Re: [PATCH 19/19] mm: filemap: Avoid unnecessary barries and waitqueue lookups in unlock_page fastpath

[Peter Zijlstra]

On Tue, May 13, 2014 at 01:53:13PM +0100, Mel Gorman wrote:
> On Tue, May 13, 2014 at 10:45:50AM +0100, Mel Gorman wrote:
> >  void unlock_page(struct page *page)
> >  {
> > +	wait_queue_head_t *wqh = clear_page_waiters(page);
> > +
> >  	VM_BUG_ON_PAGE(!PageLocked(page), page);
> > +
> > +	/*
> > +	 * No additional barrier needed due to clear_bit_unlock barriering all updates
> > +	 * before waking waiters
> > +	 */
> >  	clear_bit_unlock(PG_locked, &page->flags);
> > -	smp_mb__after_clear_bit();
> > -	wake_up_page(page, PG_locked);
> 
> This is wrong. The smp_mb__after_clear_bit() is still required to ensure
> that the cleared bit is visible before the wakeup on all architectures.

wakeup implies a mb, and I just noticed that our Documentation is
'obsolete' and only mentions it implies a wmb.

Also, if you're going to use smp_mb__after_atomic() you can use
clear_bit() and not use clear_bit_unlock().



---
Subject: doc: Update wakeup barrier documentation

As per commit e0acd0a68ec7 ("sched: fix the theoretical signal_wake_up()
vs schedule() race") both wakeup and schedule now imply a full barrier.

Furthermore, the barrier is unconditional when calling try_to_wake_up()
and has been for a fair while.

Cc: Oleg Nesterov <oleg@xxxxxxxxxx>
Cc: Linus Torvalds <torvalds@xxxxxxxxxxxxxxxxxxxx>
Cc: David Howells <dhowells@xxxxxxxxxx>
Cc: Paul E. McKenney <paulmck@xxxxxxxxxxxxxxxxxx>
Signed-off-by: Peter Zijlstra <peterz@xxxxxxxxxxxxx>
---
 Documentation/memory-barriers.txt | 6 +++---
 1 file changed, 3 insertions(+), 3 deletions(-)

diff --git a/Documentation/memory-barriers.txt b/Documentation/memory-barriers.txt
index 46412bded104..dae5158c2382 100644
--- a/Documentation/memory-barriers.txt
+++ b/Documentation/memory-barriers.txt
@@ -1881,9 +1881,9 @@ The whole sequence above is available in various canned forms, all of which
 	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:
+A full memory barrier is implied by wake_up() and co. 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
 	===============================	===============================
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