To: Wu Fengguang <fengguang.wu@intel.com>
Cc: Andrew Morton <akpm@linux-foundation.org>,
       Quentin Barnes <qbarnes+nfs@yahoo-inc.com>,
       "linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>,
       linux-fsdevel@vger.kernel.org, Nick Piggin <npiggin@suse.de>,
       Steven Whitehouse <swhiteho@redhat.com>
Subject: Re: [RFC][PATCH 2/2] readahead: avoid page-by-page reads on POSIX_FADV_RANDOM
From: Andi Kleen <andi@firstfloor.org>
References: <20091225000717.GA26949@yahoo-inc.com>
	<87aax18xms.fsf@basil.nowhere.org> <20091230051540.GA16308@localhost>
	<20091230052402.GB26364@localhost>
Date: Wed, 30 Dec 2009 19:02:38 +0100
In-Reply-To: <20091230052402.GB26364@localhost> (Wu Fengguang's message of "Wed, 30 Dec 2009 13:24:02 +0800")
Message-ID: <873a2s8hmp.fsf@basil.nowhere.org>
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Wu Fengguang <fengguang.wu@intel.com> writes:
>   * the ra fields can be accessed concurrently in a racy way.
> --- linux.orig/mm/fadvise.c	2009-12-30 13:02:03.000000000 +0800
> +++ linux/mm/fadvise.c	2009-12-30 13:23:05.000000000 +0800
> @@ -77,12 +77,14 @@ SYSCALL_DEFINE(fadvise64_64)(int fd, lof
>  	switch (advice) {
>  	case POSIX_FADV_NORMAL:
>  		file->f_ra.ra_pages = bdi->ra_pages;
> +		file->f_ra.flags &= ~RA_FLAG_RANDOM;
>  		break;
>  	case POSIX_FADV_RANDOM:
> -		file->f_ra.ra_pages = 0;
> +		file->f_ra.flags |= RA_FLAG_RANDOM;

What prevents this from racing with a parallel readahead
state modification, losing the bits?

-Andi

-- 
ak@linux.intel.com -- Speaking for myself only.
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