Re: [syzbot] [net?] possible deadlock in ptp_clock_unregister

[syzbot <syzbot+eb6f218811a9d721fd53@syzkaller.appspotmail.com>]

syzbot has found a reproducer for the following issue on:

HEAD commit:    19272b37aa4f Linux 6.16-rc1
git tree:       upstream
console output: https://syzkaller.appspot.com/x/log.txt?x=13bc5a0c580000
kernel config:  https://syzkaller.appspot.com/x/.config?x=c4c8362784bb7796
dashboard link: https://syzkaller.appspot.com/bug?extid=eb6f218811a9d721fd53
compiler:       gcc (Debian 12.2.0-14) 12.2.0, GNU ld (GNU Binutils for Debian) 2.40
syz repro:      https://syzkaller.appspot.com/x/repro.syz?x=15848d70580000
C reproducer:   https://syzkaller.appspot.com/x/repro.c?x=17bc5a0c580000

Downloadable assets:
disk image: https://storage.googleapis.com/syzbot-assets/0ceded60de88/disk-19272b37.raw.xz
vmlinux: https://storage.googleapis.com/syzbot-assets/51f4990c81f8/vmlinux-19272b37.xz
kernel image: https://storage.googleapis.com/syzbot-assets/7addf73549b9/bzImage-19272b37.xz

IMPORTANT: if you fix the issue, please add the following tag to the commit:
Reported-by: syzbot+eb6f218811a9d721fd53@syzkaller.appspotmail.com

ptp ptp0: delete virtual clock ptp15
============================================
WARNING: possible recursive locking detected
6.16.0-rc1-syzkaller #0 Not tainted
--------------------------------------------
syz-executor421/5891 is trying to acquire lock:
ffff888079fcc868 (&ptp->n_vclocks_mux){+.+.}-{4:4}, at: ptp_vclock_in_use drivers/ptp/ptp_private.h:103 [inline]
ffff888079fcc868 (&ptp->n_vclocks_mux){+.+.}-{4:4}, at: ptp_clock_unregister+0x21/0x250 drivers/ptp/ptp_clock.c:415

but task is already holding lock:
ffff8880300cc868 (&ptp->n_vclocks_mux){+.+.}-{4:4}, at: n_vclocks_store+0xf1/0x6d0 drivers/ptp/ptp_sysfs.c:215

other info that might help us debug this:
 Possible unsafe locking scenario:

       CPU0
       ----
  lock(&ptp->n_vclocks_mux);
  lock(&ptp->n_vclocks_mux);

 *** DEADLOCK ***

 May be due to missing lock nesting notation

4 locks held by syz-executor421/5891:
 #0: ffff88803631c428 (sb_writers#7){.+.+}-{0:0}, at: ksys_write+0x12a/0x250 fs/read_write.c:738
 #1: ffff888031c2c488 (&of->mutex){+.+.}-{4:4}, at: kernfs_fop_write_iter+0x28f/0x510 fs/kernfs/file.c:325
 #2: ffff8880303d4878 (kn->active#57){.+.+}-{0:0}, at: kernfs_fop_write_iter+0x2b2/0x510 fs/kernfs/file.c:326
 #3: ffff8880300cc868 (&ptp->n_vclocks_mux){+.+.}-{4:4}, at: n_vclocks_store+0xf1/0x6d0 drivers/ptp/ptp_sysfs.c:215

stack backtrace:
CPU: 1 UID: 0 PID: 5891 Comm: syz-executor421 Not tainted 6.16.0-rc1-syzkaller #0 PREEMPT(full) 
Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 05/07/2025
Call Trace:
 <TASK>
 __dump_stack lib/dump_stack.c:94 [inline]
 dump_stack_lvl+0x116/0x1f0 lib/dump_stack.c:120
 print_deadlock_bug+0x1e9/0x240 kernel/locking/lockdep.c:3044
 check_deadlock kernel/locking/lockdep.c:3096 [inline]
 validate_chain kernel/locking/lockdep.c:3898 [inline]
 __lock_acquire+0x1106/0x1c90 kernel/locking/lockdep.c:5240
 lock_acquire kernel/locking/lockdep.c:5871 [inline]
 lock_acquire+0x179/0x350 kernel/locking/lockdep.c:5828
 __mutex_lock_common kernel/locking/mutex.c:602 [inline]
 __mutex_lock+0x199/0xb90 kernel/locking/mutex.c:747
 ptp_vclock_in_use drivers/ptp/ptp_private.h:103 [inline]
 ptp_clock_unregister+0x21/0x250 drivers/ptp/ptp_clock.c:415
 ptp_vclock_unregister+0x11a/0x160 drivers/ptp/ptp_vclock.c:228
 unregister_vclock+0x108/0x1a0 drivers/ptp/ptp_sysfs.c:177
 device_for_each_child_reverse+0x136/0x1a0 drivers/base/core.c:4051
 n_vclocks_store+0x4b6/0x6d0 drivers/ptp/ptp_sysfs.c:241
 dev_attr_store+0x58/0x80 drivers/base/core.c:2440
 sysfs_kf_write+0xf2/0x150 fs/sysfs/file.c:145
 kernfs_fop_write_iter+0x351/0x510 fs/kernfs/file.c:334
 new_sync_write fs/read_write.c:593 [inline]
 vfs_write+0x6c4/0x1150 fs/read_write.c:686
 ksys_write+0x12a/0x250 fs/read_write.c:738
 do_syscall_x64 arch/x86/entry/syscall_64.c:63 [inline]
 do_syscall_64+0xcd/0x490 arch/x86/entry/syscall_64.c:94
 entry_SYSCALL_64_after_hwframe+0x77/0x7f
RIP: 0033:0x7f577c6b6f29
Code: 28 00 00 00 75 05 48 83 c4 28 c3 e8 e1 1d 00 00 90 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 b0 ff ff ff f7 d8 64 89 01 48
RSP: 002b:00007ffefa44cc98 EFLAGS: 00000246 ORIG_RAX: 0000000000000001
RAX: ffffffffffffffda RBX: 0000000000000003 RCX: 00007f577c6b6f29
RDX: 00000000000005c8 RSI: 0000000000000000 RDI: 0000000000000003
RBP: 0000000000000000 R08: 0000000000000000 R09: 0000000000000000
R10: 0000000000000000 R11: 0000000000000246 R12: 00007ffefa44ccdc
R13: 00007ffefa44ccf0 R14: 00007ffefa44cd30 R15: 0000000000000004
 </TASK>
ptp ptp0: guarantee physical clock free running


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