linux-kernel - Re: [PATCH] fuse: hold i_mutex in fuse_file

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Message-ID: <51B8C457.5050502@redhat.com>
Date:	Wed, 12 Jun 2013 11:56:23 -0700
From:	Anand Avati <avati@...hat.com>
To:	Brian Foster <bfoster@...hat.com>
CC:	Maxim Patlasov <MPatlasov@...allels.com>, miklos@...redi.hu,
	fuse-devel@...ts.sourceforge.net, linux-kernel@...r.kernel.org,
	devel@...nvz.org
Subject: Re: [PATCH] fuse: hold i_mutex in fuse_file_fallocate()

On 6/12/13 4:40 AM, Brian Foster wrote:
> On 06/11/2013 06:59 AM, Maxim Patlasov wrote:
>> Changing size of a file on server and local update (fuse_write_update_size)
>> should be always protected by inode->i_mutex. Otherwise a race like this is
>> possible:
>>
>> 1. Process 'A' calls fallocate(2) to extend file (~FALLOC_FL_KEEP_SIZE).
>> fuse_file_fallocate() sends FUSE_FALLOCATE request to the server.
>> 2. Process 'B' performs ordinary buffered write(2) with a length big enough
>> to extend the file beyond "offset + length" of fallocate call.
>> 3. Process 'A' resumes execution of fuse_file_fallocate() and calls
>> fuse_write_update_size(inode, offset + length). But 'offset + length' was
>> obsoleted by write from previous step.
>>
>
> Hi Maxim,
>
> Doesn't fuse_write_update_size() already handle this particular case by
> only ever extending the size?
>


As you say, fuse_write_update_size() does seem to protect against the 
case Maxim writes in the commit log.

However, there is still an issue with with truncate(shrinking_offset) 
and fallocate(growing_offset,~FALLOC_FL_KEEP_SIZE) racing, and changing 
inode size in opposing order between file server and in core ->i_size. 
Therefore, grabbing i_mutex is making fallocate and truncate atomic 
against each other.

I guess we just need an updated commit log, and same code change?

Avati


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