Re: nfs: infinite loop in fcntl(F_SETLKW)

[Trond Myklebust <Trond.Myklebust@netapp.com>]

On Thu, 2008-04-10 at 17:02 -0400, Trond Myklebust wrote:
> On Thu, 2008-04-10 at 21:51 +0200, Miklos Szeredi wrote:
> > Another infinite loop, this one involving both client and server.
> > 
> > Basically what happens is that on the server nlm_fopen() calls
> > nfsd_open() which returns -EACCES, to which nlm_fopen() returns
> > NLM_LCK_DENIED.
> > 
> > On the client this will turn into a -EAGAIN (nlm_stat_to_errno()),
> > which in will cause fcntl_setlk() to retry forever.
> > 
> > I _think_ the solution is to turn NLM_LCK_DENIED into ENOLCK for
> > blocking locks, as NLM_LCK_BLOCKED is for the contended case.  For
> > testing the lock leave NLM_LCK_DENIED as EAGAIN.  That still could be
> > misleading, but at least there's no infinite loop in that case.
> > 
> > I've minimally tested this patch to verify that it cures the lockup,
> > and that simple blocking locks keep working.
> > 
> > Signed-off-by: Miklos Szeredi <mszeredi@suse.cz>
> > ---
> >  fs/lockd/clntproc.c |    3 +++
> >  1 file changed, 3 insertions(+)
> > 
> > Index: linux/fs/lockd/clntproc.c
> > ===================================================================
> > --- linux.orig/fs/lockd/clntproc.c	2008-04-02 13:34:57.000000000 +0200
> > +++ linux/fs/lockd/clntproc.c	2008-04-10 21:23:46.000000000 +0200
> > @@ -536,6 +536,9 @@ again:
> >  		up_read(&host->h_rwsem);
> >  	}
> >  	status = nlm_stat_to_errno(resp->status);
> > +	/* Don't return EAGAIN, as that would make fcntl_setlk() loop */
> > +	if (status == -EAGAIN)
> > +		status = -ENOLCK;
> >  out_unblock:
> >  	nlmclnt_finish_block(block);
> >  	/* Cancel the blocked request if it is still pending */
> 
> 
> Wait. There is something really weird going on here.
> 
> According to the spec, LCK_DENIED means 'the request failed' (i.e.
> ENOLCK is definitely correct)
> 
> OTOH, LCK_DENIED_NOLOCKS and LCK_DENIED_GRACE_PERIOD are both temporary
> failures, the first because the server had a resource problem, and the
> second because the server rebooted and is in the grace period (i.e.
> EAGAIN would appear to be more appropriate). See
> 
> http://www.opengroup.org/onlinepubs/9629799/chap10.htm#tagcjh_11_02_02_02
> 
> AFAICS, the correct thing to do is to fix nlm_stat_to_errno() by
> swapping the return values for NLM_LCK_DENIED and
> NLM_LCK_DENIED_NOLOCKS/NLM_LCK_DENIED_GRACE_PERIOD.
> 
> The problem is that there appears to be a similar confusion on the Linux
> server side in nlmsvc_lock(). :-(

Duh... Sorry, EAGAIN is indeed the correct return value for fcntl() when
the lock attempt failed. I should have reread the manpage/posix spec
before replying.

-- 
Trond Myklebust
Linux NFS client maintainer

NetApp
Trond.Myklebust@netapp.com
www.netapp.com