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

Hillf Danton
2025-08-30T06:56:51Z

> Date: Fri, 29 Aug 2025 20:03:05 -0700
> Hello,
>
> syzbot has tested the proposed patch but the reproducer is still triggering an issue:
> possible deadlock in __snd_pcm_lib_xfer
>
> ======================================================
> WARNING: possible circular locking dependency detected
> syzkaller #0 Not tainted
> ------------------------------------------------------
> syz.0.46/6843 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
>
Given softirq_ctrl is percpu, this report is false positive.

> but task is already holding lock:
> ffff88814ccb7150 (&group->lock#2){+.+.}-{3:3}, at: __snd_pcm_lib_xfer+0x386/0x1ce0 sound/core/pcm_lib.c:2319
>
> which lock already depends on the new lock.
>
>
> the existing dependency chain (in reverse order) is:
>
> -> #1 (&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:1763 [inline]
> __hrtimer_run_queues+0x590/0xda0 kernel/time/hrtimer.c:1829
> hrtimer_run_softirq+0x1a3/0x2e0 kernel/time/hrtimer.c:1847
> handle_softirqs+0x22c/0x710 kernel/softirq.c:579
> __do_softirq kernel/softirq.c:613 [inline]
> run_ktimerd+0xcf/0x190 kernel/softirq.c:1043
> smpboot_thread_fn+0x542/0xa60 kernel/smpboot.c:160
> kthread+0x711/0x8a0 kernel/kthread.c:463
> ret_from_fork+0x3f9/0x770 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+0x12a/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_xrun+0x27f/0x7c0 sound/core/pcm_lib.c:180
> snd_pcm_update_state+0x342/0x430 sound/core/pcm_lib.c:224
> snd_pcm_update_hw_ptr0+0x10b2/0x1b00 sound/core/pcm_lib.c:493
> snd_pcm_update_hw_ptr sound/core/pcm_lib.c:499 [inline]
> __snd_pcm_lib_xfer+0x510/0x1ce0 sound/core/pcm_lib.c:2326
> snd_pcm_oss_write3+0x1bc/0x320 sound/core/oss/pcm_oss.c:1241
> snd_pcm_plug_write_transfer+0x2cb/0x4c0 sound/core/oss/pcm_plugin.c:630
> snd_pcm_oss_write2 sound/core/oss/pcm_oss.c:1373 [inline]
> snd_pcm_oss_write1 sound/core/oss/pcm_oss.c:1439 [inline]
> snd_pcm_oss_write+0xba2/0x11a0 sound/core/oss/pcm_oss.c:2795
> vfs_write+0x287/0xb40 fs/read_write.c:684
> ksys_write+0x14b/0x260 fs/read_write.c:738
> do_syscall_x64 arch/x86/entry/syscall_64.c:63 [inline]
> do_syscall_64+0xfa/0x3b0 arch/x86/entry/syscall_64.c:94
> entry_SYSCALL_64_after_hwframe+0x77/0x7f
>
> other info that might help us debug this:
>
> Possible unsafe locking scenario:
>
> CPU0 CPU1
> ---- ----
> lock(&group->lock#2);
> lock((softirq_ctrl.lock));
> lock(&group->lock#2);
> lock((softirq_ctrl.lock));
>
> *** DEADLOCK ***
>
> 2 locks held by syz.0.46/6843:
> #0: ffff88814ccb7150 (&group->lock#2){+.+.}-{3:3}, at: __snd_pcm_lib_xfer+0x386/0x1ce0 sound/core/pcm_lib.c:2319
> #1: ffffffff8d9a8b80 (rcu_read_lock){....}-{1:3}, at: rcu_lock_acquire include/linux/rcupdate.h:331 [inline]
> #1: ffffffff8d9a8b80 (rcu_read_lock){....}-{1:3}, at: rcu_read_lock include/linux/rcupdate.h:841 [inline]
> #1: ffffffff8d9a8b80 (rcu_read_lock){....}-{1:3}, at: __rt_spin_lock kernel/locking/spinlock_rt.c:50 [inline]
> #1: ffffffff8d9a8b80 (rcu_read_lock){....}-{1:3}, at: rt_spin_lock+0x1bb/0x2c0 kernel/locking/spinlock_rt.c:57
>
> stack backtrace:
> CPU: 0 UID: 0 PID: 6843 Comm: syz.0.46 Not tainted syzkaller #0 PREEMPT_{RT,(full)}
> Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 07/12/2025
> 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+0x12a/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_xrun+0x27f/0x7c0 sound/core/pcm_lib.c:180
> snd_pcm_update_state+0x342/0x430 sound/core/pcm_lib.c:224
> snd_pcm_update_hw_ptr0+0x10b2/0x1b00 sound/core/pcm_lib.c:493
> snd_pcm_update_hw_ptr sound/core/pcm_lib.c:499 [inline]
> __snd_pcm_lib_xfer+0x510/0x1ce0 sound/core/pcm_lib.c:2326
> snd_pcm_oss_write3+0x1bc/0x320 sound/core/oss/pcm_oss.c:1241
> snd_pcm_plug_write_transfer+0x2cb/0x4c0 sound/core/oss/pcm_plugin.c:630
> snd_pcm_oss_write2 sound/core/oss/pcm_oss.c:1373 [inline]
> snd_pcm_oss_write1 sound/core/oss/pcm_oss.c:1439 [inline]
> snd_pcm_oss_write+0xba2/0x11a0 sound/core/oss/pcm_oss.c:2795
> vfs_write+0x287/0xb40 fs/read_write.c:684
> ksys_write+0x14b/0x260 fs/read_write.c:738
> do_syscall_x64 arch/x86/entry/syscall_64.c:63 [inline]
> do_syscall_64+0xfa/0x3b0 arch/x86/entry/syscall_64.c:94
> entry_SYSCALL_64_after_hwframe+0x77/0x7f
> RIP: 0033:0x7f9528fcebe9
> 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:00007f952863e038 EFLAGS: 00000246 ORIG_RAX: 0000000000000001
> RAX: ffffffffffffffda RBX: 00007f95291f5fa0 RCX: 00007f9528fcebe9
> RDX: 000000000000fc36 RSI: 0000200000000500 RDI: 0000000000000003
> RBP: 00007f9529051e19 R08: 0000000000000000 R09: 0000000000000000
> R10: 0000000000000000 R11: 0000000000000246 R12: 0000000000000000
> R13: 00007f95291f6038 R14: 00007f95291f5fa0 R15: 00007ffe192e0268
> </TASK>
>
>
> Tested on:
>
> commit: 11e7861d Merge tag 'for-linus' of git://git.kernel.org..
> git tree: upstream
> console output: https://syzkaller.appspot.com/x/log.txt?x=15d8d634580000
> kernel config: https://syzkaller.appspot.com/x/.config?x=bd9738e00c1bbfb4
> dashboard link: https://syzkaller.appspot.com/bug?extid=10b4363fb0f46527f3f3
> compiler: Debian clang version 20.1.8 (++20250708063551+0c9f909b7976-1~exp1~20250708183702.136), Debian LLD 20.1.8
> patch: https://syzkaller.appspot.com/x/patch.diff?x=11b7fef0580000
>
>