Messages in this thread Patch in this message | | | From | David Howells <> | Subject | [PATCH] It may not be assumed that wake_up(), finish_wait() and co. imply a memory barrier | Date | Thu, 23 Apr 2009 17:32:24 +0100 |
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Ingo Molnar <mingo@elte.hu> wrote:
> Why would an unlock be needed before a call to wake_up() variants?
Good point. I've amended my patch again (see attached).
> In fact i'd encourage to _not_ document try_to_lock() as a write barrier > either
Did you mean try_to_wake_up()? Or did you mean things like spin_lock_trylock()? If the latter, it *has* to be a LOCK-class barrier if it succeeds - otherwise what's the point?
> - but rather have explicit barriers where they are needed. Then we > could remove that barrier from try_to_wake_up() too ;-)
I was wondering if wake_up() and friends should in fact imply smp_wmb(), but I guess that they're often used in conjunction with spinlocks - and in such a situation a barrier is unnecessary overhead.
David --- From: David Howells <dhowells@redhat.com> Subject: [PATCH] It may not be assumed that wake_up(), finish_wait() and co. imply a memory barrier
Add to the memory barriers document to note that try_to_wake_up(), wake_up(), complete(), finish_wait() and co. should not be assumed to imply any sort of memory barrier.
This is because:
(1) A lot of the time, memory barriers in the wake-up and sleeper paths would be superfluous due to the use of locks.
(2) It is possible to pass right through wake_up() and co. without hitting anything at all or anything other than a spin_lock/spin_unlock (if try_to_wake_up() isn't invoked).
(3) The smp_wmb() should probably move out of try_to_wake_up() and into suitable places in its callers.
Signed-off-by: David Howells <dhowells@redhat.com> ---
Documentation/memory-barriers.txt | 34 +++++++++++++++++++++++++++++++++- kernel/sched.c | 11 +++++++++++ 2 files changed, 44 insertions(+), 1 deletions(-)
diff --git a/Documentation/memory-barriers.txt b/Documentation/memory-barriers.txt index f5b7127..8c32e23 100644 --- a/Documentation/memory-barriers.txt +++ b/Documentation/memory-barriers.txt @@ -42,6 +42,7 @@ Contents: - Interprocessor interaction. - Atomic operations. + - Wake up of processes - Accessing devices. - Interrupts. @@ -1366,13 +1367,15 @@ 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, five circumstances in which reordering definitely _could_ be a problem: (*) Interprocessor interaction. (*) Atomic operations. + (*) Wake up of processes. + (*) Accessing devices. (*) Interrupts. @@ -1568,6 +1571,35 @@ and in such cases the special barrier primitives will be no-ops. See Documentation/atomic_ops.txt for more information. +WAKE UP OF PROCESSES +-------------------- + +If locking is not used, and if the waker sets some state that the sleeper will +need to see, a write memory barrier or a full memory barrier may be needed +before one of the following calls is used to wake up another process: + + complete(); + try_to_wake_up(); + 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(); + +After waking, and assuming it doesn't take a matching lock, the sleeper may +need to interpolate a read or full memory barrier before accessing that state +as finish_wait() does not imply a barrier either, and schedule() only implies a +barrier on entry. + + ACCESSING DEVICES ----------------- diff --git a/kernel/sched.c b/kernel/sched.c index b902e58..2ef0479 100644 --- a/kernel/sched.c +++ b/kernel/sched.c @@ -2337,6 +2337,9 @@ static int sched_balance_self(int cpu, int flag) * runnable without the overhead of this. * * returns failure only if the task is already active. + * + * It should not be assumed that this function implies any sort of memory + * barrier. */ static int try_to_wake_up(struct task_struct *p, unsigned int state, int sync) { @@ -5241,6 +5244,8 @@ 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 not be assumed that this function implies any sort of memory barrier. */ void __wake_up(wait_queue_head_t *q, unsigned int mode, int nr_exclusive, void *key) @@ -5279,6 +5284,8 @@ 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 not be assumed that this function implies any sort of memory barrier. */ void __wake_up_sync_key(wait_queue_head_t *q, unsigned int mode, int nr_exclusive, void *key) @@ -5315,6 +5322,8 @@ 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 not be assumed that this function implies any sort of memory barrier. */ void complete(struct completion *x) { @@ -5332,6 +5341,8 @@ 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 not be assumed that this function implies any sort of memory barrier. */ void complete_all(struct completion *x) {
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