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| -rw-r--r-- | Documentation/pi-futex.txt | 121 | ||||
| -rw-r--r-- | Documentation/rt-mutex-design.txt | 781 | ||||
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diff --git a/Documentation/pi-futex.txt b/Documentation/pi-futex.txt new file mode 100644 index 00000000000..5d61dacd21f --- /dev/null +++ b/Documentation/pi-futex.txt @@ -0,0 +1,121 @@ +Lightweight PI-futexes +---------------------- + +We are calling them lightweight for 3 reasons: + + - in the user-space fastpath a PI-enabled futex involves no kernel work + (or any other PI complexity) at all. No registration, no extra kernel + calls - just pure fast atomic ops in userspace. + + - even in the slowpath, the system call and scheduling pattern is very + similar to normal futexes. + + - the in-kernel PI implementation is streamlined around the mutex + abstraction, with strict rules that keep the implementation + relatively simple: only a single owner may own a lock (i.e. no + read-write lock support), only the owner may unlock a lock, no + recursive locking, etc. + +Priority Inheritance - why? +--------------------------- + +The short reply: user-space PI helps achieving/improving determinism for +user-space applications. In the best-case, it can help achieve +determinism and well-bound latencies. Even in the worst-case, PI will +improve the statistical distribution of locking related application +delays. + +The longer reply: +----------------- + +Firstly, sharing locks between multiple tasks is a common programming +technique that often cannot be replaced with lockless algorithms. As we +can see it in the kernel [which is a quite complex program in itself], +lockless structures are rather the exception than the norm - the current +ratio of lockless vs. locky code for shared data structures is somewhere +between 1:10 and 1:100. Lockless is hard, and the complexity of lockless +algorithms often endangers to ability to do robust reviews of said code. +I.e. critical RT apps often choose lock structures to protect critical +data structures, instead of lockless algorithms. Furthermore, there are +cases (like shared hardware, or other resource limits) where lockless +access is mathematically impossible. + +Media players (such as Jack) are an example of reasonable application +design with multiple tasks (with multiple priority levels) sharing +short-held locks: for example, a highprio audio playback thread is +combined with medium-prio construct-audio-data threads and low-prio +display-colory-stuff threads. Add video and decoding to the mix and +we've got even more priority levels. + +So once we accept that synchronization objects (locks) are an +unavoidable fact of life, and once we accept that multi-task userspace +apps have a very fair expectation of being able to use locks, we've got +to think about how to offer the option of a deterministic locking +implementation to user-space. + +Most of the technical counter-arguments against doing priority +inheritance only apply to kernel-space locks. But user-space locks are +different, there we cannot disable interrupts or make the task +non-preemptible in a critical section, so the 'use spinlocks' argument +does not apply (user-space spinlocks have the same priority inversion +problems as other user-space locking constructs). Fact is, pretty much +the only technique that currently enables good determinism for userspace +locks (such as futex-based pthread mutexes) is priority inheritance: + +Currently (without PI), if a high-prio and a low-prio task shares a lock +[this is a quite common scenario for most non-trivial RT applications], +even if all critical sections are coded carefully to be deterministic +(i.e. all critical sections are short in duration and only execute a +limited number of instructions), the kernel cannot guarantee any +deterministic execution of the high-prio task: any medium-priority task +could preempt the low-prio task while it holds the shared lock and +executes the critical section, and could delay it indefinitely. + +Implementation: +--------------- + +As mentioned before, the userspace fastpath of PI-enabled pthread +mutexes involves no kernel work at all - they behave quite similarly to +normal futex-based locks: a 0 value means unlocked, and a value==TID +means locked. (This is the same method as used by list-based robust +futexes.) Userspace uses atomic ops to lock/unlock these mutexes without +entering the kernel. + +To handle the slowpath, we have added two new futex ops: + + FUTEX_LOCK_PI + FUTEX_UNLOCK_PI + +If the lock-acquire fastpath fails, [i.e. an atomic transition from 0 to +TID fails], then FUTEX_LOCK_PI is called. The kernel does all the +remaining work: if there is no futex-queue attached to the futex address +yet then the code looks up the task that owns the futex [it has put its +own TID into the futex value], and attaches a 'PI state' structure to +the futex-queue. The pi_state includes an rt-mutex, which is