Hi Steven & Sebastian,
If there are no more comments, could you like to give reviewed-by? :)
Regards
Alex
On 05/25/2017 01:26 PM, Alex Shi wrote:
> The rt-mutex-design documents didn't gotten meaningful update from its
> first version. Even after owner's pending bit was removed in commit 8161239a8bcc
> ("rtmutex: Simplify PI algorithm and make highest prio task get lock")
> and priority list 'plist' changed to rbtree. And Peter Zijlstra did some
> clean up and fix for deadline task changes on tip tree.
>
> So update it to latest code and make it meaningful.
>
> Signed-off-by: Alex Shi <[email protected]>
> Cc: Steven Rostedt <[email protected]>
> Cc: Sebastian Siewior <[email protected]>
> Cc: Mathieu Poirier <[email protected]>
> Cc: Juri Lelli <[email protected]>
> Cc: Thomas Gleixner <[email protected]>
> To: [email protected]
> To: [email protected]
> To: Jonathan Corbet <[email protected]>
> To: Ingo Molnar <[email protected]>
> To: Peter Zijlstra <[email protected]>
> ---
> Documentation/locking/rt-mutex-design.txt | 418 +++++++-----------------------
> 1 file changed, 97 insertions(+), 321 deletions(-)
>
> diff --git a/Documentation/locking/rt-mutex-design.txt b/Documentation/locking/rt-mutex-design.txt
> index 8666070..1a0da32 100644
> --- a/Documentation/locking/rt-mutex-design.txt
> +++ b/Documentation/locking/rt-mutex-design.txt
> @@ -97,9 +97,9 @@ waiter - A waiter is a struct that is stored on the stack of a blocked
> 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).
> + the task is blocked on. It also has a rbtree node structures to
> + place the task in waiters rbtree of a mutex as well as the
> + pi_waiters rbtree 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.
> @@ -179,53 +179,35 @@ again.
> |
> 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.
> -
> +If process G has the highest priority in the chain, then all the tasks up
> +the chain (A and B in this example), must have their priorities increased
> +to that of G.
>
> 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
> +has a rbtree to store these waiters by priority. This tree 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
> +wait_lock. Since the modification of the waiter tree is never done in
> interrupt context, the wait_lock can be taken without disabling interrupts.
>
>
> -Task PI List
> +Task PI Tree
> ------------
>
> -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
> +To keep track of the PI chains, each process has its own PI rbtree. This is
> +a tree of all top waiters of the mutexes that are owned by the process.
> +Note that this tree 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
> +The top of the task's PI tree 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.
> +at the top of this tree.
>
> -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,
> +This tree is stored in the task structure of a process as a rbtree called
> +pi_waiters. It 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.
>
> @@ -312,15 +294,12 @@ 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.
> +have the task structure on at least a two byte alignment (and if this is
> +not true, the rtmutex.c code will be broken!), this allows for the least
> +significant bit to be used as flag. Bit 0 is used as the "Has Waiters"
> +flag. It's set whenever there are waiters on a mutex.
>
> +See Documentation/rt-mutex.txt for further details.
>
> cmpxchg Tricks
> --------------
> @@ -359,40 +338,30 @@ 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 must adjust its priority. With the help of the pi_waiters 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.
> +The functions implementing the task adjustments are rt_mutex_adjust_prio
> +and rt_mutex_setprio. rt_mutex_setprio is 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.
> +rt_mutex_adjust_prio examines the priority of the task has, and the highest
> +priority of processes which are waiting on a mutex owned by the task. Since
> +the pi_waiters of a task holds an order by priority of all the top waiters
> +of all the mutexes that the task owns, we just simply needs to compare the
> +top pi waiter to its own normal/deadline priority, to get the higher one.
> +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/core.c
> +to implement the actual change in priority.
>
> -(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.)
> +(Note, the low number of prio has higher 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/core.c to implement the
> -actual change in priority.
> -
> -It is interesting to note that __rt_mutex_adjust_prio can either increase
> +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
> +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
> +would decrease/unboost the priority of the task. That is because the pi_waiters
> 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.
