Re: [syzbot] [bpf?] possible deadlock in __bpf_ringbuf_reserve (2)

[Leon Hwang <hffilwlqm@gmail.com>]

On 26/8/25 10:23, Alexei Starovoitov wrote:
> On Mon, Aug 25, 2025 at 7:20 PM Leon Hwang <hffilwlqm@gmail.com> wrote:
>>
>>
>>
>> On 26/8/25 01:39, syzbot wrote:
>>> Hello,
>>>
>>> syzbot found the following issue on:
>>>
>>> HEAD commit:    dd9de524183a xsk: Fix immature cq descriptor production
>>> git tree:       bpf
>>> console output: https://syzkaller.appspot.com/x/log.txt?x=102da862580000
>>> kernel config:  https://syzkaller.appspot.com/x/.config?x=c321f33e4545e2a1
>>> dashboard link: https://syzkaller.appspot.com/bug?extid=fa5c2814795b5adca240
>>> compiler:       Debian clang version 20.1.7 (++20250616065708+6146a88f6049-1~exp1~20250616065826.132), Debian LLD 20.1.7
>>> syz repro:      https://syzkaller.appspot.com/x/repro.syz?x=142da862580000
>>> C reproducer:   https://syzkaller.appspot.com/x/repro.c?x=1588aef0580000
>>>
>>> Downloadable assets:
>>> disk image: https://storage.googleapis.com/syzbot-assets/5a3389c1558f/disk-dd9de524.raw.xz
>>> vmlinux: https://storage.googleapis.com/syzbot-assets/c97133192a27/vmlinux-dd9de524.xz
>>> kernel image: https://storage.googleapis.com/syzbot-assets/3ae5a1a88637/bzImage-dd9de524.xz
>>>
>>> IMPORTANT: if you fix the issue, please add the following tag to the commit:
>>> Reported-by: syzbot+fa5c2814795b5adca240@syzkaller.appspotmail.com
>>>
>>> ============================================
>>> WARNING: possible recursive locking detected
>>> syzkaller #0 Not tainted
>>> --------------------------------------------
>>> syz-execprog/5866 is trying to acquire lock:
>>> ffffc900048c10d8 (&rb->spinlock){-.-.}-{2:2}, at: __bpf_ringbuf_reserve+0x1c7/0x5a0 kernel/bpf/ringbuf.c:423
>>>
>>> but task is already holding lock:
>>> ffffc900048e90d8 (&rb->spinlock){-.-.}-{2:2}, at: __bpf_ringbuf_reserve+0x1c7/0x5a0 kernel/bpf/ringbuf.c:423
>>>
>>> other info that might help us debug this:
>>>  Possible unsafe locking scenario:
>>>
>>>        CPU0
>>>        ----
>>>   lock(&rb->spinlock);
>>>   lock(&rb->spinlock);
>>>
>>>  *** DEADLOCK ***
>>>
>>>  May be due to missing lock nesting notation
>>
>> Confirmed.
>>
>> I can reproduce this deadlock issue and will work on a fix.
> 
> Don't.
> 
> It's due to revert.
> Once rqspinlock is fixed the revert will be reverted.

OK. I'll test it after the fixing.

Thanks,
Leon