linux-ext4 - Re: [PATCH 5/5] ext4: add locking for O

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Message-ID: <20140415193641.GF4456@thunk.org>
Date:	Tue, 15 Apr 2014 15:36:41 -0400
From:	Theodore Ts'o <tytso@....edu>
To:	Jan Kara <jack@...e.cz>
Cc:	Ext4 Developers List <linux-ext4@...r.kernel.org>,
	viro@...IV.linux.org.uk
Subject: Re: [PATCH 5/5] ext4: add locking for O_APPEND writes

On Tue, Apr 15, 2014 at 07:05:40PM +0200, Jan Kara wrote:
> On Sat 12-04-14 13:02:41, Ted Tso wrote:
> > Al Viro pointed out that we need to make sure we only allow one
> > O_APPEND write to proceed at a time so that the the s_bitmap_maxbytes
> > check can be properly checked.
>   But this introduces lock inversion between aio_mutex and i_mutex, doesn't
> it?

Doh!  Thanks for pointing that out.  Fortunately, we don't need to
care about optimizing the AIO/DIO O_APPEND write case, so probably the
best thing to do is to unconditionally take aio_mutex and call
ext4_unwritten_wait() early, before we grab i_mutex.  So in effect
we'll treat all O_APPEND writes as being unaligned in and in need of
serialization.

I'll send out a revised version for this last patch.

     	      			    	      - Ted
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