linux-kernel - Re: [syzbot] WARNING in trc_read_check

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Date:   Sat, 4 Dec 2021 10:50:47 +0100
From:   Dmitry Vyukov <dvyukov@...gle.com>
To:     paulmck@...nel.org
Cc:     syzbot <syzbot+fe9d8c955bd1d0f02dc1@...kaller.appspotmail.com>,
        bigeasy@...utronix.de, jgross@...e.com, jiangshanlai@...il.com,
        joel@...lfernandes.org, josh@...htriplett.org,
        linux-kernel@...r.kernel.org, mathieu.desnoyers@...icios.com,
        mingo@...nel.org, namit@...are.com, netdev@...r.kernel.org,
        peterz@...radead.org, rcu@...r.kernel.org, rdunlap@...radead.org,
        rostedt@...dmis.org, syzkaller-bugs@...glegroups.com
Subject: Re: [syzbot] WARNING in trc_read_check_handler

On Wed, 1 Dec 2021 at 22:09, Paul E. McKenney <paulmck@...nel.org> wrote:
>
> On Wed, Dec 01, 2021 at 12:50:07PM -0800, syzbot wrote:
> > syzbot suspects this issue was fixed by commit:
> >
> > commit 96017bf9039763a2e02dcc6adaa18592cd73a39d
> > Author: Paul E. McKenney <paulmck@...nel.org>
> > Date:   Wed Jul 28 17:53:41 2021 +0000
> >
> >     rcu-tasks: Simplify trc_read_check_handler() atomic operations
> >
> > bisection log:  https://syzkaller.appspot.com/x/bisect.txt?x=1281d89db00000
> > start commit:   5319255b8df9 selftests/bpf: Skip verifier tests that fail ..
> > git tree:       bpf-next
> > kernel config:  https://syzkaller.appspot.com/x/.config?x=9290a409049988d4
> > dashboard link: https://syzkaller.appspot.com/bug?extid=fe9d8c955bd1d0f02dc1
> > syz repro:      https://syzkaller.appspot.com/x/repro.syz?x=14990477300000
> > C reproducer:   https://syzkaller.appspot.com/x/repro.c?x=105ebd84b00000
> >
> > If the result looks correct, please mark the issue as fixed by replying with:
>
> #syz fix: rcu-tasks: Simplify trc_read_check_handler() atomic operations
>
> > For information about bisection process see: https://goo.gl/tpsmEJ#bisection
>
> Give or take.  There were quite a few related bugs, so some or all of
> the following commits might also have helped:
>
> cbe0d8d91415c rcu-tasks: Wait for trc_read_check_handler() IPIs
> 18f08e758f34e rcu-tasks: Add trc_inspect_reader() checks for exiting critical section
> 46aa886c483f5 rcu-tasks: Fix IPI failure handling in trc_wait_for_one_reader

Thanks for checking. If we don't have one exact fix, let's go with
what syzbot suggested. At this point it does not matter much since all
of them are in most trees I assume. We just need to close the bug with
something.

#syz fix: rcu-tasks: Simplify trc_read_check_handler() atomic operations

> Quibbles aside, it is nice to get an automated email about having fixed
> a bug as opposed to having added one.  ;-)

Yes, but one is not possible without the other :-)