Re: [syzbot] [kernel?] WARNING in flush_cpu_slab

Thomas Gleixner
2024-05-23T22:32:52Z

On Thu, May 23 2024 at 23:03, Vlastimil Babka wrote:
> On 5/23/24 12:36 PM, Thomas Gleixner wrote:
>>> ------------[ cut here ]------------
>>> DEBUG_LOCKS_WARN_ON(l->owner)
>>> WARNING: CPU: 3 PID: 5221 at include/linux/local_lock_internal.h:30 local_lock_acquire include/linux/local_lock_internal.h:30 [inline]
>>> WARNING: CPU: 3 PID: 5221 at include/linux/local_lock_internal.h:30 flush_slab mm/slub.c:3088 [inline]
>>> WARNING: CPU: 3 PID: 5221 at include/linux/local_lock_internal.h:30 flush_cpu_slab+0x37f/0x410 mm/slub.c:3146
>
> I'm puzzled by this. We use local_lock_irqsave() on !PREEMPT_RT everywhere.
> IIUC this warning says we did the irqsave() and then found out somebody else
> already set the owner? But that means they also did that irqsave() and set
> themselves as l->owner. Does that mey there would be a spurious irq enable
> that didn't go through local_unlock_irqrestore()?
>
> Also this particular stack is from the work, which is scheduled by
> queue_work_on() in flush_all_cpus_locked(), which also has a
> lockdep_assert_cpus_held() so it should fullfill the "the caller must ensure
> the cpu doesn't go away" property. But I think even if this ended up on the
> wrong cpu (for the full duration or migrated while processing the work item)
> somehow, it wouldn't be able to cause such warning, but rather corrupt
> something else

Indeed. There is another report which makes no sense either:

https://lore.kernel.org/lkml/000000000000fa09d906191c3ee5@google.com

Both look like data corropution issues caused by whatever...

Thanks,

tglx