linux-kernel - Re: [PATCH] 9p/client: fix data race on req->status

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Date:   Fri, 9 Dec 2022 08:50:46 +0900
From:   Dominique Martinet <asmadeus@...ewreck.org>
To:     Christian Schoenebeck <linux_oss@...debyte.com>
Cc:     Naresh Kamboju <naresh.kamboju@...aro.org>,
        v9fs-developer@...ts.sourceforge.net, linux-kernel@...r.kernel.org,
        Marco Elver <elver@...gle.com>
Subject: Re: [PATCH] 9p/client: fix data race on req->status

Christian Schoenebeck wrote on Thu, Dec 08, 2022 at 04:51:27PM +0100:
> Right, looks like most of it should be fine. Maybe p9_client_zc_rpc() needs a
> barrier as well?

Good point, the request is used without any other lock after the
wait_event on req->status in trans_virtio.c;
I'll send a separate patch for it later today.

> > I think we're just protecting against compiler
> > reordering or if on some arch the store isn't actually atomic.
> 
> And access order within the same thread.

In this case afaik the barrier also does that? There would be no point
if a write barrier allowed a write placed before the barrier to be
reordered after it...

> > This code path actually was broken before I added the barrier a while
> > ago (2b6e72ed747f68a03), as I was observing some rare but very real
> > errors on a big server so I'm fairly confident that for at least x86_64
> > the generated code isn't too bad, but if KCSAN helps catching stuff I
> > won't complain.
> 
> What about p9_tag_alloc()?

I think that one's ok: it happens during the allocation before the
request is enqueued in the idr, so it should be race-free by defition.

tools/memory-model/Documentation/access-marking.txt says
"Initialization-time and cleanup-time accesses" should use plain
C-language accesses, so I stuck to that.

cheers,
-- 
Dominique