On 04/13/2017 11:54 PM, Steven Rostedt wrote:
> On Thu, 13 Apr 2017 22:02:52 +0800
> Alex Shi <alex.shi@xxxxxxxxxx> wrote:
>
>> The rt-mutex 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.
>
> Yes it needs some loving.
>
>>
>> So 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 | 438 ++++++++++--------------------
>> Documentation/locking/rt-mutex.txt | 32 ++-
>> 2 files changed, 153 insertions(+), 317 deletions(-)
>>
>> diff --git a/Documentation/locking/rt-mutex-design.txt b/Documentation/locking/rt-mutex-design.txt
>> index 8666070..5a1c0ca 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 the waiters of a mutex as well as the pi_waiters
>
> I would say "waiters rbtree of a mutex" as "the waiters of a mutex"
> sounds confusing. Same for pi_waiters.
Yes. thanks for suggestion!
>
>> + 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 +179,34 @@ 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, any right lock owners
>
> "any right lock owners" doesn't make sense. You mean owners to the
> right side of the tree of G?
Yes, how about this?
+If the G process has highest priority in the chain, any rightside lock owners
+in the tree branch need to increase its' priority as high as G.
>
>> +need to increase its' priority as high as 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 +293,12 @@ 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 bits to be used as flag. Bit 0 is used as the "Has Waiters"
>
> s/bits/bit/ "the least significant bit to be used as a flag."
Thanks!
>
>
>> +flag. It's set whenever there are waiters on a mutex.
>>
>> +Documentation/rt-mutex.txt described this for details.
>
> See Documentation/rt-mutex.txt for further details.
Yes.
>
>>
>> cmpxchg Tricks
>> --------------
>> @@ -359,7 +337,7 @@ 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 +349,10 @@ 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 +370,7 @@ 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 +392,8 @@ 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 +403,70 @@ 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 as
>> +following:
>
> "as the following:"
>
> Hmm, this is cut and pasted from the comments in rtmutex.c, which means
> that if something changes, this will not be updated. I don't like the
> duplication. Perhaps just add a reference to the comments in the code?
>
> "See the 'Chain walk basics and protection scope' comment in rtmutex.c
> for further details"
>
> And don't add the text here.
Yes, that make more sense here.
>
>> +
>> + Step Description Protected by
>> + function arguments:
>> + @task [R]
>> + @orig_lock if != NULL @top_task is blocked on it
>> + @next_lock Unprotected. Cannot be
>> + dereferenced. Only used for
>> + comparison.
>> + @orig_waiter if != NULL @top_task is blocked on it
>> + @top_task current, or in case of proxy
>> + locking protected by calling
>> + code
>> + again:
>> + loop_sanity_check();
>> + retry:
>> + [1] lock(task->pi_lock); [R] acquire [P]
>> + [2] waiter = task->pi_blocked_on; [P]
>> + [3] check_exit_conditions_1(); [P]
>> + [4] lock = waiter->lock; [P]
>> + [5] if (!try_lock(lock->wait_lock)) { [P] try to acquire [L]
>> + unlock(task->pi_lock); release [P]
>> + goto retry;
>> + }
>> + [6] check_exit_conditions_2(); [P] + [L]
>> + [7] requeue_lock_waiter(lock, waiter); [P] + [L]
>> + [8] unlock(task->pi_lock); release [P]
>> + put_task_struct(task); release [R]
>> + [9] check_exit_conditions_3(); [L]
>> + [10] task = owner(lock); [L]
>> + get_task_struct(task); [L] acquire [R]
>> + lock(task->pi_lock); [L] acquire [P]
>> + [11] requeue_pi_waiter(tsk, waiters(lock));[P] + [L]
>> + [12] check_exit_conditions_4(); [P] + [L]
>> + [13] unlock(task->pi_lock); release [P]
>> + unlock(lock->wait_lock); release [L]
>> + goto again;
>> +
>> +The first 5 steps are used to hold the task and the lock on which is the task
>> +blocked. It will try to get both of them in loop, or return when some exit
>> +conditions happened, some of them like:
>> + 1) Get the end of the boosting chain
>> + 2) Task or lock moved on or changed during retry
>> + 3) The original waiter isn't the highest in the lock owner's pi_waiters
>> + 4) The waiter's prio is same as the lock owner's prio
>> +
>> +After hold the task and the lock which task blocked on. We update the task
>> +blocked_on waiter's prio to lock owner's prio and requeue it in the lock's
>> +waiters tree. That is done from steps 6 to 8.
