From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1753572AbdDMPym (ORCPT ); Thu, 13 Apr 2017 11:54:42 -0400 Received: from mail.kernel.org ([198.145.29.136]:50746 "EHLO mail.kernel.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1751242AbdDMPyk (ORCPT ); Thu, 13 Apr 2017 11:54:40 -0400 Date: Thu, 13 Apr 2017 11:54:30 -0400 From: Steven Rostedt To: Alex Shi Cc: peterz@infradead.org, mingo@redhat.com, corbet@lwn.net, linux-kernel@vger.kernel.org (open list:LOCKING PRIMITIVES), linux-doc@vger.kernel.org (open list:DOCUMENTATION), Sebastian Siewior , Thomas Gleixner Subject: Re: [PATCH 1/3] rtmutex: comments update Message-ID: <20170413115430.05a94664@gandalf.local.home> In-Reply-To: <1492092174-31734-2-git-send-email-alex.shi@linaro.org> References: <1492092174-31734-1-git-send-email-alex.shi@linaro.org> <1492092174-31734-2-git-send-email-alex.shi@linaro.org> X-Mailer: Claws Mail 3.14.0 (GTK+ 2.24.31; x86_64-pc-linux-gnu) MIME-Version: 1.0 Content-Type: text/plain; charset=US-ASCII Content-Transfer-Encoding: 7bit Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Thu, 13 Apr 2017 22:02:52 +0800 Alex Shi 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 > Cc: Steven Rostedt > Cc: Sebastian Siewior > To: linux-doc@vger.kernel.org > To: linux-kernel@vger.kernel.org > To: Jonathan Corbet > To: Ingo Molnar > To: Peter Zijlstra > Cc: Thomas Gleixner > --- > 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. > + 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? > +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." > +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. > > 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. > + > + 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" > +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" > + > +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. > > 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. > + > > 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// > + 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. > > -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. > > > 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 > +Updater: Alex Shi Updated: Alex Shi - 4/13/2017 > > 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. -- Steve