Re: WARNING in percpu_ref_kill_and_confirm

[Linus Torvalds]

On Mon, Apr 22, 2019 at 9:38 AM Jens Axboe <axboe@xxxxxxxxx> 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