linux-kernel - Re: [PATCH 3/3] 9p: Add mempools for RPCs

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Date:   Sun, 10 Jul 2022 05:50:30 +0900
From:   Dominique Martinet <asmadeus@...ewreck.org>
To:     Christian Schoenebeck <linux_oss@...debyte.com>
Cc:     Kent Overstreet <kent.overstreet@...il.com>,
        linux-kernel@...r.kernel.org, v9fs-developer@...ts.sourceforge.net,
        Eric Van Hensbergen <ericvh@...il.com>,
        Latchesar Ionkov <lucho@...kov.net>
Subject: Re: [PATCH 3/3] 9p: Add mempools for RPCs

Christian Schoenebeck wrote on Sat, Jul 09, 2022 at 08:08:41PM +0200:
> Mmm, I "think" that wouldn't be something new. There is no guarantee that 
> client would not get a late response delivery by server of a request that 
> client has already thrown away.

No. Well, it shouldn't -- responding to tflush should guarantee that the
associated request is thrown away by the server

https://9fans.github.io/plan9port/man/man9/flush.html

Order is not explicit, but I read this:
> If it recognizes oldtag as the tag of a pending transaction, it should
> abort any pending response and discard that tag.

late replies to the oldtag are no longer allowed once rflush has been
sent.

But I guess that also depends on the transport being sequential -- that
is the case for TCP but is it true for virtio as well? e.g. if a server
replies something and immediately replies rflush are we guaranteed
rflush is received second by the client?

There's also this bit:
> When the client sends a Tflush, it must wait to receive the
> corresponding Rflush before reusing oldtag for subsequent messages

if we free the request at this point we'd reuse the tag immediately,
which definitely lead to troubles.

> What happens on server side is: requests come in sequentially, and are started 
> to be processed exactly in that order. But then they are actually running in 
> parallel on worker threads, dispatched back and forth between threads several 
> times. And Tflush itself is really just another request. So there is no 
> guarantee that the response order corresponds to the order of requests 
> originally sent by client, and if client sent a Tflush, it might still get a 
> response to its causal, abolished "normal" request.

yes and processing flush ought to get a lock or something and look for
oldtag.
Looking at qemu code it does it right: processing flush find the old
request and marks it as cancelled, then it waits for the request to
finish (and possibly get discarded) during which (pdu_complete) it'll
wake the flush up; so spurrious replies of a tag after flush should not
be possible.

--
Dominique