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