a PI-aware, +kernel-based synchronization object. The 'other' task is made the owner +of the rt-mutex, and the FUTEX_WAITERS bit is atomically set in the +futex value. Then this task tries to lock the rt-mutex, on which it +blocks. Once it returns, it has the mutex acquired, and it sets the +futex value to its own TID and returns. Userspace has no other work to +perform - it now owns the lock, and futex value contains +FUTEX_WAITERS|TID. + +If the unlock side fastpath succeeds, [i.e. userspace manages to do a +TID -> 0 atomic transition of the futex value], then no kernel work is +triggered. + +If the unlock fastpath fails (because the FUTEX_WAITERS bit is set), +then FUTEX_UNLOCK_PI is called, and the kernel unlocks the futex on the +behalf of userspace - and it also unlocks the attached +pi_state->rt_mutex and thus wakes up any potential waiters. + +Note that under this approach, contrary to previous PI-futex approaches, +there is no prior 'registration' of a PI-futex. [which is not quite +possible anyway, due to existing ABI properties of pthread mutexes.] + +Also, under this scheme, 'robustness' and 'PI' are two orthogonal +properties of futexes, and all four combinations are possible: futex, +robust-futex, PI-futex, robust+PI-futex. + +More details about priority inheritance can be found in +Documentation/rtmutex.txt. diff --git a/Documentation/rt-mutex-design.txt b/Documentation/rt-mutex-design.txt new file mode 100644 index 00000000000..c472ffacc2f --- /dev/null +++ b/Documentation/rt-mutex-design.txt @@ -0,0 +1,781 @@ +# +# Copyright (c) 2006 Steven Rostedt +# Licensed under the GNU Free Documentation License, Version 1.2 +# + +RT-mutex implementation design +------------------------------ + +This document tries to describe the design of the rtmutex.c implementation. +It doesn't describe the reasons why rtmutex.c exists. For that please see +Documentation/rt-mutex.txt. Although this document does explain problems +that happen without this code, but that is in the concept to understand +what the code actually is doing. + +The goal of this document is to help others understand the priority +inheritance (PI) algorithm that is used, as well as reasons for the +decisions that were made to implement PI in the manner that was done. + + +Unbounded Priority Inversion +---------------------------- + +Priority inversion is when a lower priority process executes while a higher +priority process wants to run. This happens for several reasons, and +most of the time it can't be helped. Anytime a high priority process wants +to use a resource that a lower priority process has (a mutex for example), +the high priority process must wait until the lower priority process is done +with the resource. This is a priority inversion. What we want to prevent +is something called unbounded priority inversion. That is when the high +priority process is prevented from running by a lower priority process for +an undetermined amount of time. + +The classic example of unbounded priority inversion is were you have three +processes, let's call them processes A, B, and C, where A is the highest +priority process, C is the lowest, and B is in between. A tries to grab a lock +that C owns and must wait and lets C run to release the lock. But in the +meantime, B executes, and since B is of a higher priority than C, it preempts C, +but by doing so, it is in fact preempting A which is a higher priority process. +Now there's no way of knowing how long A will be sleeping waiting for C +to release the lock, because for all we know, B is a CPU hog and will +never give C a chance to release the lock. This is called unbounded priority +inversion. + +Here's a little ASCII art to show the problem. + + grab lock L1 (owned by C) + | +A ---+ + C preempted by B + | +C +----+ + +B +--------> + B now keeps A from running. + + +Priority Inheritance (PI) +------------------------- + +There are several ways to solve this issue, but other ways are out of scope +for this document. Here we only discuss PI. + +PI is where a process inherits the priority of another process if the other +process blocks on a lock owned by the current process. To make this easier +to understand, let's use the previous example, with processes A, B, and C again. + +This time, when A blocks on the lock owned by C, C would inherit the priority +of A. So now if B becomes runnable, it would not preempt C, since C now has +the high priority of A. As soon as C releases the lock, it loses its +inherited priority, and A then can continue with the resource that C had. + +Terminology +----------- + +Here I explain some terminology that is used in this document to help describe +the design that is used to implement PI. + +PI chain - The PI chain is an ordered series of locks and processes that cause + processes to inherit priorities from a previous process that is + blocked on one of its locks. This is described in more detail + later in this document. + +mutex - In this document, to differentiate