Re: [syzbot] [sound?] possible deadlock in snd_pcm_drop

[Takashi Iwai <tiwai@suse.de>]

On Mon, 29 Sep 2025 21:33:34 +0200,
syzbot wrote:
> 
> Hello,
> 
> syzbot found the following issue on:
> 
> HEAD commit:    4ff71af020ae Merge tag 'net-6.17-rc8' of git://git.kernel...
> git tree:       upstream
> console output: https://syzkaller.appspot.com/x/log.txt?x=1191c2e2580000
> kernel config:  https://syzkaller.appspot.com/x/.config?x=f5b21423ca3f0a96
> dashboard link: https://syzkaller.appspot.com/bug?extid=c9ab1eb0f8f5ab1f18be
> compiler:       Debian clang version 20.1.8 (++20250708063551+0c9f909b7976-1~exp1~20250708183702.136), Debian LLD 20.1.8
> syz repro:      https://syzkaller.appspot.com/x/repro.syz?x=1644dd34580000
> C reproducer:   https://syzkaller.appspot.com/x/repro.c?x=156d6f12580000
> 
> Downloadable assets:
> disk image: https://storage.googleapis.com/syzbot-assets/3fb4198b5763/disk-4ff71af0.raw.xz
> vmlinux: https://storage.googleapis.com/syzbot-assets/1499875eb2cc/vmlinux-4ff71af0.xz
> kernel image: https://storage.googleapis.com/syzbot-assets/182864fb8af7/bzImage-4ff71af0.xz
> 
> IMPORTANT: if you fix the issue, please add the following tag to the commit:
> Reported-by: syzbot+c9ab1eb0f8f5ab1f18be@syzkaller.appspotmail.com
> 
> ======================================================
> WARNING: possible circular locking dependency detected
> syzkaller #0 Not tainted
> ------------------------------------------------------
> syz.2.2869/8991 is trying to acquire lock:
> ffff8880b8823d90 ((softirq_ctrl.lock)){+.+.}-{3:3}, at: spin_lock include/linux/spinlock_rt.h:44 [inline]
> ffff8880b8823d90 ((softirq_ctrl.lock)){+.+.}-{3:3}, at: __local_bh_disable_ip+0x264/0x400 kernel/softirq.c:168
> 
> but task is already holding lock:
> ffff8880302d0150 (&group->lock#2){+.+.}-{3:3}, at: spin_lock_irq include/linux/spinlock_rt.h:93 [inline]
> ffff8880302d0150 (&group->lock#2){+.+.}-{3:3}, at: snd_pcm_group_lock_irq sound/core/pcm_native.c:98 [inline]
> ffff8880302d0150 (&group->lock#2){+.+.}-{3:3}, at: snd_pcm_stream_lock_irq sound/core/pcm_native.c:137 [inline]
> ffff8880302d0150 (&group->lock#2){+.+.}-{3:3}, at: class_pcm_stream_lock_irq_constructor include/sound/pcm.h:679 [inline]
> ffff8880302d0150 (&group->lock#2){+.+.}-{3:3}, at: snd_pcm_drop+0x10d/0x270 sound/core/pcm_native.c:2223
> 
> which lock already depends on the new lock.
> 
> 
> the existing dependency chain (in reverse order) is:
> 
> -> #2 (&group->lock#2){+.+.}-{3:3}:
>        lock_acquire+0x120/0x360 kernel/locking/lockdep.c:5868
>        rt_spin_lock+0x88/0x2c0 kernel/locking/spinlock_rt.c:56
>        spin_lock include/linux/spinlock_rt.h:44 [inline]
>        _snd_pcm_stream_lock_irqsave+0x7c/0xa0 sound/core/pcm_native.c:171
>        class_pcm_stream_lock_irqsave_constructor include/sound/pcm.h:682 [inline]
>        snd_pcm_period_elapsed+0x1e/0x80 sound/core/pcm_lib.c:1938
>        dummy_hrtimer_callback+0x80/0x180 sound/drivers/dummy.c:386
>        __run_hrtimer kernel/time/hrtimer.c:1761 [inline]
>        __hrtimer_run_queues+0x54f/0xd40 kernel/time/hrtimer.c:1825
>        hrtimer_run_softirq+0x1a3/0x2e0 kernel/time/hrtimer.c:1842
>        handle_softirqs+0x22f/0x710 kernel/softirq.c:579
>        __do_softirq kernel/softirq.c:613 [inline]
>        run_ktimerd+0xcf/0x190 kernel/softirq.c:1043
>        smpboot_thread_fn+0x53f/0xa60 kernel/smpboot.c:160
>        kthread+0x70e/0x8a0 kernel/kthread.c:463
>        ret_from_fork+0x436/0x7d0 arch/x86/kernel/process.c:148
>        ret_from_fork_asm+0x1a/0x30 arch/x86/entry/entry_64.S:245
> 
> -> #1 (&base->softirq_expiry_lock){+...}-{3:3}:
>        lock_acquire+0x120/0x360 kernel/locking/lockdep.c:5868
