Re: NFS TCP race condition with SOCK_ASYNC_NOSPACE

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

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

-- 
Trond Myklebust
Linux NFS client maintainer

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