from locks that implement + PI and spin locks that are used in the PI code, from now on + the PI locks will be called a mutex. + +lock - In this document from now on, I will use the term lock when + referring to spin locks that are used to protect parts of the PI + algorithm. These locks disable preemption for UP (when + CONFIG_PREEMPT is enabled) and on SMP prevents multiple CPUs from + entering critical sections simultaneously. + +spin lock - Same as lock above. + +waiter - A waiter is a struct that is stored on the stack of a blocked + process. Since the scope of the waiter is within the code for + a process being blocked on the mutex, it is fine to allocate + the waiter on the process's stack (local variable). This + structure holds a pointer to the task, as well as the mutex that + the task is blocked on. It also has the plist node structures to + place the task in the waiter_list of a mutex as well as the + pi_list of a mutex owner task (described below). + + waiter is sometimes used in reference to the task that is waiting + on a mutex. This is the same as waiter->task. + +waiters - A list of processes that are blocked on a mutex. + +top waiter - The highest priority process waiting on a specific mutex. + +top pi waiter - The highest priority process waiting on one of the mutexes + that a specific process owns. + +Note: task and process are used interchangeably in this document, mostly to + differentiate between two processes that are being described together. + + +PI chain +-------- + +The PI chain is a list of processes and mutexes that may cause priority +inheritance to take place. Multiple chains may converge, but a chain +would never diverge, since a process can't be blocked on more than one +mutex at a time. + +Example: + + Process: A, B, C, D, E + Mutexes: L1, L2, L3, L4 + + A owns: L1 + B blocked on L1 + B owns L2 + C blocked on L2 + C owns L3 + D blocked on L3 + D owns L4 + E blocked on L4 + +The chain would be: + + E->L4->D->L3->C->L2->B->L1->A + +To show where two chains merge, we could add another process F and +another mutex L5 where B owns L5 and F is blocked on mutex L5. + +The chain for F would be: + + F->L5->B->L1->A + +Since a process may own more than one mutex, but never be blocked on more than +one, the chains merge. + +Here we show both chains: + + E->L4->D->L3->C->L2-+ + | + +->B->L1->A + | + F->L5-+ + +For PI to work, the processes at the right end of these chains (or we may +also call it the Top of the chain) must be equal to or higher in priority +than the processes to the left or below in the chain. + +Also since a mutex may have more than one process blocked on it, we can +have multiple chains merge at mutexes. If we add another process G that is +blocked on mutex L2: + + G->L2->B->L1->A + +And once again, to show how this can grow I will show the merging chains +again. + + E->L4->D->L3->C-+ + +->L2-+ + | | + G-+ +->B->L1->A + | + F->L5-+ + + +Plist +----- + +Before I go further and talk about how the PI chain is stored through lists +on both mutexes and processes, I'll explain the plist. This is similar to +the struct list_head functionality that is already in the kernel. +The implementation of plist is out of scope for this document, but it is +very important to understand what it does. + +There are a few differences between plist and list, the most important one +being that plist is a priority sorted linked list. This means that the +priorities of the plist are sorted, such that it takes O(1) to retrieve the +highest priority item in the list. Obviously this is useful to store processes +based on their priorities. + +Another difference, which is important for implementation, is that, unlike +list, the head of the list is a different element than the nodes of a list. +So the head of the list is declared as struct plist_head and nodes that will +be added to the list are declared as struct plist_node. + + +Mutex Waiter List +----------------- + +Every mutex keeps track of all the waiters that are blocked on itself. The mutex +has a plist to store these waiters by priority. This list is protected by +a spin lock that is located in the struct of the mutex. This lock is called +wait_lock. Since the modification of the waiter list is never done in +interrupt context, the wait_lock can be taken without disabling interrupts. + + +Task PI List +------------ + +To keep track of the PI chains, each process has its own PI list. This is +a list of all top waiters of the mutexes that are owned by the process. +Note that this list only holds the top waiters and not all waiters that are +blocked on mutexes owned by the process. + +The top of the task's PI list is always the highest priority task that +is waiting on a mutex that is owned by the task. So if the task has +inherited a priority, it will always be the priority of the task that is +at the top of this list. + +This list is stored in the task structure of a process as a plist called +pi_list. This list is protected by a spin lock