Re: [PATCH] SUNRPC: Fix backchannel reply, again

[Trond Myklebust <trondmy@hammerspace.com>]

On Tue, 2024-06-18 at 16:31 -0400, Chuck Lever wrote:
> On Fri, Jun 14, 2024 at 10:18:52AM -0400, cel@kernel.org wrote:
> > From: Chuck Lever <chuck.lever@oracle.com>
> > 
> > I still see "RPC: Could not send backchannel reply error: -110"
> > quite often, along with slow-running tests. Debugging shows that
> > the
> > backchannel is still stumbling when it has to queue a callback
> > reply
> > on a busy transport.
> > 
> > Note that every one of these timeouts causes a connection loss by
> > virtue of the xprt_conditional_disconnect() call in that arm of
> > call_cb_transmit_status().
> > 
> > I found that setting to_maxval is necessary to get the RPC timeout
> > logic to behave whenever to_exponential is not set.
> > 
> > Fixes: 57331a59ac0d ("NFSv4.1: Use the nfs_client's rpc timeouts
> > for backchannel")
> > Signed-off-by: Chuck Lever <chuck.lever@oracle.com>
> > ---
> >  net/sunrpc/svc.c | 1 +
> >  1 file changed, 1 insertion(+)
> > 
> > diff --git a/net/sunrpc/svc.c b/net/sunrpc/svc.c
> > index 965a27806bfd..f4ddb2961042 100644
> > --- a/net/sunrpc/svc.c
> > +++ b/net/sunrpc/svc.c
> > @@ -1643,6 +1643,7 @@ void svc_process_bc(struct rpc_rqst *req,
> > struct svc_rqst *rqstp)
> >  		timeout.to_initval = req->rq_xprt->timeout-
> > >to_initval;
> >  		timeout.to_retries = req->rq_xprt->timeout-
> > >to_retries;
> >  	}
> > +	timeout.to_maxval = timeout.to_initval;
> >  	memcpy(&req->rq_snd_buf, &rqstp->rq_res, sizeof(req-
> > >rq_snd_buf));
> >  	task = rpc_run_bc_task(req, &timeout);
> >  
> > -- 
> > 2.45.1
> > 
> 
> Hi - would love to see this in 6.10-rc. Is there a chance that
> could happen?

Hmm... Can we please also set the remaining fields in timeout to 0?
Otherwise, we're still playing roulette with what actually ends up
happening in xprt_calc_majortimeo(). If to_increment happens to be
large enough, we could overflow and end up with a silly small timeout
value on a retry.


-- 
Trond Myklebust
Linux NFS client maintainer, Hammerspace
trond.myklebust@hammerspace.com