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

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Message-ID: <Pine.LNX.4.64.0704231202390.29015@blonde.wat.veritas.com>
Date:	Mon, 23 Apr 2007 12:16:31 +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:
> 
> PageLRU flag operation is protected by zone->lru_lock, so
> SetPageLRU/ClearPageLRU
> can be replaced to __SetPageLRU/__ClearPageLRU non-atomic bit operation.

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.

Hugh
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