linux-ext4 - Re: [PATCH] ext4: fix race between write and fcntl(F

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Message-Id: <948E780F-14EB-4E38-9BAC-A5410F339441@dilger.ca>
Date:	Thu, 9 Oct 2014 14:50:34 -0600
From:	Andreas Dilger <adilger@...ger.ca>
To:	Dmitry Monakhov <dmonakhov@...nvz.org>
Cc:	linux-ext4@...r.kernel.org, sasha.levin@...cle.com
Subject: Re: [PATCH] ext4: fix race between write and fcntl(F_SETFL)

On Oct 9, 2014, at 5:14 AM, Dmitry Monakhov <dmonakhov@...nvz.org> wrote:
> O_DIRECT flags can be toggeled via fcntl(F_SETFL).
> But this value checked twice inside ext4_file_write_iter() and __generic_file_write()
> which result in BUG_ON (see typical stack trace below)
> In order to fix this we have to use our own copy of __generic_file_write
> and pass o_direct status explicitly.

This seems like a generic problem that would be better served by fixing
the core code instead of making a private copy of such a large function
for ext4.  I expect other filesystems might have similar issues if the
O_DIRECT state is changed in the middle of IO?

One option is to flush pending IO on the file if the O_DIRECT flag is
changed in setfl(). This is a bit heavyweight but I can't imagine any
sane app that is changing the O_DIRECT state on a file repeatedly.

Cheers, Andreas

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