linux-kernel - Re: More parallel atomic_open/d_splice_alias fun with NFS and possibly more FSes.

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Message-ID: <20160705135149.GM14480@ZenIV.linux.org.uk>
Date:	Tue, 5 Jul 2016 14:51:49 +0100
From:	Al Viro <viro@...IV.linux.org.uk>
To:	Oleg Drokin <green@...uxhacker.ru>
Cc:	Mailing List <linux-kernel@...r.kernel.org>,
	"<linux-fsdevel@...r.kernel.org>" <linux-fsdevel@...r.kernel.org>
Subject: Re: More parallel atomic_open/d_splice_alias fun with NFS and
 possibly more FSes.

On Tue, Jul 05, 2016 at 01:31:10PM +0100, Al Viro wrote:
> On Tue, Jul 05, 2016 at 02:22:48AM -0400, Oleg Drokin wrote:
> 
> > > +	if (!(open_flags & O_CREAT) && !d_unhashed(dentry)) {
> 
> s/d_unhashed/d_in_lookup/ in that.
> 
> > So we come racing here from multiple threads (say 3 or more - we have seen this
> > in the older crash reports, so totally possible)
> > 
> > > +		d_drop(dentry);
> > 
> > One lucky one does this first before the others perform the !d_unhashed check above.
> > This makes the other ones to not enter here.
> > 
> > And we are back to the original problem of multiple threads trying to instantiate
> > same dentry as before.
> 
> Yep.  See above - it should've been using d_in_lookup() in the first place,
> through the entire nfs_atomic_open().  Same in the Lustre part of fixes,
> obviously.

See current #for-linus for hopefully fixed variants (both lustre and nfs)