also in the task structure, +called pi_lock. This lock may also be taken in interrupt context, so when +locking the pi_lock, interrupts must be disabled. + + +Depth of the PI Chain +--------------------- + +The maximum depth of the PI chain is not dynamic, and could actually be +defined. But is very complex to figure it out, since it depends on all +the nesting of mutexes. Let's look at the example where we have 3 mutexes, +L1, L2, and L3, and four separate functions func1, func2, func3 and func4. +The following shows a locking order of L1->L2->L3, but may not actually +be directly nested that way. + +void func1(void) +{ + mutex_lock(L1); + + /* do anything */ + + mutex_unlock(L1); +} + +void func2(void) +{ + mutex_lock(L1); + mutex_lock(L2); + + /* do something */ + + mutex_unlock(L2); + mutex_unlock(L1); +} + +void func3(void) +{ + mutex_lock(L2); + mutex_lock(L3); + + /* do something else */ + + mutex_unlock(L3); + mutex_unlock(L2); +} + +void func4(void) +{ + mutex_lock(L3); + + /* do something again */ + + mutex_unlock(L3); +} + +Now we add 4 processes that run each of these functions separately. +Processes A, B, C, and D which run functions func1, func2, func3 and func4 +respectively, and such that D runs first and A last. With D being preempted +in func4 in the "do something again" area, we have a locking that follows: + +D owns L3 + C blocked on L3 + C owns L2 + B blocked on L2 + B owns L1 + A blocked on L1 + +And thus we have the chain A->L1->B->L2->C->L3->D. + +This gives us a PI depth of 4 (four processes), but looking at any of the +functions individually, it seems as though they only have at most a locking +depth of two. So, although the locking depth is defined at compile time, +it still is very difficult to find the possibilities of that depth. + +Now since mutexes can be defined by user-land applications, we don't want a DOS +type of application that nests large amounts of mutexes to create a large +PI chain, and have the code holding spin locks while looking at a large +amount of data. So to prevent this, the implementation not only implements +a maximum lock depth, but also only holds at most two different locks at a +time, as it walks the PI chain. More about this below. + + +Mutex owner and flags +--------------------- + +The mutex structure contains a pointer to the owner of the mutex. If the +mutex is not owned, this owner is set to NULL. Since all architectures +have the task structure on at least a four byte alignment (and if this is +not true, the rtmutex.c code will be broken!), this allows for the two +least significant bits to be used as flags. This part is also described +in Documentation/rt-mutex.txt, but will also be briefly described here. + +Bit 0 is used as the "Pending Owner" flag. This is described later. +Bit 1 is used as the "Has Waiters" flags. This is also described later + in more detail, but is set whenever there are waiters on a mutex. + + +cmpxchg Tricks +-------------- + +Some architectures implement an atomic cmpxchg (Compare and Exchange). This +is used (when applicable) to keep the fast path of grabbing and releasing +mutexes short. + +cmpxchg is basically the following function performed atomically: + +unsigned long _cmpxchg(unsigned long *A, unsigned long *B, unsigned long *C) +{ + unsigned long T = *A; + if (*A == *B) { + *A = *C; + } + return T; +} +#define cmpxchg(a,b,c) _cmpxchg(&a,&b,&c) + +This is really nice to have, since it allows you to only update a variable +if the variable is what you expect it to be. You know if it succeeded if +the return value (the old value of A) is equal to B. + +The macro rt_mutex_cmpxchg is used to try to lock and unlock mutexes. If +the architecture does not support CMPXCHG, then this macro is simply set +to fail every time. But if CMPXCHG is supported, then this will +help out extremely to keep the fast path short. + +The use of rt_mutex_cmpxchg with the flags in the owner field help optimize +the system for architectures that support it. This will also be explained +later in this document. + + +Priority adjustments +-------------------- + +The implementation of the PI code in rtmutex.c has several places that a +process must adjust its priority. With the help of the pi_list of a +process this is rather easy to know what needs to be adjusted. + +The functions implementing the task adjustments are rt_mutex_adjust_prio, +__rt_mutex_adjust_prio (same as the former, but expects the task pi_lock +to already be taken), rt_mutex_get_prio, and rt_mutex_setprio. + +rt_mutex_getprio and rt_mutex_setprio are only used in __rt_mutex_adjust_prio. + +rt_mutex_getprio returns the priority that the task should have. Either the +task's own normal priority, or if a process of a higher priority is waiting on +a mutex owned by the task, then that higher priority should be returned. +Since the pi_list of a task holds an order by priority list of all the top +waiters of all the mutexes that the task owns, rt_mutex_getprio simply needs +to