[BUG 5.15-rc3] kernel BUG at drivers/gpu/drm/i915/i915_sw_fence.c:245!

[Hugh Dickins]

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