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Signed-off-by: Tom Saeger <tom.saeger@oracle.com> Signed-off-by: Jonathan Corbet <corbet@lwn.net>
560 lines
21 KiB
Plaintext
560 lines
21 KiB
Plaintext
#
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# Copyright (c) 2006 Steven Rostedt
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# Licensed under the GNU Free Documentation License, Version 1.2
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#
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RT-mutex implementation design
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------------------------------
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This document tries to describe the design of the rtmutex.c implementation.
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It doesn't describe the reasons why rtmutex.c exists. For that please see
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Documentation/locking/rt-mutex.txt. Although this document does explain problems
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that happen without this code, but that is in the concept to understand
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what the code actually is doing.
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The goal of this document is to help others understand the priority
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inheritance (PI) algorithm that is used, as well as reasons for the
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decisions that were made to implement PI in the manner that was done.
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Unbounded Priority Inversion
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----------------------------
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Priority inversion is when a lower priority process executes while a higher
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priority process wants to run. This happens for several reasons, and
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most of the time it can't be helped. Anytime a high priority process wants
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to use a resource that a lower priority process has (a mutex for example),
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the high priority process must wait until the lower priority process is done
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with the resource. This is a priority inversion. What we want to prevent
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is something called unbounded priority inversion. That is when the high
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priority process is prevented from running by a lower priority process for
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an undetermined amount of time.
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The classic example of unbounded priority inversion is where you have three
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processes, let's call them processes A, B, and C, where A is the highest
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priority process, C is the lowest, and B is in between. A tries to grab a lock
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that C owns and must wait and lets C run to release the lock. But in the
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meantime, B executes, and since B is of a higher priority than C, it preempts C,
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but by doing so, it is in fact preempting A which is a higher priority process.
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Now there's no way of knowing how long A will be sleeping waiting for C
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to release the lock, because for all we know, B is a CPU hog and will
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never give C a chance to release the lock. This is called unbounded priority
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inversion.
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Here's a little ASCII art to show the problem.
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grab lock L1 (owned by C)
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A ---+
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C preempted by B
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C +----+
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B +-------->
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B now keeps A from running.
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Priority Inheritance (PI)
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-------------------------
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There are several ways to solve this issue, but other ways are out of scope
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for this document. Here we only discuss PI.
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PI is where a process inherits the priority of another process if the other
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process blocks on a lock owned by the current process. To make this easier
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to understand, let's use the previous example, with processes A, B, and C again.
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This time, when A blocks on the lock owned by C, C would inherit the priority
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of A. So now if B becomes runnable, it would not preempt C, since C now has
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the high priority of A. As soon as C releases the lock, it loses its
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inherited priority, and A then can continue with the resource that C had.
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Terminology
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-----------
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Here I explain some terminology that is used in this document to help describe
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the design that is used to implement PI.
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PI chain - The PI chain is an ordered series of locks and processes that cause
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processes to inherit priorities from a previous process that is
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blocked on one of its locks. This is described in more detail
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later in this document.
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mutex - In this document, to differentiate from locks that implement
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PI and spin locks that are used in the PI code, from now on
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the PI locks will be called a mutex.
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lock - In this document from now on, I will use the term lock when
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referring to spin locks that are used to protect parts of the PI
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algorithm. These locks disable preemption for UP (when
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CONFIG_PREEMPT is enabled) and on SMP prevents multiple CPUs from
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entering critical sections simultaneously.
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spin lock - Same as lock above.
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waiter - A waiter is a struct that is stored on the stack of a blocked
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process. Since the scope of the waiter is within the code for
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a process being blocked on the mutex, it is fine to allocate
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the waiter on the process's stack (local variable). This
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structure holds a pointer to the task, as well as the mutex that
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the task is blocked on. It also has rbtree node structures to
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place the task in the waiters rbtree of a mutex as well as the
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pi_waiters rbtree of a mutex owner task (described below).
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waiter is sometimes used in reference to the task that is waiting
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on a mutex. This is the same as waiter->task.
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waiters - A list of processes that are blocked on a mutex.
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top waiter - The highest priority process waiting on a specific mutex.
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top pi waiter - The highest priority process waiting on one of the mutexes
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that a specific process owns.
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Note: task and process are used interchangeably in this document, mostly to
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differentiate between two processes that are being described together.
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PI chain
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--------
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The PI chain is a list of processes and mutexes that may cause priority
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inheritance to take place. Multiple chains may converge, but a chain
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would never diverge, since a process can't be blocked on more than one
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mutex at a time.