compare the top pi waiter to its own normal priority, and return the higher +priority back. + +(Note: if looking at the code, you will notice that the lower number of + prio is returned. This is because the prio field in the task structure + is an inverse order of the actual priority. So a "prio" of 5 is + of higher priority than a "prio" of 10.) + +__rt_mutex_adjust_prio examines the result of rt_mutex_getprio, and if the +result does not equal the task's current priority, then rt_mutex_setprio +is called to adjust the priority of the task to the new priority. +Note that rt_mutex_setprio is defined in kernel/sched.c to implement the +actual change in priority. + +It is interesting to note that __rt_mutex_adjust_prio can either increase +or decrease the priority of the task. In the case that a higher priority +process has just blocked on a mutex owned by the task, __rt_mutex_adjust_prio +would increase/boost the task's priority. But if a higher priority task +were for some reason to leave the mutex (timeout or signal), this same function +would decrease/unboost the priority of the task. That is because the pi_list +always contains the highest priority task that is waiting on a mutex owned +by the task, so we only need to compare the priority of that top pi waiter +to the normal priority of the given task. + + +High level overview of the PI chain walk +---------------------------------------- + +The PI chain walk is implemented by the function rt_mutex_adjust_prio_chain. + +The implementation has gone through several iterations, and has ended up +with what we believe is the best. It walks the PI chain by only grabbing +at most two locks at a time, and is very efficient. + +The rt_mutex_adjust_prio_chain can be used either to boost or lower process +priorities. + +rt_mutex_adjust_prio_chain is called with a task to be checked for PI +(de)boosting (the owner of a mutex that a process is blocking on), a flag to +check for deadlocking, the mutex that the task owns, and a pointer to a waiter +that is the process's waiter struct that is blocked on the mutex (although this +parameter may be NULL for deboosting). + +For this explanation, I will not mention deadlock detection. This explanation +will try to stay at a high level. + +When this function is called, there are no locks held. That also means +that the state of the owner and lock can change when entered into this function. + +Before this function is called, the task has already had rt_mutex_adjust_prio +performed on it. This means that the task is set to the priority that it +should be at, but the plist nodes of the task's waiter have not been updated +with the new priorities, and that this task may not be in the proper locations +in the pi_lists and wait_lists that the task is blocked on. This function +solves all that. + +A loop is entered, where task is the owner to be checked for PI changes that +was passed by parameter (for the first iteration). The pi_lock of this task is +taken to prevent any more changes to the pi_list of the task. This also +prevents new tasks from completing the blocking on a mutex that is owned by this +task. + +If the task is not blocked on a mutex then the loop is exited. We are at +the top of the PI chain. + +A check is now done to see if the original waiter (the process that is blocked +on the current mutex) is the top pi waiter of the task. That is, is this +waiter on the top of the task's pi_list. If it is not, it either means that +there is another process higher in priority that is blocked on one of the +mutexes that the task owns, or that the waiter has just woken up via a signal +or timeout and has left the PI chain. In either case, the loop is exited, since +we don't need to do any more changes to the priority of the current task, or any +task that owns a mutex that this current task is waiting on. A priority chain +walk is only needed when a new top pi waiter is made to a task. + +The next check sees if the task's waiter plist node has the priority equal to +the priority the task is set at. If they are equal, then we are done with +the loop. Remember that the function started with the priority of the +task adjusted, but the plist nodes that hold the task in other processes +pi_lists have not been adjusted. + +Next, we look at the mutex that the task is blocked on. The mutex's wait_lock +is taken. This is done by a spin_trylock, because the locking order of the +pi_lock and wait_lock goes in the opposite direction. If we fail to grab the +lock, the pi_lock is released, and we restart the loop. + +Now that we have both the pi_lock of the task as well as the wait_lock of +the mutex the task is blocked on, we update the task's waiter's plist node +that is located on the mutex's wait_list. + +Now we release the pi_lock of the task. + +Next the owner of the mutex has its pi_lock taken, so we can update the +task's entry in the owner's pi_list. If the task is the highest priority +process on the mutex's wait_list, then we remove the