[syzbot] [dri?] WARNING in __ww_mutex_wound

[syzbot]

Hello,

syzbot found the following issue on:

HEAD commit: d086c886ceb9 Add linux-next specific files for 20250718
git tree: linux-next
console output: https://syzkaller.appspot.com/x/log.txt?x=161204a2580000
kernel config: https://syzkaller.appspot.com/x/.config?x=69896dd7b8c4e81e
dashboard link: https://syzkaller.appspot.com/bug?extid=602c4720aed62576cd79
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=16fff4f0580000
C reproducer: https://syzkaller.appspot.com/x/repro.c?x=111204a2580000

Downloadable assets:
disk image: https://storage.googleapis.com/syzbot-assets/54504fbc2437/disk-d086c886.raw.xz
vmlinux: https://storage.googleapis.com/syzbot-assets/b427b00abffe/vmlinux-d086c886.xz
kernel image: https://storage.googleapis.com/syzbot-assets/5a87731b006b/bzImage-d086c886.xz

IMPORTANT: if you fix the issue, please add the following tag to the commit:
Reported-by: syzbot+602c47...@syzkaller.appspotmail.com

------------[ cut here ]------------
WARNING: ./include/linux/sched.h:2173 at __clear_task_blocked_on include/linux/sched.h:2173 [inline], CPU#1: syz.1.8698/395
WARNING: ./include/linux/sched.h:2173 at __ww_mutex_wound+0x21a/0x2b0 kernel/locking/ww_mutex.h:346, CPU#1: syz.1.8698/395
Modules linked in:
CPU: 1 UID: 0 PID: 395 Comm: syz.1.8698 Not tainted 6.16.0-rc6-next-20250718-syzkaller #0 PREEMPT(full)
Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 07/12/2025
RIP: 0010:__clear_task_blocked_on include/linux/sched.h:2173 [inline]
RIP: 0010:__ww_mutex_wound+0x21a/0x2b0 kernel/locking/ww_mutex.h:346
Code: 5f 5d c3 cc cc cc cc cc 90 0f 0b 90 e9 89 fe ff ff 90 0f 0b 90 e9 39 ff ff ff 90 0f 0b 90 4d 85 ff 0f 85 67 ff ff ff eb 95 90 <0f> 0b 90 eb 8f 48 c7 c1 70 00 e4 8f 80 e1 07 80 c1 03 38 c1 0f 8c
RSP: 0018:ffffc900030e7720 EFLAGS: 00010046
RAX: ffff888024a1b000 RBX: dffffc0000000000 RCX: 8f1a7ab232ebe500
RDX: 00000000000003ef RSI: ffffffff8de5b067 RDI: ffffffff8c04d400
RBP: 0000000000000001 R08: ffff888024a224bf R09: 1ffff11004944497
R10: dffffc0000000000 R11: ffffed1004944498 R12: ffff88802dee8a78
R13: ffffc900030e7ae8 R14: ffff88802dee8000 R15: ffff888024a224b8
FS: 00007fe6e4a7f6c0(0000) GS:ffff8881258ab000(0000) knlGS:0000000000000000
CS: 0010 DS: 0000 ES: 0000 CR0: 0000000080050033
CR2: 00007fe6e4a5ed58 CR3: 000000003115e000 CR4: 00000000003526f0
Call Trace:
<TASK>
__ww_mutex_add_waiter kernel/locking/ww_mutex.h:574 [inline]
__mutex_lock_common kernel/locking/mutex.c:642 [inline]
__ww_mutex_lock+0xba3/0x2930 kernel/locking/mutex.c:771
ww_mutex_lock_interruptible+0x3f/0x1c0 kernel/locking/mutex.c:904
modeset_lock+0x21a/0x650 drivers/gpu/drm/drm_modeset_lock.c:-1
drm_modeset_lock drivers/gpu/drm/drm_modeset_lock.c:398 [inline]
drm_modeset_lock_all_ctx+0x62/0x300 drivers/gpu/drm/drm_modeset_lock.c:459
setplane_internal drivers/gpu/drm/drm_plane.c:1118 [inline]
drm_mode_setplane+0x577/0xba0 drivers/gpu/drm/drm_plane.c:1175
drm_ioctl_kernel+0x2cc/0x390 drivers/gpu/drm/drm_ioctl.c:796
drm_ioctl+0x67f/0xb10 drivers/gpu/drm/drm_ioctl.c:893
vfs_ioctl fs/ioctl.c:51 [inline]
__do_sys_ioctl fs/ioctl.c:598 [inline]
__se_sys_ioctl+0xf9/0x170 fs/ioctl.c:584
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: 0033:0x7fe6e3b8e9a9
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:00007fe6e4a7f038 EFLAGS: 00000246 ORIG_RAX: 0000000000000010
RAX: ffffffffffffffda RBX: 00007fe6e3db5fa0 RCX: 00007fe6e3b8e9a9
RDX: 0000200000000080 RSI: 00000000c03064b7 RDI: 0000000000000003
RBP: 00007fe6e3c10d69 R08: 0000000000000000 R09: 0000000000000000
R10: 0000000000000000 R11: 0000000000000246 R12: 0000000000000000
R13: 0000000000000000 R14: 00007fe6e3db5fa0 R15: 00007ffdf1fa90a8
</TASK>


