structure holds the nodes to store
-the task on the wait_list of the mutex, and if need be, the pi_list of
-the owner.
-
-The wait_lock of the mutex is taken since the slow path of unlocking the
-mutex also takes this lock.
-
-We then call try_to_take_rt_mutex. This is where the architecture that
-does not implement CMPXCHG would always grab the lock (if there's no
-contention).
-
-try_to_take_rt_mutex is used every time the task tries to grab a mutex in the
-slow path. The first thing that is done here is an atomic setting of
-the "Has Waiters" flag of the mutex's owner field. Yes, this could really
-be false, because if the mutex has no owner, there are no waiters and
-the current task also won't have any waiters. But we don't have the lock
-yet, so we assume we are going to be a waiter. The reason for this is to
-play nice for those architectures that do have CMPXCHG. By setting this flag
-now, the owner of the mutex can't release the mutex without going into the
-slow unlock path, and it would then need to grab the wait_lock, which this
-code currently holds. So setting the "Has Waiters" flag forces the owner
-to synchronize with this code.
-
-Now that we know that we can't have any races with the owner releasing the
-mutex, we check to see if we can take the ownership. This is done if the
-mutex doesn't have a owner, or if we can steal the mutex from a pending
-owner. Let's look at the situations we have here.
-
- 1) Has owner that is pending
- ----------------------------
-
- The mutex has a owner, but it hasn't woken up and the mutex flag
- "Pending Owner" is set. The first check is to see if the owner isn't the
- current task. This is because this function is also used for the pending
- owner to grab the mutex. When a pending owner wakes up, it checks to see
- if it can take the mutex, and this is done if the owner is already set to
- itself. If so, we succeed and leave the function, clearing the "Pending
- Owner" bit.
-
- If the pending owner is not current, we check to see if the current priority is
- higher than the pending owner. If not, we fail the function and return.
-
- There's also something special about a pending owner. That is a pending owner
- is never blocked on a mutex. So there is no PI chain to worry about. It also
- means that if the mutex doesn't have any waiters, there's no accounting needed
- to update the pending owner's pi_list, since we only worry about processes
- blocked on the current mutex.
-
- If there are waiters on this mutex, and we just stole the ownership, we need
- to take the top waiter, remove it from the pi_list of the pending owner, and
- add it to the current pi_list. Note that at this moment, the pending owner
- is no longer on the list of waiters. This is fine, since the pending owner
- would add itself back when it realizes that it had the ownership stolen
- from itself. When the pending owner tries to grab the mutex, it will fail
- in try_to_take_rt_mutex if the owner field points to another process.
-
- 2) No owner
- -----------
-
- If there is no owner (or we successfully stole the lock), we set the owner
- of the mutex to current, and set the flag of "Has Waiters" if the current
- mutex actually has waiters, or we clear the flag if it doesn't. See, it was
- OK that we set that flag early, since now it is cleared.
-
- 3) Failed to grab ownership
- ---------------------------
-
- The most interesting case is when we fail to take ownership. This means that
- there exists an owner, or there's a pending owner with equal or higher
- priority than the current task.
-
-We'll continue on the failed case.
-
-If the mutex has a timeout, we set up a timer to go off to break us out
-of this mutex if we failed to get it after a specified amount of time.
-
-Now we enter a loop that will continue to try to take ownership of the mutex, or
-fail from a timeout or signal.
-
-Once again we try to take the mutex. This will usually fail the first time
-in the loop, since it had just failed to get the mutex. But the second time
-in the loop, this would likely succeed, since the task would likely be
-the pending owner.
-
-If the mutex is TASK_INTERRUPTIBLE a check for signals and timeout is done
-here.
-
-The waiter structure has a "task" field that points to the task that is blocked
-on the mutex. This field can be NULL the first time it goes through the loop
-or if the task is a pending owner and had its mutex stolen. If the "task"
-field is NULL then we need to set up the accounting for it.
-
-Task blocks on mutex
---------------------
-
-The accounting of a mutex and process is done with the waiter structure of
-the process. The "task" field is set to the process, and the "lock" field
-to the mutex. The plist nodes are initialized to the processes current
-priority.
-
-Since the wait_lock was taken at the entry of the slow lock, we can safely
-add the waiter to the wait_list. If the current process is the highest
-priority process currently waiting on this mutex, then we remove the
-previous top waiter process (if it exists) from the pi_list of the owner,
-and add the current process to that list. Since the pi_list of the owner
-has changed, we call rt_mutex_adjust_prio on the owner to see if the owner
-should adjust its priority accordingly.
