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Date:   Sat, 2 Oct 2021 10:10:40 -0700 (PDT)
From:   Hugh Dickins <>
To:     Linus Torvalds <>
cc:     Steven Rostedt <>,
        Hugh Dickins <>,
        Daniele Ceraolo Spurio <>,
        Matt Roper <>,
        Lucas De Marchi <>,
        Tvrtko Ursulin <>,
        Caz Yokoyama <>,
        LKML <>,
        Jani Nikula <>,
        Matthew Brost <>,
        intel-gfx <>,
        dri-devel <>
Subject: Re: [BUG 5.15-rc3] kernel BUG at

On Sat, 2 Oct 2021, Linus Torvalds wrote:
> On Sat, Oct 2, 2021 at 5:17 AM Steven Rostedt <> wrote:
> > On Sat, 2 Oct 2021 03:17:29 -0700 (PDT)
> > Hugh Dickins <> wrote:
> >
> > > Yes (though bisection doesn't work right on this one): the fix
> >
> > Interesting, as it appeared to be very reliable. But I didn't do the
> > "try before / after" on the patch.
> Well, even the before/after might well have worked, since the problem
> depended on how that sw_fence_dummy_notify() function ended up
> aligned. So random unrelated changes could re-align it just by
> mistake.


> Patch applied directly.

Great, thanks a lot.

> I'd also like to point out how that BUG_ON() actually made things
> worse, and made this harder to debug. If it had been a WARN_ON_ONCE(),
> this would presumably not even have needed bisecting, it would have
> been obvious.
> BUG_ON() really is pretty much *always* the wrong thing to do. It
> onl;y results in problems being harder to see because you end up with
> a dead machine and the message is often hidden.

Jani made the same point. But I guess they then went off into the weeds
of how to recover when warning, that the fix itself did not progress.


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