linux-kernel - Re: [PATCH, RFC] xip: use i_mutex for xip_file

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Date:	Sun, 11 Sep 2011 17:57:36 +0200
From:	Marco Stornelli <marco.stornelli@...il.com>
To:	Al Viro <viro@...IV.linux.org.uk>
CC:	Linux FS Devel <linux-fsdevel@...r.kernel.org>,
	Linux Kernel <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH, RFC] xip: use i_mutex for xip_file_fault



Il 11/09/2011 13:25, Al Viro ha scritto:
> On Sun, Sep 11, 2011 at 12:15:04PM +0100, Al Viro wrote:
>
>> write() grabs ->i_mutex on the file it's going to write to.  It uses
>> copy_from_user() while holding ->i_mutex; that can end up calling ->fault().
>> If your data comes from the same file mmapped in your address space, you
>> have xip_write_fault() called while you are in xip_file_write() and *already*
>> are holding ->i_mutex on the same inode.  With your patch it will, AFAICS,
>> cheerfully deadlock.
>
> Oh, wait...  You are only doing that to write side of pagefault?  That's
> better, but not much:
>
> thread 1: mmap the file, modify mapping
> thread 2: write() to file
>
> The former will do xip_write_fault() while holding ->mmap_sem.
> The latter will do copy_from_user() from xip_file_write(), getting
> pagefaults while holding ->i_mutex.
>
> Note that we are grabbing ->mmap_sem and ->i_mutex in opposite orders.
> I.e. that will deadlock on you - all you need is threads sharing the
> address space.
>

Ok, thank you very much for the on-line debug :) So i_mutex is not a 
good lock to use in this situation. It was a common sync point, but it 
has some collateral effect on the write path that we must avoid. At this 
point, what can be a good strategy? Any opinion is welcome.

Marco
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