-
-If the owner is also blocked on a lock, and had its pi_list changed
-(or deadlock checking is on), we unlock the wait_lock of the mutex and go ahead
-and run rt_mutex_adjust_prio_chain on the owner, as described earlier.
-
-Now all locks are released, and if the current process is still blocked on a
-mutex (waiter "task" field is not NULL), then we go to sleep (call schedule).
-
-Waking up in the loop
----------------------
-
-The schedule can then wake up for a few reasons.
- 1) we were given pending ownership of the mutex.
- 2) we received a signal and was TASK_INTERRUPTIBLE
- 3) we had a timeout and was TASK_INTERRUPTIBLE
-
-In any of these cases, we continue the loop and once again try to grab the
-ownership of the mutex. If we succeed, we exit the loop, otherwise we continue
-and on signal and timeout, will exit the loop, or if we had the mutex stolen
-we just simply add ourselves back on the lists and go back to sleep.
-
-Note: For various reasons, because of timeout and signals, the steal mutex
- algorithm needs to be careful. This is because the current process is
- still on the wait_list. And because of dynamic changing of priorities,
- especially on SCHED_OTHER tasks, the current process can be the
- highest priority task on the wait_list.
-
-Failed to get mutex on Timeout or Signal
-----------------------------------------
-
-If a timeout or signal occurred, the waiter's "task" field would not be
-NULL and the task needs to be taken off the wait_list of the mutex and perhaps
-pi_list of the owner. If this process was a high priority process, then
-the rt_mutex_adjust_prio_chain needs to be executed again on the owner,
-but this time it will be lowering the priorities.
-
-
-Unlocking the Mutex
--------------------
-
-The unlocking of a mutex also has a fast path for those architectures with
-CMPXCHG. Since the taking of a mutex on contention always sets the
-"Has Waiters" flag of the mutex's owner, we use this to know if we need to
-take the slow path when unlocking the mutex. If the mutex doesn't have any
-waiters, the owner field of the mutex would equal the current process and
-the mutex can be unlocked by just replacing the owner field with NULL.
-
-If the owner field has the "Has Waiters" bit set (or CMPXCHG is not available),
-the slow unlock path is taken.
-
-The first thing done in the slow unlock path is to take the wait_lock of the
-mutex. This synchronizes the locking and unlocking of the mutex.
-
-A check is made to see if the mutex has waiters or not. On architectures that
-do not have CMPXCHG, this is the location that the owner of the mutex will
-determine if a waiter needs to be awoken or not. On architectures that
-do have CMPXCHG, that check is done in the fast path, but it is still needed
-in the slow path too. If a waiter of a mutex woke up because of a signal
-or timeout between the time the owner failed the fast path CMPXCHG check and
-the grabbing of the wait_lock, the mutex may not have any waiters, thus the
-owner still needs to make this check. If there are no waiters then the mutex
-owner field is set to NULL, the wait_lock is released and nothing more is
-needed.
-
-If there are waiters, then we need to wake one up and give that waiter
-pending ownership.
-
-On the wake up code, the pi_lock of the current owner is taken. The top
-waiter of the lock is found and removed from the wait_list of the mutex
-as well as the pi_list of the current owner. The task field of the new
-pending owner's waiter structure is set to NULL, and the owner field of the
-mutex is set to the new owner with the "Pending Owner" bit set, as well
-as the "Has Waiters" bit if there still are other processes blocked on the
-mutex.
-
-The pi_lock of the previous owner is released, and the new pending owner's
-pi_lock is taken. Remember that this is the trick to prevent the race
-condition in rt_mutex_adjust_prio_chain from adding itself as a waiter
-on the mutex.
-
-We now clear the "pi_blocked_on" field of the new pending owner, and if
-the mutex still has waiters pending, we add the new top waiter to the pi_list
-of the pending owner.
-
-Finally we unlock the pi_lock of the pending owner and wake it up.
-
-
-Contact
--------
-
-For updates on this document, please email Steven Rostedt <rostedt@goodmis.org>
-
-
-Credits
--------
-
-Author: Steven Rostedt <rostedt@goodmis.org>
-
-Reviewers: Ingo Molnar, Thomas Gleixner, Thomas Duetsch, and Randy Dunlap
-
-Updates
--------
-
-This document was originally written for 2.6.17-rc3-mm1