linux-kernel - Re: [syzbot] [kernel?] WARNING in flush_cpu

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Message-ID: <87r0dsdur6.ffs@tglx>
Date: Fri, 24 May 2024 00:32:45 +0200
From: Thomas Gleixner <tglx@...utronix.de>
To: Vlastimil Babka <vbabka@...e.cz>, syzbot
 <syzbot+50e25cfa4f917d41749f@...kaller.appspotmail.com>, bp@...en8.de,
 dave.hansen@...ux.intel.com, hpa@...or.com, linux-kernel@...r.kernel.org,
 mingo@...hat.com, syzkaller-bugs@...glegroups.com, x86@...nel.org,
 linux-mm@...ck.org, Sebastian Andrzej Siewior <bigeasy@...utronix.de>,
 Tejun Heo <tj@...nel.org>, Lai Jiangshan <jiangshanlai@...il.com>
Subject: Re: [syzbot] [kernel?] WARNING in flush_cpu_slab

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