linux-kernel - Re: Regression in next with ext4 oops

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Message-ID: <20161004145915.GZ19539@ZenIV.linux.org.uk>
Date:   Tue, 4 Oct 2016 15:59:15 +0100
From:   Al Viro <viro@...IV.linux.org.uk>
To:     Theodore Ts'o <tytso@....edu>, Jan Kara <jack@...e.cz>,
        Tony Lindgren <tony@...mide.com>,
        Eric Biggers <ebiggers@...gle.com>,
        Andreas Dilger <adilger@...ger.ca>,
        Jaegeuk Kim <jaegeuk@...nel.org>, linux-ext4@...r.kernel.org,
        linux-kernel@...r.kernel.org, linux-omap@...r.kernel.org
Subject: Re: Regression in next with ext4 oops

On Tue, Oct 04, 2016 at 10:02:31AM -0400, Theodore Ts'o wrote:
> On Tue, Oct 04, 2016 at 11:00:41AM +0200, Jan Kara wrote:
> > Never seen this but I suspect it is a fallout from Al's directory locking
> > changes. In particular ext4_htree_fill_tree() builds rb-tree of found
> > directory entries in file->private_data (and generally modifies the
> > structure stored there) but after Al's changes we don't have exclusive
> > access to struct file if I'm right so if two processes end up calling
> > getdents() for the same 'struct file' we are doomed.
> 
> I haven't seen it either, and I've been doing a lot of testing on the
> ext4 test branch.  So I'm guessing Tony has the only reliable repro
> for the problem at the moment.  That being said, it shouldn't be that
> hard to create a test case for this and add it to xfstests.
> 
> I'm pretty sure Jan is right about this, though, but it would be great
> to a get a quick confirmation from Tony if at all possible.

Jan is wrong - we do have per-struct-file serialization for getdents()
et.al.  It might be a race between getdents() on *different* struct
file for the same directory, but ->private_data is not a problem.