Re: [syzbot] [sound?] possible deadlock in snd_seq_deliver_event (2)

[Takashi Iwai <tiwai@suse.de>]

On Thu, 30 Jul 2026 03:59:25 +0200,
syzbot wrote:
> 
> Hello,
> 
> syzbot found the following issue on:
> 
> HEAD commit:    fc02acf6ac0c Merge tag 'platform-drivers-x86-v7.2-4' of gi..
> git tree:       upstream
> console output: https://syzkaller.appspot.com/x/log.txt?x=17a0b8c6580000
> kernel config:  https://syzkaller.appspot.com/x/.config?x=145fa60d73086782
> dashboard link: https://syzkaller.appspot.com/bug?extid=d4d7ac4a87bb2d52a822
> compiler:       gcc (Debian 14.2.0-19) 14.2.0, GNU ld (GNU Binutils for Debian) 2.44
> 
> Unfortunately, I don't have any reproducer for this issue yet.
> 
> Downloadable assets:
> disk image: https://storage.googleapis.com/syzbot-assets/67251350dd03/disk-fc02acf6.raw.xz
> vmlinux: https://storage.googleapis.com/syzbot-assets/e4edd57a8c61/vmlinux-fc02acf6.xz
> kernel image: https://storage.googleapis.com/syzbot-assets/04ff166cc853/bzImage-fc02acf6.xz
> 
> IMPORTANT: if you fix the issue, please add the following tag to the commit:
> Reported-by: syzbot+d4d7ac4a87bb2d52a822@syzkaller.appspotmail.com
> 
> ============================================
> WARNING: possible recursive locking detected
> syzkaller #0 Not tainted
> --------------------------------------------
> kworker/1:3/5615 is trying to acquire lock:
> ffff888032c85128 (&grp->list_lock){.+.+}-{3:3}, at: __deliver_to_subscribers sound/core/seq/seq_clientmgr.c:723 [inline]
> ffff888032c85128 (&grp->list_lock){.+.+}-{3:3}, at: deliver_to_subscribers sound/core/seq/seq_clientmgr.c:763 [inline]
> ffff888032c85128 (&grp->list_lock){.+.+}-{3:3}, at: snd_seq_deliver_event+0x230/0x860 sound/core/seq/seq_clientmgr.c:813
> 
> but task is already holding lock:
> ffff8880269b0128 (&grp->list_lock){.+.+}-{3:3}, at: __deliver_to_subscribers sound/core/seq/seq_clientmgr.c:723 [inline]
> ffff8880269b0128 (&grp->list_lock){.+.+}-{3:3}, at: deliver_to_subscribers sound/core/seq/seq_clientmgr.c:763 [inline]
> ffff8880269b0128 (&grp->list_lock){.+.+}-{3:3}, at: snd_seq_deliver_event+0x230/0x860 sound/core/seq/seq_clientmgr.c:813
> 
> other info that might help us debug this:
>  Possible unsafe locking scenario:
> 
>        CPU0
>        ----
>   lock(&grp->list_lock);
>   lock(&grp->list_lock);
> 
>  *** DEADLOCK ***
> 
>  May be due to missing lock nesting notation
> 
> 3 locks held by kworker/1:3/5615:
>  #0: ffff88801bc0f140 ((wq_completion)events){+.+.}-{0:0}, at: process_one_work+0x12b1/0x1940 kernel/workqueue.c:3297
>  #1: ffffc900039c7d08 ((work_completion)(&runtime->event_work)){+.+.}-{0:0}, at: process_one_work+0x988/0x1940 kernel/workqueue.c:3298
>  #2: ffff8880269b0128 (&grp->list_lock){.+.+}-{3:3}, at: __deliver_to_subscribers sound/core/seq/seq_clientmgr.c:723 [inline]
>  #2: ffff8880269b0128 (&grp->list_lock){.+.+}-{3:3}, at: deliver_to_subscribers sound/core/seq/seq_clientmgr.c:763 [inline]
>  #2: ffff8880269b0128 (&grp->list_lock){.+.+}-{3:3}, at: snd_seq_deliver_event+0x230/0x860 sound/core/seq/seq_clientmgr.c:813
> 
> stack backtrace:
> CPU: 1 UID: 0 PID: 5615 Comm: kworker/1:3 Not tainted syzkaller #0 PREEMPT(full) 
> Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 07/16/2026