previous top waiter +from the owner's pi_list, and replace it with the task. + +Note: It is possible that the task was the current top waiter on the mutex, + in which case the task is not yet on the pi_list of the waiter. This + is OK, since plist_del does nothing if the plist node is not on any + list. + +If the task was not the top waiter of the mutex, but it was before we +did the priority updates, that means we are deboosting/lowering the +task. In this case, the task is removed from the pi_list of the owner, +and the new top waiter is added. + +Lastly, we unlock both the pi_lock of the task, as well as the mutex's +wait_lock, and continue the loop again. On the next iteration of the +loop, the previous owner of the mutex will be the task that will be +processed. + +Note: One might think that the owner of this mutex might have changed + since we just grab the mutex's wait_lock. And one could be right. + The important thing to remember is that the owner could not have + become the task that is being processed in the PI chain, since + we have taken that task's pi_lock at the beginning of the loop. + So as long as there is an owner of this mutex that is not the same + process as the tasked being worked on, we are OK. + + Looking closely at the code, one might be confused. The check for the + end of the PI chain is when the task isn't blocked on anything or the + task's waiter structure "task" element is NULL. This check is + protected only by the task's pi_lock. But the code to unlock the mutex + sets the task's waiter structure "task" element to NULL with only + the protection of the mutex's wait_lock, which was not taken yet. + Isn't this a race condition if the task becomes the new owner? + + The answer is No! The trick is the spin_trylock of the mutex's + wait_lock. If we fail that lock, we release the pi_lock of the + task and continue the loop, doing the end of PI chain check again. + + In the code to release the lock, the wait_lock of the mutex is held + the entire time, and it is not let go when we grab the pi_lock of the + new owner of the mutex. So if the switch of a new owner were to happen + after the check for end of the PI chain and the grabbing of the + wait_lock, the unlocking code would spin on the new owner's pi_lock + but never give up the wait_lock. So the PI chain loop is guaranteed to + fail the spin_trylock on the wait_lock, release the pi_lock, and + try again. + + If you don't quite understand the above, that's OK. You don't have to, + unless you really want to make a proof out of it ;) + + +Pending Owners and Lock stealing +-------------------------------- + +One of the flags in the owner field of the mutex structure is "Pending Owner". +What this means is that an owner was chosen by the process releasing the +mutex, but that owner has yet to wake up and actually take the mutex. + +Why is this important? Why can't we just give the mutex to another process +and be done with it? + +The PI code is to help with real-time processes, and to let the highest +priority process run as long as possible with little latencies and delays. +If a high priority process owns a mutex that a lower priority process is +blocked on, when the mutex is released it would be given to the lower priority +process. What if the higher priority process wants to take that mutex again. +The high priority process would fail to take that mutex that it just gave up +and it would need to boost the lower priority process to run with full +latency of that critical section (since the low priority process just entered +it). + +There's no reason a high priority process that gives up a mutex should be +penalized if it tries to take that mutex again. If the new owner of the +mutex has not woken up yet, there's no reason that the higher priority process +could not take that mutex away. + +To solve this, we introduced Pending Ownership and Lock Stealing. When a +new process is given a mutex that it was blocked on, it is only given +pending ownership. This means that it's the new owner, unless a higher +priority process comes in and tries to grab that mutex. If a higher priority +process does come along and wants that mutex, we let the higher priority +process "steal" the mutex from the pending owner (only if it is still pending) +and continue with the mutex. + + +Taking of a mutex (The walk through) +------------------------------------ + +OK, now let's take a look at the detailed walk through of what happens when +taking a mutex. + +The first thing that is tried is the fast taking of the mutex. This is +done when we have CMPXCHG enabled (otherwise the fast taking automatically +fails). Only when the owner field of the mutex is NULL can the lock be +taken with the CMPXCHG and nothing else needs to be done. + +If there is contention on the lock, whether it is owned or pending owner +we go about the slow path (rt_mutex_slowlock). + +The slow path function is where the task's waiter structure is created on +the stack. This is because