linux-kernel - Re: [BUG 5.15-rc3] kernel BUG at drivers/gpu/drm/i915/i915_sw

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Date:   Sat, 2 Oct 2021 10:10:40 -0700 (PDT)
From:   Hugh Dickins <hughd@...gle.com>
To:     Linus Torvalds <torvalds@...ux-foundation.org>
cc:     Steven Rostedt <rostedt@...dmis.org>,
        Hugh Dickins <hughd@...gle.com>,
        Daniele Ceraolo Spurio <daniele.ceraolospurio@...el.com>,
        Matt Roper <matthew.d.roper@...el.com>,
        Lucas De Marchi <lucas.demarchi@...el.com>,
        Tvrtko Ursulin <tvrtko.ursulin@...ux.intel.com>,
        Caz Yokoyama <caz.yokoyama@...el.com>,
        LKML <linux-kernel@...r.kernel.org>,
        Jani Nikula <jani.nikula@...ux.intel.com>,
        Matthew Brost <matthew.brost@...el.com>,
        intel-gfx <intel-gfx@...ts.freedesktop.org>,
        dri-devel <dri-devel@...ts.freedesktop.org>
Subject: Re: [BUG 5.15-rc3] kernel BUG at
 drivers/gpu/drm/i915/i915_sw_fence.c:245!

On Sat, 2 Oct 2021, Linus Torvalds wrote:
> On Sat, Oct 2, 2021 at 5:17 AM Steven Rostedt <rostedt@...dmis.org> wrote:
> > On Sat, 2 Oct 2021 03:17:29 -0700 (PDT)
> > Hugh Dickins <hughd@...gle.com> 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.

Yup.

> 
> 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.

Hugh