linux-kernel - Re: Write is not atomic?

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Message-ID: <20121015231302.GF2739@dastard>
Date:	Tue, 16 Oct 2012 10:13:02 +1100
From:	Dave Chinner <david@...morbit.com>
To:	Juliusz Chroboczek <jch@....jussieu.fr>
Cc:	linux-kernel@...r.kernel.org
Subject: Re: Write is not atomic?

On Mon, Oct 15, 2012 at 11:36:15PM +0200, Juliusz Chroboczek wrote:
> Hi,
> 
> The Linux manual page for write(2) says:
> 
>     The adjustment of the file offset and the write operation are
>     performed as an atomic step.

That's wrong. The file offset update is not synchronised at all with
the write, and for a shared fd the update will race.


> This is apparently an extension to POSIX, which says
> 
>     This volume of IEEE Std 1003.1-2001 does not specify behavior of
>     concurrent writes to a file from multiple processes. Applications
>     should use some form of concurrency control.

This is how Linux behaves.

> The following fragment of code
> 
>     int fd;
>     fd = open("exemple", O_CREAT | O_WRONLY | O_TRUNC, 0666);
>     fork();
>     write(fd, "Ouille", 6);
>     close(fd);
> 
> produces "OuilleOuille", as expected, on ext4 on two machines running
> Linux 3.2 AMD64.  However, over XFS on an old Pentium III at 500 MHz
> running 2.6.32, it produces just "Ouille" roughly once in three times.

ext4, on 3.6:

$ for i in `seq 0 10000`; do ./a.out ; cat /mnt/scratch/foo ; echo ; done | sort |uniq -c
     39 Ouille
   9962 OuilleOuille
$

XFS, on the same kernel, hardware and block device:

$ for i in `seq 0 10000`; do ./a.out ; cat /mnt/scratch/foo ; echo ; done | sort |uniq -c
     40 Ouille
   9961 OuilleOuille
$

So both filesystems behave according to the POSIX definition of
concurrent writes....

Cheers,

Dave.
-- 
Dave Chinner
david@...morbit.com
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