Re: nfs: infinite loop in fcntl(F_SETLKW)

[Trond Myklebust]

On Thu, 2008-04-10 at 17:07 -0400, Trond Myklebust wrote:
> 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@xxxxxxx>
> > > ---
> > >  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.

OK. So the correct fix here should really be applied to fcntl_setlk().
There is absolutely no reason why we should be looping at all if the
filesystem has a ->lock() method.

In fact, this looping behaviour was introduced recently in commit
7723ec9777d9832849b76475b1a21a2872a40d20. Marc, Bruce, why was this
done, and how are filesystems now supposed to behave?

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