linux-kernel - Re: [RESEND] [PATCH] lseek: remove i

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Message-Id: <6.0.0.20.2.20090206091511.0764d0a8@172.19.0.2>
Date:	Fri, 06 Feb 2009 09:20:30 +0900
From:	Hisashi Hifumi <hifumi.hisashi@....ntt.co.jp>
To:	Andrew Morton <akpm@...ux-foundation.org>
Cc:	linux-kernel@...r.kernel.org, linux-fsdevel@...r.kernel.org
Subject: Re: [RESEND] [PATCH] lseek: remove i_mutex


At 05:05 09/02/06, Andrew Morton wrote:
>On Tue, 03 Feb 2009 17:04:40 +0900
>Hisashi Hifumi <hifumi.hisashi@....ntt.co.jp> wrote:
>
>> I removed i_mutex from generic_file_llseek.
>> I think that the reason of protecting lseek with i_mutex is just
>> touching i_size atomically.
>> 
>> So I introduce i_size_read here so i_mutex is no longer needed.
>> 
>> Following patch removes i_mutex from generic_file_llseek, and deletes 
>> generic_file_llseek_nolock totally.
>> 
>> Currently there is i_mutex contention not only around lseek, but also 
>fsync or write.
>> So,  I think we can mitigate i_mutex contention between fsync lseek and 
>write by
>> removing i_mutex.
>
>Prior to this change, generic_file_llseek() modified file->f_pos
>atomically with respect to other i_mutex holders.
>
>After this change, it doesn't.

Hi Andrew.

Even before this change is applied, file->f_pos access is not atomic.
sys_read change f_pos value through file_pos_write without i_mutex.
I think seqlock is needed to make f_pos access atomic.

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