Re: [PATCH v2 1/3] rtmutex: update rt-mutex-design

[Mathieu Poirier]

On 21 April 2017 at 08:12, Alex Shi <alex.shi@xxxxxxxxxx> 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. So the documents are far
> late of real code.
>
> Update it to latest code and make it meaningful.
>
> Signed-off-by: Alex Shi <alex.shi@xxxxxxxxxx>
> Cc: Steven Rostedt <rostedt@xxxxxxxxxxx>
> Cc: Sebastian Siewior <bigeasy@xxxxxxxxxxxxx>
> To: linux-doc@xxxxxxxxxxxxxxx
> To: linux-kernel@xxxxxxxxxxxxxxx
> To: Jonathan Corbet <corbet@xxxxxxx>
> To: Ingo Molnar <mingo@xxxxxxxxxx>
> To: Peter Zijlstra <peterz@xxxxxxxxxxxxx>
> Cc: Thomas Gleixner <tglx@xxxxxxxxxxxxx>
> ---
>  Documentation/locking/rt-mutex-design.txt | 390 +++++++-----------------------
>  1 file changed, 88 insertions(+), 302 deletions(-)
>
> diff --git a/Documentation/locking/rt-mutex-design.txt b/Documentation/locking/rt-mutex-design.txt
> index 8666070..11beb55 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

Here I assume we are talking about struct rt_mutex_waiter[1].  If so I
suggest to replace rbtree with rb_node.

> +          place the task in waiters rbtree of a mutex as well as the
> +          pi_waiters rbtree of a mutex owner task (described below).

Also following the comment for @pi_tree_entry, s/"a mutex owner
task"/"a mutex owner waiters tree" .

[1]. http://lxr.free-electrons.com/source/kernel/locking/rtmutex_common.h#L25


>
>             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 the G process has highest priority in the chain, then all the tasks up

If process G has the highest priority in the chain, ...

> +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
>  --------------
> @@ -359,7 +338,7 @@ 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,
> @@ -371,10 +350,10 @@ 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.
> +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, 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
> @@ -392,7 +371,7 @@ 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
> +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 +393,8 @@ 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 +404,14 @@ 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 +424,13 @@ 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 +442,71 @@ 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).
> +mutex (waiter "task" field is not NULL), then we go to sleep (call schedule)
> +

This change was likely not done on purpose.

>
>  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.
> +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
>
> -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 +536,12 @@ 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 +556,7 @@ Credits
>  -------
>
>  Author:  Steven Rostedt <rostedt@xxxxxxxxxxx>
> +Updated: Alex Shi <alex.shi@xxxxxxxxxx>        - 4/14/2017
>
>  Reviewers:  Ingo Molnar, Thomas Gleixner, Thomas Duetsch, and Randy Dunlap
>
> @@ -779,3 +564,4 @@ Updates
>  -------
>
>  This document was originally written for 2.6.17-rc3-mm1
> +was updated on 4.11
> --
> 1.9.1
>