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Example:
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Process: A, B, C, D, E
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Mutexes: L1, L2, L3, L4
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A owns: L1
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B blocked on L1
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B owns L2
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C blocked on L2
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C owns L3
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D blocked on L3
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D owns L4
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E blocked on L4
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The chain would be:
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E->L4->D->L3->C->L2->B->L1->A
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To show where two chains merge, we could add another process F and
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another mutex L5 where B owns L5 and F is blocked on mutex L5.
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The chain for F would be:
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F->L5->B->L1->A
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Since a process may own more than one mutex, but never be blocked on more than
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one, the chains merge.
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Here we show both chains:
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E->L4->D->L3->C->L2-+
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+->B->L1->A
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F->L5-+
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For PI to work, the processes at the right end of these chains (or we may
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also call it the Top of the chain) must be equal to or higher in priority
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than the processes to the left or below in the chain.
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Also since a mutex may have more than one process blocked on it, we can
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have multiple chains merge at mutexes. If we add another process G that is
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blocked on mutex L2:
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G->L2->B->L1->A
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And once again, to show how this can grow I will show the merging chains
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again.
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E->L4->D->L3->C-+
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+->L2-+
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G-+ +->B->L1->A
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F->L5-+
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If process G has the highest priority in the chain, then all the tasks up
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the chain (A and B in this example), must have their priorities increased
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to that of G.
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Mutex Waiters Tree
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-----------------
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Every mutex keeps track of all the waiters that are blocked on itself. The
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mutex has a rbtree to store these waiters by priority. This tree is protected
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by a spin lock that is located in the struct of the mutex. This lock is called
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wait_lock.
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Task PI Tree
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------------
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To keep track of the PI chains, each process has its own PI rbtree. This is
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a tree of all top waiters of the mutexes that are owned by the process.
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Note that this tree only holds the top waiters and not all waiters that are
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blocked on mutexes owned by the process.
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The top of the task's PI tree is always the highest priority task that
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is waiting on a mutex that is owned by the task. So if the task has
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inherited a priority, it will always be the priority of the task that is
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at the top of this tree.
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This tree is stored in the task structure of a process as a rbtree called
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pi_waiters. It is protected by a spin lock also in the task structure,
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called pi_lock. This lock may also be taken in interrupt context, so when
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locking the pi_lock, interrupts must be disabled.
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Depth of the PI Chain
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---------------------
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The maximum depth of the PI chain is not dynamic, and could actually be
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defined. But is very complex to figure it out, since it depends on all
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the nesting of mutexes. Let's look at the example where we have 3 mutexes,
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L1, L2, and L3, and four separate functions func1, func2, func3 and func4.
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The following shows a locking order of L1->L2->L3, but may not actually
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be directly nested that way.
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void func1(void)
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{
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mutex_lock(L1);
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/* do anything */
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mutex_unlock(L1);
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}
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void func2(void)
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{
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mutex_lock(L1);
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mutex_lock(L2);
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/* do something */
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mutex_unlock(L2);
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mutex_unlock(L1);
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}
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void func3(void)
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{
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mutex_lock(L2);
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mutex_lock(L3);
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/* do something else */
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mutex_unlock(L3);
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mutex_unlock(L2);
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}
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void func4(void)
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{
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mutex_lock(L3);
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/* do something again */
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mutex_unlock(L3);
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}
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Now we add 4 processes that run each of these functions separately.
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Processes A, B, C, and D which run functions func1, func2, func3 and func4
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respectively, and such that D runs first and A last. With D being preempted
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in func4 in the "do something again" area, we have a locking that follows:
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D owns L3
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C blocked on L3
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C owns L2
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B blocked on L2
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B owns L1
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A blocked on L1
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And thus we have the chain A->L1->B->L2->C->L3->D.
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This gives us a PI depth of 4 (four processes), but looking at any of the
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functions individually, it seems as though they only have at most a locking
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depth of two. So, although the locking depth is defined at compile time,
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it still is very difficult to find the possibilities of that depth.
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Now since mutexes can be defined by user-land applications, we don't want a DOS
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type of application that nests large amounts of mutexes to create a large
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PI chain, and have the code holding spin locks while looking at a large
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amount of data. So to prevent this, the implementation not only implements
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a maximum lock depth, but also only holds at most two different locks at a
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time, as it walks the PI chain. More about this below.
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Mutex owner and flags
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---------------------
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The mutex structure contains a pointer to the owner of the mutex. If the
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mutex is not owned, this owner is set to NULL. Since all architectures
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have the task structure on at least a two byte alignment (and if this is
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not true, the rtmutex.c code will be broken!), this allows for the least
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significant bit to be used as a flag. Bit 0 is used as the "Has Waiters"
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flag. It's set whenever there are waiters on a mutex.
