Re: [syzbot] WARNING in __percpu_ref_exit (2)

From: Pavel Begunkov <asml.silence@gmail.com>
To: syzbot <syzbot+d6218cb2fae0b2411e9d@syzkaller.appspotmail.com>,
	axboe@kernel.dk, io-uring@vger.kernel.org,
	linux-kernel@vger.kernel.org, syzkaller-bugs@googlegroups.com
Subject: Re: [syzbot] WARNING in __percpu_ref_exit (2)
Date: Mon, 15 Mar 2021 12:18:20 +0000	[thread overview]
Message-ID: <a3420ea6-8f91-f97f-9887-1de9aa5bb02e@gmail.com> (raw)
In-Reply-To: <0000000000006e9e0705bd91f762@google.com>

On 15/03/2021 11:58, syzbot wrote:
> Hello,
> 
> syzbot found the following issue on:
> 
> HEAD commit:    75013c6c Merge tag 'perf_urgent_for_v5.12-rc3' of git://gi..
> git tree:       upstream
> console output: https://syzkaller.appspot.com/x/log.txt?x=174df32ad00000
> kernel config:  https://syzkaller.appspot.com/x/.config?x=844457676c06b88c
> dashboard link: https://syzkaller.appspot.com/bug?extid=d6218cb2fae0b2411e9d
> userspace arch: i386
> 
> Unfortunately, I don't have any reproducer for this issue yet.
> 
> IMPORTANT: if you fix the issue, please add the following tag to the commit:
> Reported-by: syzbot+d6218cb2fae0b2411e9d@syzkaller.appspotmail.com
> ------------[ cut here ]------------
> WARNING: CPU: 1 PID: 53 at lib/percpu-refcount.c:113 __percpu_ref_exit+0x98/0x100 lib/percpu-refcount.c:113

if (percpu_count) {
	/* non-NULL confirm_switch indicates switching in progress */
	WARN_ON_ONCE(ref->data && ref->data->confirm_switch);
	...
}

Points to this warning. Not sure, but not yet included
"io_uring: halt SQO submission on ctx exit" may fix it or at least is
related.

> Modules linked in:
> CPU: 1 PID: 53 Comm: kworker/u4:2 Not tainted 5.12.0-rc2-syzkaller #0
> Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 01/01/2011
> Workqueue: events_unbound io_ring_exit_work
> RIP: 0010:__percpu_ref_exit+0x98/0x100 lib/percpu-refcount.c:113
> Code: fd 49 8d 7c 24 10 48 b8 00 00 00 00 00 fc ff df 48 89 fa 48 c1 ea 03 80 3c 02 00 75 61 49 83 7c 24 10 00 74 07 e8 28 42 ac fd <0f> 0b e8 21 42 ac fd 48 89 ef e8 e9 fa da fd 48 89 da 48 b8 00 00
> RSP: 0018:ffffc90000f1fb78 EFLAGS: 00010293
> RAX: 0000000000000000 RBX: ffff88805c976000 RCX: 0000000000000000
> RDX: ffff888011839bc0 RSI: ffffffff83c76be8 RDI: ffff88802b2a9010
> RBP: 0000607f46077778 R08: 0000000000000000 R09: ffffffff8fab0967
> R10: ffffffff83c76b88 R11: 0000000000000009 R12: ffff88802b2a9000
> R13: 0000000000000001 R14: ffff88802b2a9000 R15: dffffc0000000000
> FS:  0000000000000000(0000) GS:ffff8880b9c00000(0000) knlGS:0000000000000000
> CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
> CR2: 00000000085a0004 CR3: 000000001896a000 CR4: 00000000001506f0
> DR0: 0000000000000000 DR1: 0000000000000000 DR2: 0000000000000000
> DR3: 0000000000000000 DR6: 00000000fffe0ff0 DR7: 0000000000000400
> Call Trace:
>  percpu_ref_exit+0x3b/0x140 lib/percpu-refcount.c:134
>  io_ring_ctx_free fs/io_uring.c:8419 [inline]
>  io_ring_exit_work+0x599/0xcf0 fs/io_uring.c:8565
>  process_one_work+0x98d/0x1600 kernel/workqueue.c:2275
>  worker_thread+0x64c/0x1120 kernel/workqueue.c:2421
>  kthread+0x3b1/0x4a0 kernel/kthread.c:292
>  ret_from_fork+0x1f/0x30 arch/x86/entry/entry_64.S:294
> 
> 
> ---
> 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@googlegroups.com.
> 
> syzbot will keep track of this issue. See:
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> 

-- 
Pavel Begunkov