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

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Date:   Thu, 08 Dec 2022 16:51:27 +0100
From:   Christian Schoenebeck <linux_oss@...debyte.com>
To:     Dominique Martinet <asmadeus@...ewreck.org>
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

On Monday, December 5, 2022 11:27:48 PM CET Dominique Martinet wrote:
> Christian Schoenebeck wrote on Mon, Dec 05, 2022 at 04:19:01PM +0100:
> > I must have missed the prior discussion, but looking at the suggested
> 
> Good point, I'll add a link to the report as well...
> It's this thread:
> https://lkml.kernel.org/r/CA+G9fYsK5WUxs6p9NaE4e3p7ew_+s0SdW0+FnBgiLWdYYOvoMg@mail.gmail.com
> 
> > solution: if there is no lock, then adding READ_ONCE() and WRITE_ONCE() would
> > not fix cross-CPU issues, as those would not have a memory barrier in that
> > case.
> > 
> > Shouldn't that therefore rather be at least smp_load_acquire() and
> > smp_store_release() at such places instead?
> 
> The barrier is here -- 

Right, looks like most of it should be fine. Maybe p9_client_zc_rpc() needs a
barrier as well?

> 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.

> 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()?