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Date:   Mon, 22 Apr 2019 10:50:56 -0600
From:   Jens Axboe <>
To:     Linus Torvalds <>
Cc:     syzbot <>,
        Arnd Bergmann <>, Borislav Petkov <>,
        "Darrick J. Wong" <>,
        Greg Kroah-Hartman <>,
        Peter Anvin <>,
        Linux API <>,
        linux-arch <>,
        linux-block <>,
        linux-fsdevel <>,
        Linux List Kernel Mailing <>,
        Andrew Lutomirski <>,
        Mathieu Desnoyers <>,
        Ingo Molnar <>,
        Michael Ellerman <>,
        syzkaller-bugs <>,
        Thomas Gleixner <>,
        Al Viro <>,
        the arch/x86 maintainers <>
Subject: Re: WARNING in percpu_ref_kill_and_confirm

On 4/22/19 10:48 AM, Linus Torvalds wrote:
> On Mon, Apr 22, 2019 at 9:38 AM Jens Axboe <> 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!
> 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.

Ah good, that makes me feel better. I'll queue the fix up, thanks.

Jens Axboe

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