> Workqueue: events snd_rawmidi_input_event_work
> Call Trace:
>  <TASK>
>  __dump_stack lib/dump_stack.c:94 [inline]
>  dump_stack_lvl+0x100/0x190 lib/dump_stack.c:120
>  print_deadlock_bug.cold+0xbd/0xca kernel/locking/lockdep.c:3041
>  check_deadlock kernel/locking/lockdep.c:3093 [inline]
>  validate_chain kernel/locking/lockdep.c:3895 [inline]
>  __lock_acquire+0x1256/0x1a40 kernel/locking/lockdep.c:5237
>  lock_acquire kernel/locking/lockdep.c:5868 [inline]
>  lock_acquire+0x1b9/0x370 kernel/locking/lockdep.c:5825
>  __raw_read_lock include/linux/rwlock_api_smp.h:163 [inline]
>  _raw_read_lock+0x5f/0x70 kernel/locking/spinlock.c:232
>  __deliver_to_subscribers sound/core/seq/seq_clientmgr.c:723 [inline]
>  deliver_to_subscribers sound/core/seq/seq_clientmgr.c:763 [inline]
>  snd_seq_deliver_event+0x230/0x860 sound/core/seq/seq_clientmgr.c:813
>  snd_seq_kernel_client_dispatch+0x16f/0x1b0 sound/core/seq/seq_clientmgr.c:2439
>  queue_broadcast_event.isra.0+0x143/0x1b0 sound/core/seq/seq_queue.c:585
>  snd_seq_queue_process_event sound/core/seq/seq_queue.c:634 [inline]
>  snd_seq_control_queue+0x709/0xb70 sound/core/seq/seq_queue.c:659
>  __snd_seq_deliver_single_event sound/core/seq/seq_clientmgr.c:626 [inline]
>  __snd_seq_deliver_single_event+0x127/0x1c0 sound/core/seq/seq_clientmgr.c:613
>  _snd_seq_deliver_single_event sound/core/seq/seq_clientmgr.c:672 [inline]
>  snd_seq_deliver_single_event+0x31f/0x8c0 sound/core/seq/seq_clientmgr.c:687
>  snd_seq_deliver_event+0x44b/0x860 sound/core/seq/seq_clientmgr.c:815
>  snd_seq_kernel_client_dispatch+0x16f/0x1b0 sound/core/seq/seq_clientmgr.c:2439
>  send_timer_event.isra.0+0x130/0x190 sound/core/seq/oss/seq_oss_timer.c:140
>  snd_seq_oss_timer_start+0x1db/0x320 sound/core/seq/oss/seq_oss_timer.c:161
>  send_midi_event sound/core/seq/oss/seq_oss_midi.c:576 [inline]
>  snd_seq_oss_midi_input+0x467/0xa30 sound/core/seq/oss/seq_oss_midi.c:501
>  snd_seq_oss_event_input+0x194/0x250 sound/core/seq/oss/seq_oss_event.c:429
>  __snd_seq_deliver_single_event sound/core/seq/seq_clientmgr.c:626 [inline]
>  __snd_seq_deliver_single_event+0x127/0x1c0 sound/core/seq/seq_clientmgr.c:613
>  _snd_seq_deliver_single_event sound/core/seq/seq_clientmgr.c:672 [inline]
>  snd_seq_deliver_single_event+0x31f/0x8c0 sound/core/seq/seq_clientmgr.c:687
>  __deliver_to_subscribers sound/core/seq/seq_clientmgr.c:735 [inline]
>  deliver_to_subscribers sound/core/seq/seq_clientmgr.c:763 [inline]
>  snd_seq_deliver_event+0x2c1/0x860 sound/core/seq/seq_clientmgr.c:813
>  snd_seq_kernel_client_dispatch+0x16f/0x1b0 sound/core/seq/seq_clientmgr.c:2439
>  snd_midi_input_event+0x32f/0x470 sound/core/seq/seq_midi.c:93
>  snd_rawmidi_input_event_work+0x61/0x80 sound/core/rawmidi.c:127
>  process_one_work+0xa23/0x1940 kernel/workqueue.c:3322
>  process_scheduled_works kernel/workqueue.c:3405 [inline]
>  worker_thread+0x5ef/0xe50 kernel/workqueue.c:3486
>  kthread+0x370/0x450 kernel/kthread.c:436
>  ret_from_fork+0x72b/0xd50 arch/x86/kernel/process.c:158
>  ret_from_fork_asm+0x1a/0x30 arch/x86/entry/entry_64.S:245
>  </TASK>
> 
> 
> ---
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#syz fix: ALSA: seq: Use RCU for the port subscriber list