>        rt_spin_lock+0x88/0x2c0 kernel/locking/spinlock_rt.c:56
>        spin_lock include/linux/spinlock_rt.h:44 [inline]
>        hrtimer_cpu_base_lock_expiry kernel/time/hrtimer.c:1383 [inline]
>        hrtimer_run_softirq+0x7c/0x2e0 kernel/time/hrtimer.c:1838
>        handle_softirqs+0x22f/0x710 kernel/softirq.c:579
>        __do_softirq kernel/softirq.c:613 [inline]
>        run_ktimerd+0xcf/0x190 kernel/softirq.c:1043
>        smpboot_thread_fn+0x53f/0xa60 kernel/smpboot.c:160
>        kthread+0x70e/0x8a0 kernel/kthread.c:463
>        ret_from_fork+0x436/0x7d0 arch/x86/kernel/process.c:148
>        ret_from_fork_asm+0x1a/0x30 arch/x86/entry/entry_64.S:245
> 
> -> #0 ((softirq_ctrl.lock)){+.+.}-{3:3}:
>        check_prev_add kernel/locking/lockdep.c:3165 [inline]
>        check_prevs_add kernel/locking/lockdep.c:3284 [inline]
>        validate_chain+0xb9b/0x2140 kernel/locking/lockdep.c:3908
>        __lock_acquire+0xab9/0xd20 kernel/locking/lockdep.c:5237
>        reacquire_held_locks+0x127/0x1d0 kernel/locking/lockdep.c:5385
>        __lock_release kernel/locking/lockdep.c:5574 [inline]
>        lock_release+0x1b4/0x3e0 kernel/locking/lockdep.c:5889
>        __local_bh_enable_ip+0x10c/0x270 kernel/softirq.c:228
>        hrtimer_cancel+0x39/0x60 kernel/time/hrtimer.c:1491
>        dummy_hrtimer_stop+0xcf/0x100 sound/drivers/dummy.c:410
>        snd_pcm_do_stop+0x127/0x1c0 sound/core/pcm_native.c:1525
>        snd_pcm_action_single sound/core/pcm_native.c:1305 [inline]
>        snd_pcm_action+0xe4/0x240 sound/core/pcm_native.c:1388
>        snd_pcm_stop sound/core/pcm_native.c:1561 [inline]
>        snd_pcm_drop+0x160/0x270 sound/core/pcm_native.c:2228
>        snd_pcm_oss_sync+0x1de/0xc30 sound/core/oss/pcm_oss.c:1733
>        snd_pcm_oss_release+0x102/0x250 sound/core/oss/pcm_oss.c:2574
>        __fput+0x45b/0xa80 fs/file_table.c:468
>        task_work_run+0x1d4/0x260 kernel/task_work.c:227
>        resume_user_mode_work include/linux/resume_user_mode.h:50 [inline]
>        exit_to_user_mode_loop+0xec/0x110 kernel/entry/common.c:43
>        exit_to_user_mode_prepare include/linux/irq-entry-common.h:225 [inline]
>        syscall_exit_to_user_mode_work include/linux/entry-common.h:175 [inline]
>        syscall_exit_to_user_mode include/linux/entry-common.h:210 [inline]
>        do_syscall_64+0x2bd/0x3b0 arch/x86/entry/syscall_64.c:100
>        entry_SYSCALL_64_after_hwframe+0x77/0x7f
> 
> other info that might help us debug this:
> 
> Chain exists of:
>   (softirq_ctrl.lock) --> &base->softirq_expiry_lock --> &group->lock#2
> 
>  Possible unsafe locking scenario:
> 
>        CPU0                    CPU1
>        ----                    ----
>   lock(&group->lock#2);
>                                lock(&base->softirq_expiry_lock);
>                                lock(&group->lock#2);
>   lock((softirq_ctrl.lock));
> 
>  *** DEADLOCK ***
> 
> 2 locks held by syz.2.2869/8991:
>  #0: ffff8880302d0150 (&group->lock#2){+.+.}-{3:3}, at: spin_lock_irq include/linux/spinlock_rt.h:93 [inline]
>  #0: ffff8880302d0150 (&group->lock#2){+.+.}-{3:3}, at: snd_pcm_group_lock_irq sound/core/pcm_native.c:98 [inline]
>  #0: ffff8880302d0150 (&group->lock#2){+.+.}-{3:3}, at: snd_pcm_stream_lock_irq sound/core/pcm_native.c:137 [inline]
>  #0: ffff8880302d0150 (&group->lock#2){+.+.}-{3:3}, at: class_pcm_stream_lock_irq_constructor include/sound/pcm.h:679 [inline]
>  #0: ffff8880302d0150 (&group->lock#2){+.+.}-{3:3}, at: snd_pcm_drop+0x10d/0x270 sound/core/pcm_native.c:2223
>  #1: ffffffff8d9a8d80 (rcu_read_lock){....}-{1:3}, at: rcu_lock_acquire include/linux/rcupdate.h:331 [inline]
