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

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Message-ID: <87mtnp2q8e.fsf@intel.com>
Date:   Mon, 04 Oct 2021 10:39:29 +0300
From:   Jani Nikula <jani.nikula@...ux.intel.com>
To:     Hugh Dickins <hughd@...gle.com>,
        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>,
        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, 02 Oct 2021, Hugh Dickins <hughd@...gle.com> wrote:
> 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.

Thanks & sorry, really looks like we managed to drop this between the
cracks. :(

>
>> 
>> 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.

Yes. That, as well as removing the entire alignment thing to reuse a
couple of bits for flags. Too fragile for its own good.

BR,
Jani.


-- 
Jani Nikula, Intel Open Source Graphics Center