======================================================
WARNING: possible circular locking dependency detected
syzkaller #0 Not tainted
------------------------------------------------------
syz.0.860/9399 is trying to acquire lock:
ffff888115da4aa0 (slock-AF_PHONET/1){+.+.}-{3:3}, at: __sk_receive_skb+0x1bf/0x9e0

but task is already holding lock:
ffff88817621b8a0 (slock-AF_PHONET){+...}-{3:3}, at: __sk_receive_skb+0x1f1/0x9e0

which lock already depends on the new lock.


the existing dependency chain (in reverse order) is:

-> #1 (slock-AF_PHONET){+...}-{3:3}:
       rt_spin_lock+0x83/0x400
       __sk_receive_skb+0x1f1/0x9e0
       phonet_rcv+0x781/0xc40
       process_backlog+0x5e1/0xc60
       __napi_poll+0xab/0x550
       net_rx_action+0x696/0xe00
       handle_softirqs+0x1de/0x6d0
       __local_bh_enable_ip+0x170/0x2b0
       netif_rx+0xb9/0xf0
       pn_send+0x62a/0x8e0
       pn_skb_send+0x218/0x530
       pipe_snd_status+0x1f1/0x320
       pipe_do_rcv+0xf15/0x16a0
       __sk_receive_skb+0x962/0x9e0
       pep_do_rcv+0x685/0xaa0
       __release_sock+0x2a9/0x3d0
       release_sock+0x1be/0x290
       pep_sock_accept+0xd47/0x11e0
       pn_socket_accept+0xc1/0x310
       do_accept+0x6ca/0x930
       __sys_accept4+0x139/0x230
       __x64_sys_accept4+0x9a/0xb0
       do_syscall_64+0x15f/0xf80
       entry_SYSCALL_64_after_hwframe+0x77/0x7f

-> #0 (slock-AF_PHONET/1){+.+.}-{3:3}:
       __lock_acquire+0x15a5/0x2d10
       lock_acquire+0x106/0x350
       rt_spin_lock_nested+0x81/0x3f0
       __sk_receive_skb+0x1bf/0x9e0
       pep_do_rcv+0x685/0xaa0
       __sk_receive_skb+0x962/0x9e0
       phonet_rcv+0x781/0xc40
       process_backlog+0x5e1/0xc60
       __napi_poll+0xab/0x550
       net_rx_action+0x696/0xe00
       handle_softirqs+0x1de/0x6d0
       __local_bh_enable_ip+0x170/0x2b0
       netif_rx+0xb9/0xf0
       pn_send+0x62a/0x8e0
       pn_skb_send+0x218/0x530
       pep_sock_close+0x2c1/0x5b0
       pn_socket_release+0x9b/0xc0
       __sock_release+0xb9/0x250
       sock_close+0x1c/0x30
       __fput+0x461/0xa70
       task_work_run+0x1d9/0x270
       exit_to_user_mode_loop+0xf3/0x4d0
       do_syscall_64+0x33e/0xf80
       entry_SYSCALL_64_after_hwframe+0x77/0x7f

other info that might help us debug this:

 Possible unsafe locking scenario:

       CPU0                    CPU1
       ----                    ----
  lock(slock-AF_PHONET);
                               lock(slock-AF_PHONET/1);
                               lock(slock-AF_PHONET);
  lock(slock-AF_PHONET/1);

 *** DEADLOCK ***

7 locks held by syz.0.860/9399:
 #0: ffff88817f0d82b8 (&sb->s_type->i_mutex_key#13){+.+.}-{4:4}, at: __sock_release+0x89/0x250
 #1: ffff88817621b058 (sk_lock-AF_PHONET){+.+.}-{0:0}, at: pep_sock_close+0x86/0x5b0
 #2: ffffffff8dfc8240 (rcu_read_lock){....}-{1:3}, at: __local_bh_disable_ip+0x3c/0x420
 #3: ffffffff8dfc8240 (rcu_read_lock){....}-{1:3}, at: process_backlog+0x271/0xc60
 #4: ffff88817621b8a0 (slock-AF_PHONET){+...}-{3:3}, at: __sk_receive_skb+0x1f1/0x9e0
 #5: ffffffff8dfc8240 (rcu_read_lock){....}-{1:3}, at: rt_spin_lock+0x1e0/0x400
 #6: ffff88817621b958 (sk_lock-AF_PHONET){+.+.}-{0:0}, at: phonet_rcv+0x781/0xc40

stack backtrace:
CPU: 1 UID: 0 PID: 9399 Comm: syz.0.860 Not tainted syzkaller #0 PREEMPT_{RT,(full)} 
Hardware name: QEMU Standard PC (Q35 + ICH9, 2009), BIOS 1.16.2-debian-1.16.2-1 04/01/2014
Call Trace:
 <TASK>
 dump_stack_lvl+0xe8/0x150
 print_circular_bug+0x2e1/0x300
 check_noncircular+0x12e/0x150
 __lock_acquire+0x15a5/0x2d10
 lock_acquire+0x106/0x350
 rt_spin_lock_nested+0x81/0x3f0
 __sk_receive_skb+0x1bf/0x9e0
 pep_do_rcv+0x685/0xaa0
 __sk_receive_skb+0x962/0x9e0
 phonet_rcv+0x781/0xc40
 process_backlog+0x5e1/0xc60
 __napi_poll+0xab/0x550
 net_rx_action+0x696/0xe00
 handle_softirqs+0x1de/0x6d0
 __local_bh_enable_ip+0x170/0x2b0
 netif_rx+0xb9/0xf0
 pn_send+0x62a/0x8e0
 pn_skb_send+0x218/0x530
 pep_sock_close+0x2c1/0x5b0
 pn_socket_release+0x9b/0xc0
 __sock_release+0xb9/0x250
 sock_close+0x1c/0x30
 __fput+0x461/0xa70
 task_work_run+0x1d9/0x270
 exit_to_user_mode_loop+0xf3/0x4d0
 do_syscall_64+0x33e/0xf80
 entry_SYSCALL_64_after_hwframe+0x77/0x7f
RIP: 0033:0x7fbf29b6cdd9
Code: ff c3 66 2e 0f 1f 84 00 00 00 00 00 0f 1f 44 00 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 e8 ff ff ff f7 d8 64 89 01 48
RSP: 002b:00007fbf27dbe028 EFLAGS: 00000246 ORIG_RAX: 000000000000002a
RAX: 0000000000000000 RBX: 00007fbf29de5fa0 RCX: 00007fbf29b6cdd9
RDX: 0000000000000010 RSI: 0000200000000040 RDI: 0000000000000007
RBP: 00007fbf29c02d69 R08: 0000000000000000 R09: 0000000000000000
R10: 0000000000000000 R11: 0000000000000246 R12: 0000000000000000
R13: 00007fbf29de6038 R14: 00007fbf29de5fa0 R15: 00007ffd5edd11c8
 </TASK>
