[pnfs] [PATCH 9/9] pnfs client prevent race in sequence slot

[Benny Halevy]

William A. (Andy) Adamson wrote:
>
>
> On 9/27/07, *Trond Myklebust* <trond.myklebust at fys.uio.no 
> <mailto:trond.myklebust at fys.uio.no>> wrote:
>
>     On Thu, 2007-09-27 at 15:12 -0400, Trond Myklebust wrote:
>     > On Thu, 2007-09-27 at 15:00 -0400, William A. (Andy) Adamson wrote:
>     > >
>     > >
>     > > On 9/26/07, Trond Myklebust < trond.myklebust at fys.uio.no
>     <mailto:trond.myklebust at fys.uio.no>> wrote:
>     > >         If we have to interrupt an RPC call, then we
>     immediately fire
>     > >         off an
>     > >         asynchronous RPC call with a single SEQUENCE call that
>     uses
>     > >         the _same_
>     > >         sa_sequence id as the synchronous RPC call that was
>     cancelled
>     > >         (and drops
>     > >         all the other arguments).
>     > >
>     > > ok. but can't we get around sending yet another rpc by looking
>     at what
>     > > the server has already sent (or not)?  we do have all the
>     information.
>     >
>     > I don't understand. If you have a reply, then you're not
>     interrupting an
>     > RPC call,
>
>     however the fact that you don't have a reply means nothing:
>     > assuming that an RPC call was actually sent by the client then the
>     > server may or may not have received it (we just don't know until
>     we have
>     > a reply).
>
>     Oh... I think I see your point. Are you perhaps suggesting that we
>     just
>     look for the NFS4ERR_SEQ_MISORDERED reply, and use that as to indicate
>     that the server didn't see our previous RPC call on that slot?
>     That might work too.
>
>
> yes, just remember in the slot that the RPC with seq# was interrupted 
> and reset the slot sequence number iff MISORDERED err on seq#+1.
The problem with doing this is that you might be racing with the 
original call in case it
was delayed in the network for some obscure reason and you could get 
MISORDERED or DELAY
on the retry.  In the DELAY case you'll need to poll the server until it 
finishes processing the previous
request, then you should get MISORDERED again and you can try seq#+1 yet 
again.

>
> -->Andy
>
>     Trond
>
>
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>
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