Re: [PATCH] nfsd: CB_RECALL can race with FREE_STATEID

[Scott Mayhew <smayhew@redhat.com>]

On Thu, 18 Apr 2019, J. Bruce Fields wrote:

> On Thu, Apr 18, 2019 at 04:50:24PM -0400, Scott Mayhew wrote:
> > On Thu, 18 Apr 2019, J. Bruce Fields wrote:
> > 
> > > On Thu, Apr 18, 2019 at 09:24:00AM -0400, Scott Mayhew wrote:
> > > > While trying to track down some issues involving large numbers of
> > > > delegations being recalled/revoked, I caught the server setting
> > > > SEQ4_STATUS_CB_PATH_DOWN while the client was actively responding to
> > > > CB_RECALLs.  It turns out that the client had already done a
> > > > TEST_STATEID and FREE_STATEID for a delegation being recalled by the
> > > > time it received the CB_RECALL.
> > > 
> > > That's interesting, thanks!
> > > 
> > > This exception seems awfully narrow, though.
> > > 
> > > If we get back any NFS-level error at all, then I think the callback
> > > channel is working (am I wrong?)
> > 
> > Correct, if the client replies with either NFS4ERR_DELAY or
> > NFS4ERR_BAD_STATEID, the server will retry 1 time (see dl_retries).
> > After that, we fall thru and nfsd4_cb_recall_done() returns -1 which
> > causes the SEQ4_STATUS_CB_PATH_DOWN flag to be set.
> > 
> > > and telling the client to set up a new
> > > one is probably not going to help.  The best we can do is probably just
> > > give up
> > 
> > That's what the patch is essentially doing.  Or are you saying don't
> > even bother with the checks but still return 1 so we don't set the
> > SEQ4_STATUS_CB_PATH_DOWN flag?
> 
> Right, I don't see any point returning -1 (which ends up setting
> CB_PATH_DOWN) in any case where we get an nfs-level error.  If the
> client got so far as returning an error, then the callback path is
> working.
> 
> I'm not sure exactly what errors *should* result in CB_PATH_DOWN,
> though.  ETIMEDOUT, ENOTCONN, EIO?

I'm not sure either.  Looking at
call_status/call_timeout/rpc_check_timeout, it looks to me like ENOTCONN
will be translated to ETIMEDOUT because nfsd4_run_cb_work sets the 
RPC_TASK_SOFTCONN flag in the call to rpc_call_async.

It looks like call_status can return EHOSTDOWN, ENETDOWN, EHOSTUNREACH,
ENETUNREACH, and EPERM... should those be handled as well?

-Scott

> And maybe we should be checking for
> those in nfsd4_cb_done, and do away with the convention that -1 means
> CB_PATH_DOWN.  I don't think there's a reason individual callback ops
> would need different rules for when to mark the callback channel down.
> 
> --b.
> 
> > 
> > > and let the client deal with the ensuing
> > > RECALLABLE_STATE_REVOKED flag.
> > 
> > The client's already dealing with the RECALLABLE_STATE_REVOKED flag,
> > that's why it sent a TEST_STATEID and FREE_STATEID before it got this
> > particular CB_RECALL.  The idea behind the patch is to not give the
> > state manager on the client additional work by setting CB_PATH_DOWN when
> > the callback channel is clearly working...
> > 
> > -Scott
> > > 
> > > --b.
> > > 
> > > > 
> > > > Signed-off-by: Scott Mayhew <smayhew@redhat.com>
> > > > ---
> > > >  fs/nfsd/nfs4state.c | 8 ++++++++
> > > >  1 file changed, 8 insertions(+)
> > > > 
> > > > diff --git a/fs/nfsd/nfs4state.c b/fs/nfsd/nfs4state.c
> > > > index 6a45fb00c5fc..e88e429133a8 100644
> > > > --- a/fs/nfsd/nfs4state.c
> > > > +++ b/fs/nfsd/nfs4state.c
> > > > @@ -3958,6 +3958,14 @@ static int nfsd4_cb_recall_done(struct nfsd4_callback *cb,
> > > >  			rpc_delay(task, 2 * HZ);
> > > >  			return 0;
> > > >  		}
> > > > +		/*
> > > > +		 * Race: client may have done a FREE_STATEID before
> > > > +		 * receiving the CB_RECALL.
> > > > +		 */
> > > > +		if (dp->dl_stid.sc_type == NFS4_REVOKED_DELEG_STID &&
> > > > +				refcount_read(&dp->dl_stid.sc_count) == 1 &&
> > > > +				list_empty(&dp->dl_recall_lru))
> > > > +			return 1;
> > > >  		/*FALLTHRU*/
> > > >  	default:
> > > >  		return -1;
> > > > -- 
> > > > 2.17.2