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See Documentation/locking/rt-mutex.txt for further details.
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cmpxchg Tricks
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--------------
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Some architectures implement an atomic cmpxchg (Compare and Exchange). This
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is used (when applicable) to keep the fast path of grabbing and releasing
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mutexes short.
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cmpxchg is basically the following function performed atomically:
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unsigned long _cmpxchg(unsigned long *A, unsigned long *B, unsigned long *C)
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{
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unsigned long T = *A;
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if (*A == *B) {
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*A = *C;
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}
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return T;
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}
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#define cmpxchg(a,b,c) _cmpxchg(&a,&b,&c)
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This is really nice to have, since it allows you to only update a variable
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if the variable is what you expect it to be. You know if it succeeded if
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the return value (the old value of A) is equal to B.
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The macro rt_mutex_cmpxchg is used to try to lock and unlock mutexes. If
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the architecture does not support CMPXCHG, then this macro is simply set
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to fail every time. But if CMPXCHG is supported, then this will
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help out extremely to keep the fast path short.
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The use of rt_mutex_cmpxchg with the flags in the owner field help optimize
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the system for architectures that support it. This will also be explained
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later in this document.
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Priority adjustments
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--------------------
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The implementation of the PI code in rtmutex.c has several places that a
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process must adjust its priority. With the help of the pi_waiters of a
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process this is rather easy to know what needs to be adjusted.
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The functions implementing the task adjustments are rt_mutex_adjust_prio
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and rt_mutex_setprio. rt_mutex_setprio is only used in rt_mutex_adjust_prio.
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rt_mutex_adjust_prio examines the priority of the task, and the highest
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priority process that is waiting any of mutexes owned by the task. Since
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the pi_waiters of a task holds an order by priority of all the top waiters
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of all the mutexes that the task owns, we simply need to compare the top
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pi waiter to its own normal/deadline priority and take the higher one.
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Then rt_mutex_setprio is called to adjust the priority of the task to the
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new priority. Note that rt_mutex_setprio is defined in kernel/sched/core.c
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to implement the actual change in priority.
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(Note: For the "prio" field in task_struct, the lower the number, the
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higher the priority. A "prio" of 5 is of higher priority than a
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"prio" of 10.)
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It is interesting to note that rt_mutex_adjust_prio can either increase
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or decrease the priority of the task. In the case that a higher priority
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process has just blocked on a mutex owned by the task, rt_mutex_adjust_prio
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would increase/boost the task's priority. But if a higher priority task
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were for some reason to leave the mutex (timeout or signal), this same function
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would decrease/unboost the priority of the task. That is because the pi_waiters
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always contains the highest priority task that is waiting on a mutex owned
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by the task, so we only need to compare the priority of that top pi waiter
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to the normal priority of the given task.
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High level overview of the PI chain walk
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----------------------------------------
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The PI chain walk is implemented by the function rt_mutex_adjust_prio_chain.
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The implementation has gone through several iterations, and has ended up
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with what we believe is the best. It walks the PI chain by only grabbing
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at most two locks at a time, and is very efficient.
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The rt_mutex_adjust_prio_chain can be used either to boost or lower process
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priorities.
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rt_mutex_adjust_prio_chain is called with a task to be checked for PI
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(de)boosting (the owner of a mutex that a process is blocking on), a flag to
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check for deadlocking, the mutex that the task owns, a pointer to a waiter
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that is the process's waiter struct that is blocked on the mutex (although this
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parameter may be NULL for deboosting), a pointer to the mutex on which the task
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is blocked, and a top_task as the top waiter of the mutex.
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For this explanation, I will not mention deadlock detection. This explanation
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will try to stay at a high level.
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When this function is called, there are no locks held. That also means
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that the state of the owner and lock can change when entered into this function.
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Before this function is called, the task has already had rt_mutex_adjust_prio
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performed on it. This means that the task is set to the priority that it
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should be at, but the rbtree nodes of the task's waiter have not been updated
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with the new priorities, and this task may not be in the proper locations
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in the pi_waiters and waiters trees that the task is blocked on. This function
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solves all that.
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The main operation of this function is summarized by Thomas Gleixner in
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rtmutex.c. See the 'Chain walk basics and protection scope' comment for further
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details.
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Taking of a mutex (The walk through)
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------------------------------------
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OK, now let's take a look at the detailed walk through of what happens when
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taking a mutex.
