Re: nfs: infinite loop in fcntl(F_SETLKW)

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

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(). :-(

-- 
Trond Myklebust
Linux NFS client maintainer

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