> @@ -414,7 +383,8 @@ 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).
> +parameter may be NULL for deboosting), a next_lock mutex on which the task
> +is blocked, and a top_task as the top waiter of the mutex.
>
> For this explanation, I will not mention deadlock detection. This explanation
> will try to stay at a high level.
> @@ -424,133 +394,14 @@ 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
> +should be at, but the rbtree nodes of the task's waiter have not been updated
> +with the new priorities, and this task may not be in the proper locations
> +in the pi_waiters and waiters 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.
> -
> +The main operation of this function is summarized by Thomas Gleixner in
> +rtmutex.c. See the 'Chain walk basics and protection scope' comment for further
> +details.
>
> Taking of a mutex (The walk through)
> ------------------------------------
> @@ -563,13 +414,13 @@ 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).
> +If there is contention on the lock, 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 task on the waiters of the mutex, and if need be, the pi_waiters of
> the owner.
>
> The wait_lock of the mutex is taken since the slow path of unlocking the
> @@ -581,135 +432,71 @@ 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.
> +the "Has Waiters" flag of the mutex's owner field. By setting this flag
> +now, the current owner of the mutex being contended for 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 current owner to synchronize with this code.
> +
> +The lock is taken if the following are true:
> + 1) The lock has no owner
> + 2) The current task is the highest priority against all other
> + waiters of the lock
> +
> +If the task succeeds to acquire the lock, then the task is set as the
> +owner of the lock, and if the lock still has waiters, the top_waiter
> +(highest priority task waiting on the lock) is added to this task's
> +pi_waiters tree.
> +
> +If the lock is not taken by try_to_take_rt_mutex(), then the
> +task_blocks_on_rt_mutex() function is called. This will add the task to
> +the lock's waiter tree and propagate the pi chain of the lock as well
> +as the lock's owner's pi_waiters tree. This is described in the next
> +section.
>
> 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.
> +to the mutex. The rbtree node of waiter 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
> +add the waiter to the task waiter tree. 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_waiters of the owner,
> +and add the current process to that tree. Since the pi_waiter 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
> +If the owner is also blocked on a lock, and had its pi_waiters 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
> +The task can then wake up for a couple of reasons:
> + 1) The previous lock owner released the lock, and the task now is top_waiter
> + 2) we received a signal or timeout
>
> -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
> -----------------------------------------
> +In the first case, the task will try again to acquire the lock. If it
> +does, then it will take itself off the waiters tree and set itself back
> +to the TASK_RUNNING state. If the lock was acquired by another task
> +before this task could get the lock, then it will go back to sleep and
> +wait to be woken again
>
> -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.
> +The second case is only applicable for tasks that are grabbing a mutex
> +that can wake up before getting the lock, either due to a signal or
> +a timeout (i.e. rt_mutex_timed_futex_lock()). When woken, it will try to
> +take the lock again, if it succeeds, then the task will return with the
> +lock held, otherwise it will return with -EINTR if the task was woken
> +by a signal, or -ETIMEDOUT if it timed out.
>
>
> Unlocking the Mutex
> @@ -739,25 +526,12 @@ 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.
> +If there are waiters, then we need to wake one up.
>
> 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.
> +waiter of the lock is found and removed from the waiters tree of the mutex
> +as well as the pi_waiters tree of the current owner. The "Has Waiters" bit is
> +marked to prevent new lower priority task to steal this lock.
>
> Finally we unlock the pi_lock of the pending owner and wake it up.
>
> @@ -772,6 +546,7 @@ Credits
> -------
>
> Author: Steven Rostedt <[email protected]>
> +Updated: Alex Shi <[email protected]> - 5/20/2017
>
> Reviewers: Ingo Molnar, Thomas Gleixner, Thomas Duetsch, and Randy Dunlap
>
> @@ -779,3 +554,4 @@ Updates
> -------
>
> This document was originally written for 2.6.17-rc3-mm1
> +was updated on 4.12-rc1
>