linux-kernel - Re: [V9fs-developer] KCSAN BUG report on p9_client_cb / p9_client

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Date:   Wed, 30 Jun 2021 21:34:30 +0200
From:   Marco Elver <elver@...gle.com>
To:     Dominique Martinet <asmadeus@...ewreck.org>
Cc:     jim.cromie@...il.com, kasan-dev@...glegroups.com,
        v9fs-developer@...ts.sourceforge.net,
        LKML <linux-kernel@...r.kernel.org>
Subject: Re: [V9fs-developer] KCSAN BUG report on p9_client_cb / p9_client_rpc

On Tue, 22 Jun 2021 at 23:03, Dominique Martinet <asmadeus@...ewreck.org> wrote:

> jim.cromie@...il.com wrote on Tue, Jun 22, 2021 at 02:55:19PM -0600:
> > heres a fuller report - Im seeing some new stuff here.

There are lots of known data races. A non-exhaustive list can be seen
here: https://syzkaller.appspot.com/upstream?manager=ci2-upstream-kcsan-gce

> Thanks, the one two should be the same as p9_client_cb / p9_client_rpc
> and p9_client_cb / p9_virtio_zc_request are very similar, and also the
> same to the first you had, so the patch didn't really work.
>
> I thought after sending it that it probably needs to be tag =
> READ_ONCE(req->tc.tag) instead of just assigning it... Would you mind
> trying that?
>
> > Im running in a vm, using virtme, which uses 9p to share host filesystems
> > since 1st report to you, Ive added --smp 2 to my testing, it seems to
> > have increased reporting
>
> I'm ashamed to say I've just never tried KCSAN... I can give it a try over
> the next few weeks* if that patch + READ_ONCE doesn't cut it

In case it helps, we have this LWN article series:
https://lwn.net/Articles/816850/

Paul McKenney also kindly wrote a summary of some parts of it:
https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/tree/tools/memory-model/Documentation/access-marking.txt

There are some upcoming changes to KCSAN that can help filter some
data races that aren't too interesting today -- see linux-next and set
CONFIG_KCSAN_PERMISSIVE=y (the opposite of that is
CONFIG_KCSAN_STRICT=y, but not recommended at this time unless you're
writing complex concurrent code).

Thanks,
-- Marco