Messages in this thread | | | Date | Thu, 23 Apr 2009 18:18:32 +0200 | From | Oleg Nesterov <> | Subject | Re: [PATCH] slow_work_thread() should do the exclusive wait |
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(add Ingo)
On 04/23, David Howells wrote: > > Andrew Morton <akpm@linux-foundation.org> wrote: > > > I wonder if slow_work_cull_timeout() should have some sort of barrier, > > so the write is suitably visible to the woken thread. Bearing in mind > > that the thread might _already_ have been woken by someone else? > > Perhaps the attached patch? > > David > --- > From: David Howells <dhowells@redhat.com> > Subject: [PATCH] slow_work_cull_timeout() should have a memory barrier > > slow_work_cull_timeout() should have a write memory barrier so that the setting > of the cull flag is seen before the wakeup takes place. This is required > because wake_up() does not guarantee any memory barriership at all. > > Concomitant to this, slow_work_thread() should have a read memory barrier > between its return from schedule() and its testing of slow_work_cull() as > finish_wait() isn't a guaranteed barrier either. > > Signed-off-by: David Howells <dhowells@redhat.com> > --- > > kernel/slow-work.c | 2 ++ > 1 files changed, 2 insertions(+), 0 deletions(-) > > > diff --git a/kernel/slow-work.c b/kernel/slow-work.c > index 521ed20..96e418d 100644 > --- a/kernel/slow-work.c > +++ b/kernel/slow-work.c > @@ -382,6 +382,7 @@ static int slow_work_thread(void *_data) > finish_wait(&slow_work_thread_wq, &wait); > > try_to_freeze(); > + smp_rmb(); > > vsmax = vslow_work_proportion; > vsmax *= atomic_read(&slow_work_thread_count); > @@ -416,6 +417,7 @@ static int slow_work_thread(void *_data) > static void slow_work_cull_timeout(unsigned long data) > { > slow_work_cull = true; > + smp_wmb(); > wake_up(&slow_work_thread_wq); > }
Confused. If we need this barrier, a lot of similar code is broken.
slow_work_cull_timeout:
slow_work_cull = true; wake_up(&slow_work_thread_wq);
slow_work_thread:
prepare_to_wait(&slow_work_thread_wq); if (!slow_work_cull) schedule(); finish_wait(&slow_work_thread_wq);
if (slow_work_cull) .....
Both wake_up() and prepare_to_wait() take the same wait_queue_head_t->lock, and prepare_to_wait() does set_current_state() under this lock.
How can we miss the event? If wake_up() happens before prepare_to_wait(), slow_work_thread() must see slow_work_cull = T, otherwise the subsequent wake_up() must see the result of list_add() + set_current_state() and wake up the sleeping thread.
Could you please clarify?
Oleg.
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