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Date:   Mon, 22 Apr 2019 09:48:08 -0700
From:   Linus Torvalds <torvalds@...ux-foundation.org>
To:     Jens Axboe <axboe@...nel.dk>
Cc:     syzbot <syzbot+10d25e23199614b7721f@...kaller.appspotmail.com>,
        Arnd Bergmann <arnd@...db.de>, Borislav Petkov <bp@...en8.de>,
        "Darrick J. Wong" <darrick.wong@...cle.com>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Peter Anvin <hpa@...or.com>,
        Linux API <linux-api@...r.kernel.org>,
        linux-arch <linux-arch@...r.kernel.org>,
        linux-block <linux-block@...r.kernel.org>,
        linux-fsdevel <linux-fsdevel@...r.kernel.org>,
        Linux List Kernel Mailing <linux-kernel@...r.kernel.org>,
        Andrew Lutomirski <luto@...nel.org>,
        Mathieu Desnoyers <mathieu.desnoyers@...icios.com>,
        Ingo Molnar <mingo@...hat.com>,
        Michael Ellerman <mpe@...erman.id.au>,
        syzkaller-bugs <syzkaller-bugs@...glegroups.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Al Viro <viro@...iv.linux.org.uk>,
        "the arch/x86 maintainers" <x86@...nel.org>
Subject: Re: WARNING in percpu_ref_kill_and_confirm

On Mon, Apr 22, 2019 at 9:38 AM Jens Axboe <axboe@...nel.dk> wrote:
>
> With the mutex change in, I can trigger it in a second or so. Just ran
> the reproducer with that change reverted, and I'm not seeing any badness.
> So I do wonder if the bisect results are accurate?

Looking at the syzbot report, it's syzbot being confused.

The actual WARNING in percpu_ref_kill_and_confirm() only happens with
recent kernels.

But then syzbot mixes it up with a completely different bug:

   crash: BUG: MAX_STACK_TRACE_ENTRIES too low!
   BUG: MAX_STACK_TRACE_ENTRIES too low!

and for some reason decides that *that* bug is the same thing entirely.

So yeah, I think the simple percpu_ref_is_dying() check is sufficient,
and that the syzbot bisection is completely bogus.

                Linus

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