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

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Message-ID: <20160705123110.GL14480@ZenIV.linux.org.uk>
Date:	Tue, 5 Jul 2016 13:31:10 +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 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.