linux-kernel - Re: [PATCH] mm: PageLRU can be non-atomic bit operation

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Message-ID: <Pine.LNX.4.64.0704231438280.3516@blonde.wat.veritas.com>
Date:	Mon, 23 Apr 2007 14:42:21 +0100 (BST)
From:	Hugh Dickins <hugh@...itas.com>
To:	Hisashi Hifumi <hifumi.hisashi@....ntt.co.jp>
cc:	akpm@...ux-foundation.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] mm: PageLRU can be non-atomic bit operation

On Mon, 23 Apr 2007, Hisashi Hifumi wrote:
> >No.  The PG_lru flag bit is just one bit amongst many others:
> >what of concurrent operations changing other bits in that same
> >unsigned long e.g. trying to lock the page by setting PG_locked?
> >There are some places where such micro-optimizations can be made
> >(typically while first allocating the page); but in general, no.
> 
> In i386 and x86_64, btsl is used to change page flag. In this case, if btsl
> without lock prefix
> set PG_locked and PG_lru flag concurrently, does only one operation
> succeed ?

That's right: on an SMP machine, without the lock prefix, the operation
is no longer atomic: what's stored back may be missing the result of
one or the other of the racing operations.

Hugh
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