Merge branch 'sched-docs-for-linus' of git://git.kernel.org/pub/scm/linux/kernel/git/tip/linux-2.6-tip
* 'sched-docs-for-linus' of git://git.kernel.org/pub/scm/linux/kernel/git/tip/linux-2.6-tip: sched: Document memory barriers implied by sleep/wake-up primitives
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Linus Torvalds
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@ -31,6 +31,7 @@ Contents:
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- Locking functions.
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- Interrupt disabling functions.
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- Sleep and wake-up functions.
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- Miscellaneous functions.
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(*) Inter-CPU locking barrier effects.
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@ -1217,6 +1218,132 @@ barriers are required in such a situation, they must be provided from some
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other means.
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SLEEP AND WAKE-UP FUNCTIONS
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---------------------------
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Sleeping and waking on an event flagged in global data can be viewed as an
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interaction between two pieces of data: the task state of the task waiting for
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the event and the global data used to indicate the event. To make sure that
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these appear to happen in the right order, the primitives to begin the process
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of going to sleep, and the primitives to initiate a wake up imply certain
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barriers.
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Firstly, the sleeper normally follows something like this sequence of events:
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for (;;) {
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set_current_state(TASK_UNINTERRUPTIBLE);
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if (event_indicated)
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break;
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schedule();
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}
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A general memory barrier is interpolated automatically by set_current_state()
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after it has altered the task state:
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CPU 1
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===============================
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set_current_state();
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set_mb();
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STORE current->state
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<general barrier>
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LOAD event_indicated
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set_current_state() may be wrapped by:
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prepare_to_wait();
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prepare_to_wait_exclusive();
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which therefore also imply a general memory barrier after setting the state.
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The whole sequence above is available in various canned forms, all of which
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interpolate the memory barrier in the right place:
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wait_event();
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wait_event_interruptible();
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wait_event_interruptible_exclusive();
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wait_event_interruptible_timeout();
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wait_event_killable();
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wait_event_timeout();
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wait_on_bit();
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wait_on_bit_lock();
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Secondly, code that performs a wake up normally follows something like this:
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event_indicated = 1;
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wake_up(&event_wait_queue);
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or:
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event_indicated = 1;
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wake_up_process(event_daemon);
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A write memory barrier is implied by wake_up() and co. if and only if they wake
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something up. The barrier occurs before the task state is cleared, and so sits
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between the STORE to indicate the event and the STORE to set TASK_RUNNING:
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CPU 1 CPU 2
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=============================== ===============================
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set_current_state(); STORE event_indicated
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set_mb(); wake_up();
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STORE current->state <write barrier>
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<general barrier> STORE current->state
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LOAD event_indicated
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The available waker functions include:
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complete();
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wake_up();
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wake_up_all();
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wake_up_bit();
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wake_up_interruptible();
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wake_up_interruptible_all();
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wake_up_interruptible_nr();
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wake_up_interruptible_poll();
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wake_up_interruptible_sync();
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wake_up_interruptible_sync_poll();
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wake_up_locked();
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wake_up_locked_poll();
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wake_up_nr();
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wake_up_poll();
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wake_up_process();
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[!] Note that the memory barriers implied by the sleeper and the waker do _not_
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order multiple stores before the wake-up with respect to loads of those stored
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values after the sleeper has called set_current_state(). For instance, if the
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sleeper does:
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set_current_state(TASK_INTERRUPTIBLE);
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if (event_indicated)
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break;
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__set_current_state(TASK_RUNNING);
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do_something(my_data);
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and the waker does:
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my_data = value;
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event_indicated = 1;
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wake_up(&event_wait_queue);
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there's no guarantee that the change to event_indicated will be perceived by
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the sleeper as coming after the change to my_data. In such a circumstance, the
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code on both sides must interpolate its own memory barriers between the
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separate data accesses. Thus the above sleeper ought to do:
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set_current_state(TASK_INTERRUPTIBLE);
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if (event_indicated) {
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smp_rmb();
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do_something(my_data);
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}
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and the waker should do:
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my_data = value;
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smp_wmb();
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event_indicated = 1;
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wake_up(&event_wait_queue);
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MISCELLANEOUS FUNCTIONS
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-----------------------
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@ -1366,7 +1493,7 @@ WHERE ARE MEMORY BARRIERS NEEDED?
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Under normal operation, memory operation reordering is generally not going to
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be a problem as a single-threaded linear piece of code will still appear to
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work correctly, even if it's in an SMP kernel. There are, however, three
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work correctly, even if it's in an SMP kernel. There are, however, four
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circumstances in which reordering definitely _could_ be a problem:
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(*) Interprocessor interaction.
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@ -2464,6 +2464,17 @@ out:
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return success;
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}
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/**
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* wake_up_process - Wake up a specific process
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* @p: The process to be woken up.
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*
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* Attempt to wake up the nominated process and move it to the set of runnable
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* processes. Returns 1 if the process was woken up, 0 if it was already
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* running.
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*
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* It may be assumed that this function implies a write memory barrier before
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* changing the task state if and only if any tasks are woken up.
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*/
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int wake_up_process(struct task_struct *p)
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{
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return try_to_wake_up(p, TASK_ALL, 0);
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@ -5425,6 +5436,9 @@ static void __wake_up_common(wait_queue_head_t *q, unsigned int mode,
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* @mode: which threads
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* @nr_exclusive: how many wake-one or wake-many threads to wake up
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* @key: is directly passed to the wakeup function
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*
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* It may be assumed that this function implies a write memory barrier before
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* changing the task state if and only if any tasks are woken up.
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*/
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void __wake_up(wait_queue_head_t *q, unsigned int mode,
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int nr_exclusive, void *key)
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@ -5463,6 +5477,9 @@ void __wake_up_locked_key(wait_queue_head_t *q, unsigned int mode, void *key)
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* with each other. This can prevent needless bouncing between CPUs.
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*
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* On UP it can prevent extra preemption.
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*
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* It may be assumed that this function implies a write memory barrier before
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* changing the task state if and only if any tasks are woken up.
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*/
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void __wake_up_sync_key(wait_queue_head_t *q, unsigned int mode,
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int nr_exclusive, void *key)
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@ -5499,6 +5516,9 @@ EXPORT_SYMBOL_GPL(__wake_up_sync); /* For internal use only */
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* awakened in the same order in which they were queued.
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*
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* See also complete_all(), wait_for_completion() and related routines.
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*
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* It may be assumed that this function implies a write memory barrier before
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* changing the task state if and only if any tasks are woken up.
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*/
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void complete(struct completion *x)
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{
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@ -5516,6 +5536,9 @@ EXPORT_SYMBOL(complete);
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* @x: holds the state of this particular completion
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*
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* This will wake up all threads waiting on this particular completion event.
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*
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* It may be assumed that this function implies a write memory barrier before
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* changing the task state if and only if any tasks are woken up.
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*/
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void complete_all(struct completion *x)
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{
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