Re: NFS TCP race condition with SOCK_ASYNC_NOSPACE

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

On Tue, 2011-11-22 at 12:16 +0000, Andrew Cooper wrote: 
> On 22/11/11 12:10, Trond Myklebust wrote:
> > On Tue, 2011-11-22 at 12:02 +0000, Andrew Cooper wrote: 
> >> On 22/11/11 11:38, Trond Myklebust wrote:
> >>> On Mon, 2011-11-21 at 18:14 +0000, Andrew Cooper wrote: 
> >>>> Following some debugging, I believe that the attached patch fixes the
> >>>> problem.
> >>>>
> >>>> Simply returning EAGAIN is not sufficient, as the task does not get
> >>>> requeued, and times out 13 seconds later (as per our mount options). 
> >>>> Setting the SOCK_ASYNC_NOSPACE bit causes the requeue to happen.
> >>>>
> >>>> I realize that this is a gross hack and I should probably not be using
> >>>> SOCK_ASYNC_NOSPACE in that way.  Is there a better way to achieve the
> >>>> same solution?
> >>>>
> >>> What you are doing will cause the request to be put to sleep with no
> >>> guarantee that it will ever be woken up. Why would we want to do that if
> >>> there is no report of a tcp window/buffer space congestion?
> >> But the reason we get to this code is because there was a report of
> >> space collision.  What would you suggest instead?  Changing
> >> xs_{tcp,udp}_send_request() to retry in this case would defeat the point
> >> of having xs_nospace().
> > I suggest doing absolutely nothing: do what you originally proposed,
> > which is to report the EAGAIN so that the client state machine retries
> > the socket write.
> >
> > My point is that this is a context which is _not_ atomic with the
> > original report of tcp window/buffer space congestion. There are no
> > locks or anything else that will guarantee that the congestion still
> > exists, and the fact that the SOCK_ASYNC_NOSPACE flag is now clear
> > indicates that this is the case.
> > The whole purpose of xs_nospace() is to wait until a congestion
> > condition clears. If the congestion clears before we get here, then we
> > have no reason to do anything special other than retry.
> >
> > Trond
> 
> I am slightly confused as to what you mean now.
> 
> When you take out the if(test_bit test and always set ret to EAGAIN and
> requeue the request, the next time it wakes up is when it is killed due
> to timeout.  This results in substantially worse effects for the
> userspace, as the NFS session is killed.

What is putting the request to sleep? It should be awake when it enters
xs_nospace(), and nothing in or after that function should be putting it
to sleep until we've retried with call_transmit().

> Did you mean something else when you said "always report EAGAIN"?

Nope.

-- 
Trond Myklebust
Linux NFS client maintainer

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