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From:   Linus Torvalds <torvalds@linux-foundation.org>
Date:   Mon, 22 Apr 2019 09:48:08 -0700
Message-ID: <CAHk-=wh=kKEb6qbuhnSWh5S55SbQBOE33991ULSFUgVrEnzivA@mail.gmail.com>
Subject: Re: WARNING in percpu_ref_kill_and_confirm
To:     Jens Axboe <axboe@kernel.dk>
Cc:     syzbot <syzbot+10d25e23199614b7721f@syzkaller.appspotmail.com>,
        Arnd Bergmann <arnd@arndb.de>, Borislav Petkov <bp@alien8.de>,
        "Darrick J. Wong" <darrick.wong@oracle.com>,
        Greg Kroah-Hartman <gregkh@linuxfoundation.org>,
        Peter Anvin <hpa@zytor.com>,
        Linux API <linux-api@vger.kernel.org>,
        linux-arch <linux-arch@vger.kernel.org>,
        linux-block <linux-block@vger.kernel.org>,
        linux-fsdevel <linux-fsdevel@vger.kernel.org>,
        Linux List Kernel Mailing <linux-kernel@vger.kernel.org>,
        Andrew Lutomirski <luto@kernel.org>,
        Mathieu Desnoyers <mathieu.desnoyers@efficios.com>,
        Ingo Molnar <mingo@redhat.com>,
        Michael Ellerman <mpe@ellerman.id.au>,
        syzkaller-bugs <syzkaller-bugs@googlegroups.com>,
        Thomas Gleixner <tglx@linutronix.de>,
        Al Viro <viro@zeniv.linux.org.uk>,
        "the arch/x86 maintainers" <x86@kernel.org>
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On Mon, Apr 22, 2019 at 9:38 AM Jens Axboe <axboe@kernel.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