c79a8d85d7
Fix some typos in five documentations, no functional change. Signed-off-by: Xishi Qiu <qiuxishi@huawei.com> Acked-by: Rob Landley <rob@landley.net> Signed-off-by: Jiri Kosina <jkosina@suse.cz>
782 lines
33 KiB
Plaintext
782 lines
33 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/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 the plist node structures to
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place the task in the waiter_list of a mutex as well as the
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pi_list 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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Plist
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-----
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Before I go further and talk about how the PI chain is stored through lists
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on both mutexes and processes, I'll explain the plist. This is similar to
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the struct list_head functionality that is already in the kernel.
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The implementation of plist is out of scope for this document, but it is
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very important to understand what it does.
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There are a few differences between plist and list, the most important one
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being that plist is a priority sorted linked list. This means that the
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priorities of the plist are sorted, such that it takes O(1) to retrieve the
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highest priority item in the list. Obviously this is useful to store processes
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based on their priorities.
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Another difference, which is important for implementation, is that, unlike
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list, the head of the list is a different element than the nodes of a list.
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So the head of the list is declared as struct plist_head and nodes that will
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be added to the list are declared as struct plist_node.
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Mutex Waiter List
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-----------------
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Every mutex keeps track of all the waiters that are blocked on itself. The mutex
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has a plist to store these waiters by priority. This list is protected by
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a spin lock that is located in the struct of the mutex. This lock is called
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wait_lock. Since the modification of the waiter list is never done in
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interrupt context, the wait_lock can be taken without disabling interrupts.
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Task PI List
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------------
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To keep track of the PI chains, each process has its own PI list. This is
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a list of all top waiters of the mutexes that are owned by the process.
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Note that this list 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 list 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 list.
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This list is stored in the task structure of a process as a plist called
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pi_list. This list 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 four byte alignment (and if this is
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not true, the rtmutex.c code will be broken!), this allows for the two
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least significant bits to be used as flags. This part is also described
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in Documentation/rt-mutex.txt, but will also be briefly described here.
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Bit 0 is used as the "Pending Owner" flag. This is described later.
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Bit 1 is used as the "Has Waiters" flags. This is also described later
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in more detail, but is set whenever there are waiters on a mutex.
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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_list 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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__rt_mutex_adjust_prio (same as the former, but expects the task pi_lock
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to already be taken), rt_mutex_getprio, and rt_mutex_setprio.
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rt_mutex_getprio and rt_mutex_setprio are only used in __rt_mutex_adjust_prio.
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rt_mutex_getprio returns the priority that the task should have. Either the
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task's own normal priority, or if a process of a higher priority is waiting on
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a mutex owned by the task, then that higher priority should be returned.
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Since the pi_list of a task holds an order by priority list of all the top
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waiters of all the mutexes that the task owns, rt_mutex_getprio simply needs
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to compare the top pi waiter to its own normal priority, and return the higher
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priority back.
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(Note: if looking at the code, you will notice that the lower number of
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prio is returned. This is because the prio field in the task structure
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is an inverse order of the actual priority. So a "prio" of 5 is
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of higher priority than a "prio" of 10.)
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__rt_mutex_adjust_prio examines the result of rt_mutex_getprio, and if the
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result does not equal the task's current priority, then rt_mutex_setprio
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is called to adjust the priority of the task to the new priority.
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Note that rt_mutex_setprio is defined in kernel/sched/core.c to implement the
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actual change in priority.
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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_list
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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, and 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).
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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 plist nodes of the task's waiter have not been updated
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with the new priorities, and that this task may not be in the proper locations
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in the pi_lists and wait_lists that the task is blocked on. This function
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solves all that.
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A loop is entered, where task is the owner to be checked for PI changes that
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was passed by parameter (for the first iteration). The pi_lock of this task is
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taken to prevent any more changes to the pi_list of the task. This also
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prevents new tasks from completing the blocking on a mutex that is owned by this
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task.
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If the task is not blocked on a mutex then the loop is exited. We are at
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the top of the PI chain.
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A check is now done to see if the original waiter (the process that is blocked
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on the current mutex) is the top pi waiter of the task. That is, is this
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waiter on the top of the task's pi_list. If it is not, it either means that
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there is another process higher in priority that is blocked on one of the
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mutexes that the task owns, or that the waiter has just woken up via a signal
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or timeout and has left the PI chain. In either case, the loop is exited, since
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we don't need to do any more changes to the priority of the current task, or any
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task that owns a mutex that this current task is waiting on. A priority chain
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walk is only needed when a new top pi waiter is made to a task.
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The next check sees if the task's waiter plist node has the priority equal to
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the priority the task is set at. If they are equal, then we are done with
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the loop. Remember that the function started with the priority of the
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task adjusted, but the plist nodes that hold the task in other processes
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pi_lists have not been adjusted.
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Next, we look at the mutex that the task is blocked on. The mutex's wait_lock
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is taken. This is done by a spin_trylock, because the locking order of the
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pi_lock and wait_lock goes in the opposite direction. If we fail to grab the
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lock, the pi_lock is released, and we restart the loop.
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Now that we have both the pi_lock of the task as well as the wait_lock of
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the mutex the task is blocked on, we update the task's waiter's plist node
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that is located on the mutex's wait_list.
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Now we release the pi_lock of the task.
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Next the owner of the mutex has its pi_lock taken, so we can update the
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task's entry in the owner's pi_list. If the task is the highest priority
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process on the mutex's wait_list, then we remove the previous top waiter
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from the owner's pi_list, and replace it with the task.
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Note: It is possible that the task was the current top waiter on the mutex,
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in which case the task is not yet on the pi_list of the waiter. This
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is OK, since plist_del does nothing if the plist node is not on any
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list.
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If the task was not the top waiter of the mutex, but it was before we
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did the priority updates, that means we are deboosting/lowering the
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task. In this case, the task is removed from the pi_list of the owner,
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and the new top waiter is added.
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Lastly, we unlock both the pi_lock of the task, as well as the mutex's
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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 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.
|
|
|
|
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 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.
|
|
|
|
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
|