linux-ext4 - Re: [PATCH] ext2/3/4: change i

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Message-ID: <20090115133530.GA30522@mit.edu>
Date:	Thu, 15 Jan 2009 08:35:30 -0500
From:	Theodore Tso <tytso@....edu>
To:	Jamie Lokier <jamie@...reable.org>
Cc:	Dave Kleikamp <shaggy@...ux.vnet.ibm.com>,
	Hisashi Hifumi <hifumi.hisashi@....ntt.co.jp>,
	akpm@...ux-foundation.org, linux-ext4@...r.kernel.org,
	linux-fsdevel@...r.kernel.org
Subject: Re: [PATCH] ext2/3/4: change i_mutex usage on lseek

On Thu, Jan 15, 2009 at 01:01:54PM +0000, Jamie Lokier wrote:
> Dave Kleikamp wrote:
> > Is there any reason you couldn't have just changed generic_file_llseek()
> > to do this rather than making identical changes to the individual file
> > systems.  I would think this optimization would be safe for any file
> > system.
> 
> Is it safe on 32-bit systems where 64-bit updates are not atomic?

Protection of 64-bit updates of i_size where 32-bit systems do not
have atomic 64-bit updates is done via i_size_seqcount, which is done
automatically as long as the code in question uses i_size_write(), an
inline function defined in include/linux/fs.h.

> You may say that doing multiple parallel lseek() calls is undefined
> behaviour, so it's ok to end up with file position that none of the
> individual lseek() calls asked for.
> 
> But if it's undefined behaviour, no programs should be doing parallel
> lseek() calls on the same open file, so why optimise it at all?

i_mutex gets used for a lot more than protecting i_size updates for
lseek(), so removing the need for i_mutex could very well be helpful
for non-micro-benchmark tests.  As Hirashi-san mentioned in his patch,
this could avoid contention between lseek() and write() (possibly via
a different file descriptor, so it wouldn't be an undefined race
condition) and fsync().

I do agree this is an optimization that should be done in
generic_file_llseek(), though.

Regards,

						- Ted
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