linux-kernel - Re: [PATCH 19/19] mm: filemap: Avoid unnecessary barries and waitqueue lookups in unlock

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Date:	Tue, 13 May 2014 20:57:42 +0200
From:	Oleg Nesterov <oleg@...hat.com>
To:	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
Cc:	Peter Zijlstra <peterz@...radead.org>,
	Mel Gorman <mgorman@...e.de>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Johannes Weiner <hannes@...xchg.org>,
	Vlastimil Babka <vbabka@...e.cz>, Jan Kara <jack@...e.cz>,
	Michal Hocko <mhocko@...e.cz>, Hugh Dickins <hughd@...gle.com>,
	Dave Hansen <dave.hansen@...el.com>,
	Linux Kernel <linux-kernel@...r.kernel.org>,
	Linux-MM <linux-mm@...ck.org>,
	Linux-FSDevel <linux-fsdevel@...r.kernel.org>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	David Howells <dhowells@...hat.com>
Subject: Re: [PATCH 19/19] mm: filemap: Avoid unnecessary barries and
	waitqueue lookups in unlock_page fastpath

On 05/13, Paul E. McKenney wrote:
>
> On Tue, May 13, 2014 at 05:44:35PM +0200, Peter Zijlstra wrote:
> >
> > Ah, yes, so I'll defer to Oleg and Linus to explain that one. As per the
> > name: smp_mb__before_spinlock() should of course imply a full barrier.
>
> How about if I queue a name change to smp_wmb__before_spinlock()?

I agree, this is more accurate, simply because it describes what it
actually does.

But just in case, as for try_to_wake_up() it does not actually need
wmb() between "CONDITION = T" and "task->state = RUNNING". It would
be fine if these 2 STORE's are re-ordered, we can rely on rq->lock.

What it actually needs is a barrier between "CONDITION = T" and
"task->state & state" check. But since we do not have a store-load
barrier, wmb() was added to ensure that "CONDITION = T" can't leak
into the critical section.

But it seems that set_tlb_flush_pending() already assumes that it
acts as wmb(), so probably smp_wmb__before_spinlock() is fine.

Oleg.

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