>  #1: ffffffff8d9a8d80 (rcu_read_lock){....}-{1:3}, at: rcu_read_lock include/linux/rcupdate.h:841 [inline]
>  #1: ffffffff8d9a8d80 (rcu_read_lock){....}-{1:3}, at: __rt_spin_lock kernel/locking/spinlock_rt.c:50 [inline]
>  #1: ffffffff8d9a8d80 (rcu_read_lock){....}-{1:3}, at: rt_spin_lock+0x1bb/0x2c0 kernel/locking/spinlock_rt.c:57
> 
> stack backtrace:
> CPU: 0 UID: 0 PID: 8991 Comm: syz.2.2869 Not tainted syzkaller #0 PREEMPT_{RT,(full)} 
> Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 08/18/2025
> Call Trace:
>  <TASK>
>  dump_stack_lvl+0x189/0x250 lib/dump_stack.c:120
>  print_circular_bug+0x2ee/0x310 kernel/locking/lockdep.c:2043
>  check_noncircular+0x134/0x160 kernel/locking/lockdep.c:2175
>  check_prev_add kernel/locking/lockdep.c:3165 [inline]
>  check_prevs_add kernel/locking/lockdep.c:3284 [inline]
>  validate_chain+0xb9b/0x2140 kernel/locking/lockdep.c:3908
>  __lock_acquire+0xab9/0xd20 kernel/locking/lockdep.c:5237
>  reacquire_held_locks+0x127/0x1d0 kernel/locking/lockdep.c:5385
>  __lock_release kernel/locking/lockdep.c:5574 [inline]
>  lock_release+0x1b4/0x3e0 kernel/locking/lockdep.c:5889
>  __local_bh_enable_ip+0x10c/0x270 kernel/softirq.c:228
>  hrtimer_cancel+0x39/0x60 kernel/time/hrtimer.c:1491
>  dummy_hrtimer_stop+0xcf/0x100 sound/drivers/dummy.c:410
>  snd_pcm_do_stop+0x127/0x1c0 sound/core/pcm_native.c:1525
>  snd_pcm_action_single sound/core/pcm_native.c:1305 [inline]
>  snd_pcm_action+0xe4/0x240 sound/core/pcm_native.c:1388
>  snd_pcm_stop sound/core/pcm_native.c:1561 [inline]
>  snd_pcm_drop+0x160/0x270 sound/core/pcm_native.c:2228
>  snd_pcm_oss_sync+0x1de/0xc30 sound/core/oss/pcm_oss.c:1733
>  snd_pcm_oss_release+0x102/0x250 sound/core/oss/pcm_oss.c:2574
>  __fput+0x45b/0xa80 fs/file_table.c:468
>  task_work_run+0x1d4/0x260 kernel/task_work.c:227
>  resume_user_mode_work include/linux/resume_user_mode.h:50 [inline]
>  exit_to_user_mode_loop+0xec/0x110 kernel/entry/common.c:43
>  exit_to_user_mode_prepare include/linux/irq-entry-common.h:225 [inline]
>  syscall_exit_to_user_mode_work include/linux/entry-common.h:175 [inline]
>  syscall_exit_to_user_mode include/linux/entry-common.h:210 [inline]
>  do_syscall_64+0x2bd/0x3b0 arch/x86/entry/syscall_64.c:100
>  entry_SYSCALL_64_after_hwframe+0x77/0x7f
> RIP: 0033:0x7fc9f7ffeec9
> Code: ff ff c3 66 2e 0f 1f 84 00 00 00 00 00 0f 1f 40 00 48 89 f8 48 89 f7 48 89 d6 48 89 ca 4d 89 c2 4d 89 c8 4c 8b 4c 24 08 0f 05 <48> 3d 01 f0 ff ff 73 01 c3 48 c7 c1 a8 ff ff ff f7 d8 64 89 01 48
> RSP: 002b:00007ffef674cdc8 EFLAGS: 00000246 ORIG_RAX: 00000000000001b4
> RAX: 0000000000000000 RBX: 0000000000036070 RCX: 00007fc9f7ffeec9
> RDX: 0000000000000000 RSI: 000000000000001e RDI: 0000000000000003
> RBP: 0000000000000000 R08: 0000000000000001 R09: 0000000bf674d0bf
> R10: 0000001b31c20000 R11: 0000000000000246 R12: 00007fc9f8255fac
> R13: 00007fc9f8255fa0 R14: ffffffffffffffff R15: 0000000000000003
>  </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@googlegroups.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
> 
> If you want syzbot to run the reproducer, reply with:
> #syz test: git://repo/address.git branch-or-commit-hash
> If you attach or paste a git patch, syzbot will apply it before testing.
> 
> If you want to overwrite report's subsystems, reply with:
> #syz set subsystems: new-subsystem
> (See the list of subsystem names on the web dashboard)
> 
> If the report is a duplicate of another one, reply with:
> #syz dup: exact-subject-of-another-report
> 
> If you want to undo deduplication, reply with:
> #syz undup

#syz fix: ALSA: pcm: Disable bottom softirqs as part of spin_lock_irq() on PREEMPT_RT