Re: [PATCH - alt-2] NFSv4: Don't try to recover NFSv4 locks when they are lost.

[NeilBrown <neilb@suse.de>]

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On Wed, 4 Sep 2013 14:31:07 +0000 "Myklebust, Trond"
<Trond.Myklebust@netapp.com> wrote:

> On Wed, 2013-09-04 at 17:04 +1000, NeilBrown wrote:
> > 
> > When an NFSv4 client loses contact with the server it can lose any
> > locks that it holds.
> > 
> > Currently when it reconnects to the server it simply tries to reclaim
> > those locks.  This might succeed even though some other client has
> > held and released a lock in the mean time.  So the first client might
> > think the file is unchanged, but it isn't.  This isn't good.
> > 
> > If, when recovery happens, the locks cannot be claimed because some
> > other client still holds the lock, then we get a message in the kernel
> > logs, but the client can still write.  So two clients can both think
> > they have a lock and can both write at the same time.  This is equally
> > not good.
> > 
> > There was a patch a while ago
> >   http://comments.gmane.org/gmane.linux.nfs/41917
> > 
> > which tried to address some of this, but it didn't seem to go
> > anywhere.  That patch would also send a signal to the process.  That
> > might be useful but for now this patch just causes writes to fail.
> > 
> > For NFSv4 (unlike v2/v3) there is a strong link between the lock and
> > the write request so we can fairly easily fail any IO of the lock is
> > gone.  While some applications might not expect this, it is still
> > safer than allowing the write to succeed.
> > 
> > Because this is a fairly big change in behaviour a module parameter,
> > "recover_locks", is introduced which defaults to true (the current
> > behaviour) but can be set to "false" to tell the client not to try to
> > recover things that were lost.
> > 
> > Signed-off-by: NeilBrown <neilb@suse.de>
> 
> Thanks!
> 
> > --
> > This alternative uses a module parameter which defaults to the current,
> > incorrect, behaviour.
> > I suspect we don't want that one..
> 
> Agreed. We also need to document the module parameter, so I'm adding a
> little blurb in Documentation/kernel-parameters.txt (see attachment).
> 
> I'd also like to change the parameter name to "recover_lost_locks" to
> make it a little more obvious.
> 
> Finally, I'd like to move the parameter to fs/nfs/super.c so that we can
> use the same modprobe.conf 'options' lines for back ports of this patch
> (yes I strongly suspect we will want to back port this patch to distro
> kernels).
> 
> See attachment.

Looks good - thanks.

NeilBrown

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