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[Maarten Lankhorst]

Hey,

This warning is introduced in linux-next as a4f0b6fef4b0 ("locking/mutex: Add p->blocked_on wrappers for correctness checks")
Adding relevant people from that commit.

Kind regards,
~Maarten

[K Prateek Nayak]

When wounding the lock owner, could it be possible that the lock
owner is blocked on a different nested lock? Lock owner implies it
is not blocked on the current lock we are trying to wound right?

I remember John mentioning seeing circular chains in find_proxy_task()
which required this but looking at this call-chain I'm wondering if
only the __ww_mutex_check_waiters() (or some other path) requires
__clear_task_blocked_on() for that case.

--
Thanks and Regards,
Prateek

[Hillf Danton]

It is buggy to read and clear owner->blocked_on without
owner->blocked_on->wait_lock held, no?

[John Stultz]

On Wed, Jul 30, 2025 at 2:50 AM K Prateek Nayak <kpratee...@amd.com> wrote:
> On 7/30/2025 1:57 PM, Maarten Lankhorst wrote:
> > Hey,
> >
> > This warning is introduced in linux-next as a4f0b6fef4b0 ("locking/mutex: Add p->blocked_on wrappers for correctness checks")
> > Adding relevant people from that commit.
> >

...

> >> ------------[ cut here ]------------
> >> WARNING: ./include/linux/sched.h:2173 at __clear_task_blocked_on include/linux/sched.h:2173 [inline], CPU#1: syz.1.8698/395
> >> WARNING: ./include/linux/sched.h:2173 at __ww_mutex_wound+0x21a/0x2b0 kernel/locking/ww_mutex.h:346, CPU#1: syz.1.8698/395
> >> Modules linked in:
> >> CPU: 1 UID: 0 PID: 395 Comm: syz.1.8698 Not tainted 6.16.0-rc6-next-20250718-syzkaller #0 PREEMPT(full)
> >> Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 07/12/2025
> >> RIP: 0010:__clear_task_blocked_on include/linux/sched.h:2173 [inline]
> >> RIP: 0010:__ww_mutex_wound+0x21a/0x2b0 kernel/locking/ww_mutex.h:346
>
> When wounding the lock owner, could it be possible that the lock
> owner is blocked on a different nested lock? Lock owner implies it
> is not blocked on the current lock we are trying to wound right?
>
> I remember John mentioning seeing circular chains in find_proxy_task()
> which required this but looking at this call-chain I'm wondering if
> only the __ww_mutex_check_waiters() (or some other path) requires
> __clear_task_blocked_on() for that case.

So yeah, I have tripped over this a few times (fixing and often later
re-introducing the problem) but usually later in my full proxy-exec
series, and somehow missed that the single-rq hit this.

Obviously with __ww_mutex_die() we are clearing the blocked on
relationship for the lock waiter, but in __ww_mutex_wound() we are
waking the lock *owner*, who might be waiting on a different lock, so
passing the held lock to the clear_task_blocked_on() checks trips
these warnings.

Passing NULL instead of lock is the right call here, I'll just need to
loosen the __clear_task_blocked_on() check for null as well.

I'll spin up a quick patch.

thanks
-john