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

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Date:	Wed, 6 Jul 2016 12:24:33 -0400
From:	Oleg Drokin <green@...uxhacker.ru>
To:	Al Viro <viro@...IV.linux.org.uk>
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 Jul 5, 2016, at 9:51 AM, Al Viro wrote:

> 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)

So returning to the NFS - the patches held up for 24 hours under various loads
with no crashes or other such ill effects.
So I think the NFS patches are good to go.

The Lustre patches I only tried my own and those seem to be doing well too.
Do you want me to send you my Lustre patches just for this issue, or send
everything to Greg as usual?

Thanks.