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

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Date: Thu, 23 May 2024 23:03:52 +0200
From: Vlastimil Babka <vbabka@...e.cz>
To: Thomas Gleixner <tglx@...utronix.de>,
 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 5/23/24 12:36 PM, Thomas Gleixner wrote:
> CC+: mm folks
> 
> On Wed, May 22 2024 at 19:27, syzbot wrote:
> 
>> Hello,
>>
>> syzbot found the following issue on:
>>
>> HEAD commit:    2a8120d7b482 Merge tag 's390-6.10-2' of git://git.kernel.o..
>> git tree:       upstream
>> console output: https://syzkaller.appspot.com/x/log.txt?x=17d42b0a980000
>> kernel config:  https://syzkaller.appspot.com/x/.config?x=5dd4fde1337a9e18
>> dashboard link: https://syzkaller.appspot.com/bug?extid=50e25cfa4f917d41749f
>> compiler:       gcc (Debian 12.2.0-14) 12.2.0, GNU ld (GNU Binutils for Debian) 2.40
>> userspace arch: i386
>>
>> Unfortunately, I don't have any reproducer for this issue yet.
>>
>> Downloadable assets:
>> disk image (non-bootable): https://storage.googleapis.com/syzbot-assets/7bc7510fe41f/non_bootable_disk-2a8120d7.raw.xz
>> vmlinux: https://storage.googleapis.com/syzbot-assets/78c72ae6bdaf/vmlinux-2a8120d7.xz
>> kernel image: https://storage.googleapis.com/syzbot-assets/99dbb805b738/bzImage-2a8120d7.xz
>>
>> IMPORTANT: if you fix the issue, please add the following tag to the commit:
>> Reported-by: syzbot+50e25cfa4f917d41749f@...kaller.appspotmail.com
>>
>> ------------[ 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

Also this code didn't change for a while.

>> Modules linked in:
>> CPU: 3 PID: 5221 Comm: kworker/3:3 Not tainted 6.9.0-syzkaller-10713-g2a8120d7b482 #0
>> Hardware name: QEMU Standard PC (Q35 + ICH9, 2009), BIOS 1.16.2-debian-1.16.2-1 04/01/2014
>> Workqueue: slub_flushwq flush_cpu_slab
>> RIP: 0010:local_lock_acquire include/linux/local_lock_internal.h:30 [inline]
>> RIP: 0010:flush_slab mm/slub.c:3088 [inline]
>> RIP: 0010:flush_cpu_slab+0x37f/0x410 mm/slub.c:3146
>> Code: ff e8 e5 b2 fc 08 e9 25 ff ff ff e8 db b2 fc 08 e9 46 ff ff ff 90 48 c7 c6 7f 68 37 8d 48 c7 c7 b6 33 37 8d e8 72 88 6f ff 90 <0f> 0b 90 90 e9 dd fe ff ff 90 48 c7 c6 88 68 37 8d 48 c7 c7 b6 33
>> RSP: 0018:ffffc90002b57c98 EFLAGS: 00010086
>> RAX: 0000000000000000 RBX: ffffe8ffac1021b0 RCX: ffffffff81510229
>> RDX: ffff88801bb7c880 RSI: ffffffff81510236 RDI: 0000000000000001
>> RBP: ffff88802790b540 R08: 0000000000000001 R09: 0000000000000000
>> R10: 0000000000000000 R11: 000000002d2d2d2d R12: 0000000000000200
>> R13: ffffe8ffac1021d0 R14: 0000000000000000 R15: ffffc90002b57d80
>> FS:  0000000000000000(0000) GS:ffff88802c300000(0000) knlGS:0000000000000000
>> CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
>> CR2: 00000000f73ffec2 CR3: 00000000268ca000 CR4: 0000000000350ef0
>> DR0: 0000000000000000 DR1: 0000000000000000 DR2: 0000000000000000
>> DR3: 0000000000000000 DR6: 00000000fffe0ff0 DR7: 0000000000000400
>> Call Trace:
>>  <TASK>
>>  process_one_work+0x958/0x1ad0 kernel/workqueue.c:3231
>>  process_scheduled_works kernel/workqueue.c:3312 [inline]
>>  worker_thread+0x6c8/0xf70 kernel/workqueue.c:3393
>>  kthread+0x2c1/0x3a0 kernel/kthread.c:389
>>  ret_from_fork+0x45/0x80 arch/x86/kernel/process.c:147
>>  ret_from_fork_asm+0x1a/0x30 arch/x86/entry/entry_64.S:244
>>  </TASK>
>>
>>
>> ---
>> This report is generated by a bot. It may contain errors.
>> See https://goo.gl/tpsmEJ for more information about syzbot.
>> syzbot engineers can be reached at syzkaller@...glegroups.com.
>>
>> syzbot will keep track of this issue. See:
>> https://goo.gl/tpsmEJ#status for how to communicate with syzbot.
>>
>> If the report is already addressed, let syzbot know by replying with:
>> #syz fix: exact-commit-title
>>
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>>
>> If the report is a duplicate of another one, reply with:
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>>
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