>> +
>> +Since the waiter's priority just was changed, it may causes pi_waiters change of
>> +the lock owner task, if the changed prio was or will be highest prio in the
>> +pi_waiters of the lock owner, the lock owner task's prio will be deboost or
>> +boost accordingly. That's what happens from steps 10 to 11.
>> +
>> +Now, the 'task' pointer is forward one step in PI chain, so we goto 'again' to
>> +repeat the PI chain walking, unless some exit conditions triggered, or the
>> +right end task of PI chain is updated to right priority.
>>
>> Taking of a mutex (The walk through)
>> ------------------------------------
>> @@ -563,13 +479,13 @@ 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 +497,64 @@ 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 owner of possible contestant of the mutex can't release the mutex
>
> "the owner of possible contestant of the mutex" makes no sense. What
> about: "the current owner of the mutex being contended for"
Yours' better. :)
>
>> +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 possible owner to synchronize with this code.
>
> "forces the current owner"
Thanks!
>
>> +
>> +If the lock has a owner already. It's one of case we should give up. Other
>> +reasons to give up this lock include:
>> + 1) This enqueued waiter isn't the top waiter, aka highest priority waiter.
>> + 2) There are other waiters on the lock, and this task prio is lower or equal
>> + to their priority.
>
> The above needs to be rewritten:
>
> 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 none of fails is triggered, we will take the lock for this task -- set the
>> +task as lock's owner. Also hook the highest waiters on this task's pi_waiters
>> +tree.
>
> This too should be rewritten:
>
> 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 we give up the mutex getting in try_to_take_rt_mutex, task_blocks_on_rt_mutex
>> +function will be called to setup waiter and progagate pi chain for the lock and
>> +task. That is introduced in following:
>
> Rewrite:
>
> 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.
Thanks a lot for suggestion of this section. will replace with your comments!
>
>>
>> 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 in
>> +function __rt_mutex_slowlock).
>
> I would keep the original way of ending with "(call schedule)", the
> extra is not needed.
Right!
>
>
>> +
>>
>> 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 schedule can then wake up for a few reasons, included:
>
> s/few/couple of/
>
> s/, included//
Thanks!
I rewrite this section as following, any comments? :)
Waking up in the loop
---------------------
The schedule can then wake up for a couple of reasons:
1) The previous lock owner released the lock, and we are top_waiter now
2) we received a signal or timeout
For the first reason, we could get the lock in acquisition retry and back to
TASK_RUNNING state. For the second reason, if task is in TASK_INTERRUPTIBLE
state, we will give up the lock acquisition, and also back to TASK_RUNNING.
Otherwise we will yield cpu and back to sleep.
>
>> + 1) we received a signal and was TASK_INTERRUPTIBLE
>> + 2) we had a timeout and was TASK_INTERRUPTIBLE
>
> What about getting the lock?
>
>>
>> -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 these above cases, we are failed to get this lock, so we break out the loop
>> +remove self from the lock waiter, if still we are.
>
> I can't make any sense of the above sentence.
rewritten.
>
>>
>> -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.
>> +For other reasons we are woken up, we will keep trying to take rt mutex in
>> +every waking up. If it isn't possible will yeild the cpu again and wait for
>> +next schedule chance.
>
> This doesn't make sense either.
dropped.
>
>>
>>
>> Unlocking the Mutex
>> @@ -739,25 +584,12 @@ 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 +604,7 @@ Credits
>> -------
>>
>> Author: Steven Rostedt <rostedt@xxxxxxxxxxx>
>> +Updater: Alex Shi <alex.shi@xxxxxxxxxx>
>
> Updated: Alex Shi <alex.shi@xxxxxxxxxx> - 4/13/2017
Thanks!
>
>>
>> Reviewers: Ingo Molnar, Thomas Gleixner, Thomas Duetsch, and Randy Dunlap
>>
>> @@ -779,3 +612,4 @@ Updates
>> -------
>>
>> This document was originally written for 2.6.17-rc3-mm1
>> +was updated on 4.11-rc4.
>> diff --git a/Documentation/locking/rt-mutex.txt b/Documentation/locking/rt-mutex.txt
>> index 243393d..1481f97 100644
>
> I'm not looking at the other document right now.
May it's better to split this document to another patch.
>
> -- Steve
>
--
To unsubscribe from this list: send the line "unsubscribe linux-doc" in
the body of a message to majordomo@xxxxxxxxxxxxxxx
More majordomo info at http://vger.kernel.org/majordomo-info.html