the waiter structure is only needed for the +scope of this function. The waiter structure holds the nodes to store +the task on the wait_list of the mutex, and if need be, the pi_list of +the owner. + +The wait_lock of the mutex is taken since the slow path of unlocking the +mutex also takes this lock. + +We then call try_to_take_rt_mutex. This is where the architecture that +does not implement CMPXCHG would always grab the lock (if there's no +contention). + +try_to_take_rt_mutex is used every time the task tries to grab a mutex in the +slow path. The first thing that is done here is an atomic setting of +the "Has Waiters" flag of the mutex's owner field. Yes, this could really +be false, because if the 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 it's mutex stolen. If the "task" +field is NULL then we need to set up the accounting for it. + +Task blocks on mutex +-------------------- + +The accounting of a mutex and process is done with the waiter structure of +the process. The "task" field is set to the process, and the "lock" field +to the mutex. The plist nodes are initialized to the processes current +priority. + +Since the wait_lock was taken at the entry of the slow lock, we can safely +add the waiter to the wait_list. If the current process is the highest +priority process currently waiting on this mutex, then we remove the +previous top waiter process (if it exists) from the pi_list of the owner, +and add the current process to that list. Since the pi_list of the owner +has changed, we call rt_mutex_adjust_prio on the owner to see if the owner +should adjust its priority accordingly. + +If the owner is also blocked on a lock, and had its pi_list changed +(or deadlock checking is on), we unlock the wait_lock of the mutex and go ahead +and run rt_mutex_adjust_prio_chain on the owner, as described earlier. + +Now all locks are released, and if the current process is still blocked on a +mutex (waiter "task" field is not NULL), then we go to sleep (call schedule). + +Waking up in the loop +--------------------- + +The schedule can then wake up for a few reasons. + 1) we were given pending ownership of the mutex. + 2) we received a signal and was TASK_INTERRUPTIBLE + 3) we had a timeout and was TASK_INTERRUPTIBLE + +In any of these cases, we continue the loop and once again try to grab the +ownership of the mutex. If we succeed, we exit the loop, otherwise we continue +and on signal and timeout, will exit the loop, or if we had the mutex stolen +we just simply add ourselves back on the lists and go back to sleep. + +Note: For various reasons, because of timeout and signals, the steal mutex + algorithm needs to be careful. This is because the current process is + still on the wait_list. And because of dynamic changing of priorities, + especially on SCHED_OTHER tasks, the current process can be the + highest priority task on the wait_list. + +Failed to get mutex on Timeout or Signal +---------------------------------------- + +If a timeout or signal occurred, the waiter's "task" field would not be +NULL and the task needs to be taken off the wait_list of the mutex and perhaps +pi_list of the owner. If this process was a high priority process, then +the rt_mutex_adjust_prio_chain needs to be executed again on the owner, +but this time it will be lowering the priorities. + + +Unlocking the Mutex +------------------- + +The unlocking of a mutex also has a fast path for those architectures with +CMPXCHG. Since the taking of a mutex on contention always sets the +"Has Waiters" flag of the mutex's owner, we use this to know if we need to +take the slow path when unlocking the mutex. If the mutex doesn't have any +waiters, the owner field of the mutex would equal the current process and +the mutex can be unlocked by just replacing the owner field with NULL. + +If the owner field has the "Has Waiters" bit set (or CMPXCHG is not available), +the slow unlock path is taken. + +The first thing done in the slow unlock path is to take the wait_lock of the +mutex. This synchronizes the locking and unlocking of the mutex. + +A check is made to see if the mutex has waiters or not. On architectures that +do not have CMPXCHG, this is the location that the owner of the mutex will +determine if a waiter needs to be awoken or not. On architectures that +do have CMPXCHG, that check is done in the fast path, but it is still needed +in the slow path too. If a waiter of a mutex woke up because of a signal +or timeout between the time the owner failed the fast path CMPXCHG check and +the grabbing of the wait_lock, the mutex may not have any waiters, thus the +owner still needs to make this check. If there are no waiters than the mutex +owner field is set to NULL, the wait_lock is released and nothing more is +needed. + +If there are waiters, then we need to wake one up and give that waiter +pending ownership. + +On the wake up code, the pi_lock of the current owner is taken. The top +waiter of the lock is found and removed