Commit 082b63ae authored by Linus Torvalds's avatar Linus Torvalds
Browse files

Merge branch 'sched-docs-for-linus' of...

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
parents 99e97b86 50fa610a
......@@ -31,6 +31,7 @@ Contents:
- Locking functions.
- Interrupt disabling functions.
- Sleep and wake-up functions.
- Miscellaneous functions.
(*) Inter-CPU locking barrier effects.
......@@ -1217,6 +1218,132 @@ barriers are required in such a situation, they must be provided from some
other means.
SLEEP AND WAKE-UP FUNCTIONS
---------------------------
Sleeping and waking on an event flagged in global data can be viewed as an
interaction between two pieces of data: the task state of the task waiting for
the event and the global data used to indicate the event. To make sure that
these appear to happen in the right order, the primitives to begin the process
of going to sleep, and the primitives to initiate a wake up imply certain
barriers.
Firstly, the sleeper normally follows something like this sequence of events:
for (;;) {
set_current_state(TASK_UNINTERRUPTIBLE);
if (event_indicated)
break;
schedule();
}
A general memory barrier is interpolated automatically by set_current_state()
after it has altered the task state:
CPU 1
===============================
set_current_state();
set_mb();
STORE current->state
<general barrier>
LOAD event_indicated
set_current_state() may be wrapped by:
prepare_to_wait();
prepare_to_wait_exclusive();
which therefore also imply a general memory barrier after setting the state.
The whole sequence above is available in various canned forms, all of which
interpolate the memory barrier in the right place:
wait_event();
wait_event_interruptible();
wait_event_interruptible_exclusive();
wait_event_interruptible_timeout();
wait_event_killable();
wait_event_timeout();
wait_on_bit();
wait_on_bit_lock();
Secondly, code that performs a wake up normally follows something like this:
event_indicated = 1;
wake_up(&event_wait_queue);
or:
event_indicated = 1;
wake_up_process(event_daemon);
A write memory barrier is implied by wake_up() and co. if and only if they wake
something up. The barrier occurs before the task state is cleared, and so sits
between the STORE to indicate the event and the STORE to set TASK_RUNNING:
CPU 1 CPU 2
=============================== ===============================
set_current_state(); STORE event_indicated
set_mb(); wake_up();
STORE current->state <write barrier>
<general barrier> STORE current->state
LOAD event_indicated
The available waker functions include:
complete();
wake_up();
wake_up_all();
wake_up_bit();
wake_up_interruptible();
wake_up_interruptible_all();
wake_up_interruptible_nr();
wake_up_interruptible_poll();
wake_up_interruptible_sync();
wake_up_interruptible_sync_poll();
wake_up_locked();
wake_up_locked_poll();
wake_up_nr();
wake_up_poll();
wake_up_process();
[!] Note that the memory barriers implied by the sleeper and the waker do _not_
order multiple stores before the wake-up with respect to loads of those stored
values after the sleeper has called set_current_state(). For instance, if the
sleeper does:
set_current_state(TASK_INTERRUPTIBLE);
if (event_indicated)
break;
__set_current_state(TASK_RUNNING);
do_something(my_data);
and the waker does:
my_data = value;
event_indicated = 1;
wake_up(&event_wait_queue);
there's no guarantee that the change to event_indicated will be perceived by
the sleeper as coming after the change to my_data. In such a circumstance, the
code on both sides must interpolate its own memory barriers between the
separate data accesses. Thus the above sleeper ought to do:
set_current_state(TASK_INTERRUPTIBLE);
if (event_indicated) {
smp_rmb();
do_something(my_data);
}
and the waker should do:
my_data = value;
smp_wmb();
event_indicated = 1;
wake_up(&event_wait_queue);
MISCELLANEOUS FUNCTIONS
-----------------------
......@@ -1366,7 +1493,7 @@ WHERE ARE MEMORY BARRIERS NEEDED?
Under normal operation, memory operation reordering is generally not going to
be a problem as a single-threaded linear piece of code will still appear to
work correctly, even if it's in an SMP kernel. There are, however, three
work correctly, even if it's in an SMP kernel. There are, however, four
circumstances in which reordering definitely _could_ be a problem:
(*) Interprocessor interaction.
......
......@@ -2464,6 +2464,17 @@ static int try_to_wake_up(struct task_struct *p, unsigned int state, int sync)
return success;
}
/**
* wake_up_process - Wake up a specific process
* @p: The process to be woken up.
*
* Attempt to wake up the nominated process and move it to the set of runnable
* processes. Returns 1 if the process was woken up, 0 if it was already
* running.
*
* It may be assumed that this function implies a write memory barrier before
* changing the task state if and only if any tasks are woken up.
*/
int wake_up_process(struct task_struct *p)
{
return try_to_wake_up(p, TASK_ALL, 0);
......@@ -5425,6 +5436,9 @@ static void __wake_up_common(wait_queue_head_t *q, unsigned int mode,
* @mode: which threads
* @nr_exclusive: how many wake-one or wake-many threads to wake up
* @key: is directly passed to the wakeup function
*
* It may be assumed that this function implies a write memory barrier before
* changing the task state if and only if any tasks are woken up.
*/
void __wake_up(wait_queue_head_t *q, unsigned int mode,
int nr_exclusive, void *key)
......@@ -5463,6 +5477,9 @@ void __wake_up_locked_key(wait_queue_head_t *q, unsigned int mode, void *key)
* with each other. This can prevent needless bouncing between CPUs.
*
* On UP it can prevent extra preemption.
*
* It may be assumed that this function implies a write memory barrier before
* changing the task state if and only if any tasks are woken up.
*/
void __wake_up_sync_key(wait_queue_head_t *q, unsigned int mode,
int nr_exclusive, void *key)
......@@ -5499,6 +5516,9 @@ EXPORT_SYMBOL_GPL(__wake_up_sync); /* For internal use only */
* awakened in the same order in which they were queued.
*
* See also complete_all(), wait_for_completion() and related routines.
*
* It may be assumed that this function implies a write memory barrier before
* changing the task state if and only if any tasks are woken up.
*/
void complete(struct completion *x)
{
......@@ -5516,6 +5536,9 @@ EXPORT_SYMBOL(complete);
* @x: holds the state of this particular completion
*
* This will wake up all threads waiting on this particular completion event.
*
* It may be assumed that this function implies a write memory barrier before
* changing the task state if and only if any tasks are woken up.
*/
void complete_all(struct completion *x)
{
......
Markdown is supported
0% or .
You are about to add 0 people to the discussion. Proceed with caution.
Finish editing this message first!
Please register or to comment