Re: NFS TCP race condition with SOCK_ASYNC_NOSPACE

On Tue, 2011-11-22 at 12:02 +0000, Andrew Cooper wrote: 

On 22/11/11 11:38, Trond Myklebust wrote:

quoted

On Mon, 2011-11-21 at 18:14 +0000, Andrew Cooper wrote: 

quoted

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