from the wait_list of the mutex +as well as the pi_list of the current owner. The task field of the new +pending owner's waiter structure is set to NULL, and the owner field of the +mutex is set to the new owner with the "Pending Owner" bit set, as well +as the "Has Waiters" bit if there still are other processes blocked on the +mutex. + +The pi_lock of the previous owner is released, and the new pending owner's +pi_lock is taken. Remember that this is the trick to prevent the race +condition in rt_mutex_adjust_prio_chain from adding itself as a waiter +on the mutex. + +We now clear the "pi_blocked_on" field of the new pending owner, and if +the mutex still has waiters pending, we add the new top waiter to the pi_list +of the pending owner. + +Finally we unlock the pi_lock of the pending owner and wake it up. + + +Contact +------- + +For updates on this document, please email Steven Rostedt <rostedt@goodmis.org> + + +Credits +------- + +Author: Steven Rostedt <rostedt@goodmis.org> + +Reviewers: Ingo Molnar, Thomas Gleixner, Thomas Duetsch, and Randy Dunlap + +Updates +------- + +This document was originally written for 2.6.17-rc3-mm1 diff --git a/Documentation/rt-mutex.txt b/Documentation/rt-mutex.txt new file mode 100644 index 00000000000..243393d882e --- /dev/null +++ b/Documentation/rt-mutex.txt @@ -0,0 +1,79 @@ +RT-mutex subsystem with PI support +---------------------------------- + +RT-mutexes with priority inheritance are used to support PI-futexes, +which enable pthread_mutex_t priority inheritance attributes +(PTHREAD_PRIO_INHERIT). [See Documentation/pi-futex.txt for more details +about PI-futexes.] + +This technology was developed in the -rt tree and streamlined for +pthread_mutex support. + +Basic principles: +----------------- + +RT-mutexes extend the semantics of simple mutexes by the priority +inheritance protocol. + +A low priority owner of a rt-mutex inherits the priority of a higher +priority waiter until the rt-mutex is released. If the temporarily +boosted owner blocks on a rt-mutex itself it propagates the priority +boosting to the owner of the other rt_mutex it gets blocked on. The +priority boosting is immediately removed once the rt_mutex has been +unlocked. + +This approach allows us to shorten the block of high-prio tasks on +mutexes which protect shared resources. Priority inheritance is not a +magic bullet for poorly designed applications, but it allows +well-designed applications to use userspace locks in critical parts of +an high priority thread, without losing determinism. + +The enqueueing of the waiters into the rtmutex waiter list is done in +priority order. For same priorities FIFO order is chosen. For each +rtmutex, only the top priority waiter is enqueued into the owner's +priority waiters list. This list too queues in priority order. Whenever +the top priority waiter of a task changes (for example it timed out or +got a signal), the priority of the owner task is readjusted. [The +priority enqueueing is handled by "plists", see include/linux/plist.h +for more details.] + +RT-mutexes are optimized for fastpath operations and have no internal +locking overhead when locking an uncontended mutex or unlocking a mutex +without waiters. The optimized fastpath operations require cmpxchg +support. [If that is not available then the rt-mutex internal spinlock +is used] + +The state of the rt-mutex is tracked via the owner field of the rt-mutex +structure: + +rt_mutex->owner holds the task_struct pointer of the owner. Bit 0 and 1 +are used to keep track of the "owner is pending" and "rtmutex has +waiters" state. + + owner bit1 bit0 + NULL 0 0 mutex is free (fast acquire possible) + NULL 0 1 invalid state + NULL 1 0 Transitional state* + NULL 1 1 invalid state + taskpointer 0 0 mutex is held (fast release possible) + taskpointer 0 1 task is pending owner + taskpointer 1 0 mutex is held and has waiters + taskpointer 1 1 task is pending owner and mutex has waiters + +Pending-ownership handling is a performance optimization: +pending-ownership is assigned to the first (highest priority) waiter of +the mutex, when the mutex is released. The thread is woken up and once +it starts executing it can acquire the mutex. Until the mutex is taken +by it (bit 0 is cleared) a competing higher priority thread can "steal" +the mutex which puts the woken up thread back on the waiters list. + +The pending-ownership optimization is especially important for the +uninterrupted workflow of high-prio tasks which repeatedly +takes/releases locks that have lower-prio waiters. Without this +optimization the higher-prio thread would ping-pong to the lower-prio +task [because at unlock time we always assign a new owner]. + +(*) The "mutex has waiters" bit gets set to take the lock. If the lock +doesn't already have an owner, this bit is quickly cleared if there are +no waiters. So this is a transitional state to synchronize with looking +at the owner field of the mutex and the mutex owner releasing the lock. |