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

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Message-ID: <CAHk-=whJsD3RaqpmAMv7yjpnQqrEuXvibXZZDY7f-nzO+PvULg@mail.gmail.com>
Date:   Sat, 2 Oct 2021 09:49:41 -0700
From:   Linus Torvalds <torvalds@...ux-foundation.org>
To:     Steven Rostedt <rostedt@...dmis.org>
Cc:     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, 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.

Patch applied directly.

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.

                  Linus