On Tue, Jan 18, 2022 at 7:07 PM Vlastimil Babka <vbabka@xxxxxxx> wrote: > > > On 1/18/22 09:00, Xin Long wrote: > > On Mon, Jan 17, 2022 at 9:13 PM Juri Lelli <juri.lelli@xxxxxxxxxx> wrote: > >> > > >> > RHEL-8 kernel seems to be 4.18, unless RT uses a newer one. Could be some > >> > silently relevant backport is missing? How about e.g. 59450bbc12be ("mm, > >> > slab, slub: stop taking cpu hotplug lock") ? > >> > >> Hummm, looks like we have backported commit 59450bbc12be in RHEL-8. > >> > >> Xin Long, would you be able to check if you still see the lockdep splat > >> with latest upstream RT? > >> > >> git://git.kernel.org/pub/scm/linux/kernel/git/rt/linux-rt-devel.git linux-5.16.y-rt > > Hi, Juri, > > > > Thanks for sharing the RT kernel repo. > > > > I just tried with this kernel, and I couldn't reproduce it on my env. > > But I don't see how the upstream RT kernel can avoid the call trace. > > > > As this warning was triggered when the system was shutting down, it might > > not be reproduced on it due to some timing change. > > As it was caught by lockdep and not as a real deadlock, I think it should be > indepenedent of a timing change. Lockdep will correlate potentially deadlock > scenarios even if they don't really occur in the same time, AFAIK. > > But let's go back to: > > > Although cpu_hotplug_lock is a RWSEM, [a] will not block in there. But as > > lockdep annotations are added for cpu_hotplug_lock, a deadlock warning > > would be detected: > > Is it possible that upstream lockdep handles this RWSEM scenario properly > and doesn't report it, but the RHEL kernel is missing some relevant lockdep fix? That's a good point. I actually think: cpus_read_lock() cpus_read_lock() shouldn't be considered as a deadlock. I will check the lockdep changes, and it may take some time. Thanks. > > >> > >> Thanks! > >> Juri > >> >