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The first thing that is tried is the fast taking of the mutex. This is
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done when we have CMPXCHG enabled (otherwise the fast taking automatically
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fails). Only when the owner field of the mutex is NULL can the lock be
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taken with the CMPXCHG and nothing else needs to be done.
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If there is contention on the lock, we go about the slow path
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(rt_mutex_slowlock).
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The slow path function is where the task's waiter structure is created on
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the stack. This is because the waiter structure is only needed for the
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scope of this function. The waiter structure holds the nodes to store
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the task on the waiters tree of the mutex, and if need be, the pi_waiters
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tree of the owner.
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The wait_lock of the mutex is taken since the slow path of unlocking the
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mutex also takes this lock.
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We then call try_to_take_rt_mutex. This is where the architecture that
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does not implement CMPXCHG would always grab the lock (if there's no
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contention).
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try_to_take_rt_mutex is used every time the task tries to grab a mutex in the
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slow path. The first thing that is done here is an atomic setting of
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the "Has Waiters" flag of the mutex's owner field. By setting this flag
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now, the current owner of the mutex being contended for can't release the mutex
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without going into the slow unlock path, and it would then need to grab the
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wait_lock, which this code currently holds. So setting the "Has Waiters" flag
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forces the current owner to synchronize with this code.
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The lock is taken if the following are true:
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1) The lock has no owner
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2) The current task is the highest priority against all other
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waiters of the lock
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If the task succeeds to acquire the lock, then the task is set as the
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owner of the lock, and if the lock still has waiters, the top_waiter
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(highest priority task waiting on the lock) is added to this task's
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pi_waiters tree.
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If the lock is not taken by try_to_take_rt_mutex(), then the
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task_blocks_on_rt_mutex() function is called. This will add the task to
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the lock's waiter tree and propagate the pi chain of the lock as well
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as the lock's owner's pi_waiters tree. This is described in the next
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section.
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Task blocks on mutex
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--------------------
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The accounting of a mutex and process is done with the waiter structure of
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the process. The "task" field is set to the process, and the "lock" field
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to the mutex. The rbtree node of waiter are initialized to the processes
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current priority.
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Since the wait_lock was taken at the entry of the slow lock, we can safely
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add the waiter to the task waiter tree. If the current process is the
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highest priority process currently waiting on this mutex, then we remove the
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previous top waiter process (if it exists) from the pi_waiters of the owner,
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and add the current process to that tree. Since the pi_waiter of the owner
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has changed, we call rt_mutex_adjust_prio on the owner to see if the owner
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should adjust its priority accordingly.
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If the owner is also blocked on a lock, and had its pi_waiters changed
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(or deadlock checking is on), we unlock the wait_lock of the mutex and go ahead
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and run rt_mutex_adjust_prio_chain on the owner, as described earlier.
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Now all locks are released, and if the current process is still blocked on a
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mutex (waiter "task" field is not NULL), then we go to sleep (call schedule).
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Waking up in the loop
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---------------------
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The task can then wake up for a couple of reasons:
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1) The previous lock owner released the lock, and the task now is top_waiter
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2) we received a signal or timeout
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In both cases, the task will try again to acquire the lock. If it
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does, then it will take itself off the waiters tree and set itself back
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to the TASK_RUNNING state.
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In first case, if the lock was acquired by another task before this task
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|
could get the lock, then it will go back to sleep and wait to be woken again.
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|
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|
The second case is only applicable for tasks that are grabbing a mutex
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|
that can wake up before getting the lock, either due to a signal or
|
|
a timeout (i.e. rt_mutex_timed_futex_lock()). When woken, it will try to
|
|
take the lock again, if it succeeds, then the task will return with the
|
|
lock held, otherwise it will return with -EINTR if the task was woken
|
|
by a signal, or -ETIMEDOUT if it timed out.
|
|
|
|
|
|
Unlocking the Mutex
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|
-------------------
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|
|
|
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 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.
|
|
|
|
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 waiters tree of the mutex
|
|
as well as the pi_waiters tree of the current owner. The "Has Waiters" bit is
|
|
marked to prevent lower priority tasks from stealing the lock.
|
|
|
|
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>
|
|
Updated: Alex Shi <alex.shi@linaro.org> - 7/6/2017
|
|
|
|
Original Reviewers: Ingo Molnar, Thomas Gleixner, Thomas Duetsch, and
|
|
Randy Dunlap
|
|
Update (7/6/2017) Reviewers: Steven Rostedt and Sebastian Siewior
|
|
|
|
Updates
|
|
-------
|
|
|
|
This document was originally written for 2.6.17-rc3-mm1
|
|
was updated on 4.12
|