[RT] 5.9-rt14 softirq_ctrl.lock vs listening_hash[i].lock lockdep splat

[Mike Galbraith]

[   47.844511] ======================================================
[   47.844511] WARNING: possible circular locking dependency detected
[   47.844512] 5.9.0.gc85fb28-rt14-rt #1 Tainted: G            E
[   47.844513] ------------------------------------------------------
[   47.844514] perl/2751 is trying to acquire lock:
[   47.844515] ffff92cadec5a410 ((softirq_ctrl.lock).lock){+.+.}-{2:2}, at: __local_bh_disable_ip+0x127/0x2c0
[   47.844521]
               but task is already holding lock:
[   47.844522] ffffffffa8871468 (&h->listening_hash[i].lock){+.+.}-{0:0}, at: listening_get_next.isra.41+0xd7/0x130
[   47.844528]
               which lock already depends on the new lock.

[   47.844528]
               the existing dependency chain (in reverse order) is:
[   47.844529]
               -> #1 (&h->listening_hash[i].lock){+.+.}-{0:0}:
[   47.844532]        rt_spin_lock+0x2b/0xc0
[   47.844536]        __inet_hash+0x68/0x320
[   47.844539]        inet_hash+0x31/0x60
[   47.844541]        inet_csk_listen_start+0xaf/0xe0
[   47.844543]        inet_listen+0x86/0x150
[   47.844546]        __sys_listen+0x58/0x80
[   47.844548]        __x64_sys_listen+0x12/0x20
[   47.844549]        do_syscall_64+0x33/0x40
[   47.844552]        entry_SYSCALL_64_after_hwframe+0x44/0xa9
[   47.844555]
               -> #0 ((softirq_ctrl.lock).lock){+.+.}-{2:2}:
[   47.844557]        __lock_acquire+0x1343/0x1890
[   47.844560]        lock_acquire+0x92/0x410
[   47.844562]        rt_spin_lock+0x2b/0xc0
[   47.844564]        __local_bh_disable_ip+0x127/0x2c0
[   47.844566]        sock_i_ino+0x22/0x60
[   47.844569]        tcp4_seq_show+0x14f/0x420
[   47.844571]        seq_read+0x27c/0x420
[   47.844574]        proc_reg_read+0x5c/0x80
[   47.844576]        vfs_read+0xd1/0x1d0
[   47.844580]        ksys_read+0x87/0xc0
[   47.844581]        do_syscall_64+0x33/0x40
[   47.844583]        entry_SYSCALL_64_after_hwframe+0x44/0xa9
[   47.844585]
               other info that might help us debug this:

[   47.844585]  Possible unsafe locking scenario:

[   47.844586]        CPU0                    CPU1
[   47.844586]        ----                    ----
[   47.844587]   lock(&h->listening_hash[i].lock);
[   47.844588]                                lock((softirq_ctrl.lock).lock);
[   47.844588]                                lock(&h->listening_hash[i].lock);
[   47.844589]   lock((softirq_ctrl.lock).lock);
[   47.844590]
                *** DEADLOCK ***

[   47.844590] 3 locks held by perl/2751:
[   47.844591]  #0: ffff92ca6525a4e0 (&p->lock){+.+.}-{0:0}, at: seq_read+0x37/0x420
[   47.844594]  #1: ffffffffa8871468 (&h->listening_hash[i].lock){+.+.}-{0:0}, at: listening_get_next.isra.41+0xd7/0x130
[   47.844597]  #2: ffffffffa74b90e0 (rcu_read_lock){....}-{1:2}, at: rt_spin_lock+0x5/0xc0
[   47.844600]
               stack backtrace:
[   47.844601] CPU: 1 PID: 2751 Comm: perl Kdump: loaded Tainted: G            E     5.9.0.gc85fb28-rt14-rt #1
[   47.844603] Hardware name: MEDION MS-7848/MS-7848, BIOS M7848W08.20C 09/23/2013
[   47.844604] Call Trace:
[   47.844606]  dump_stack+0x77/0x9b
[   47.844611]  check_noncircular+0x148/0x160
[   47.844616]  ? __lock_acquire+0x1343/0x1890
[   47.844617]  __lock_acquire+0x1343/0x1890
[   47.844621]  lock_acquire+0x92/0x410
[   47.844623]  ? __local_bh_disable_ip+0x127/0x2c0
[   47.844626]  ? sock_i_ino+0x5/0x60
[   47.844628]  rt_spin_lock+0x2b/0xc0
[   47.844630]  ? __local_bh_disable_ip+0x127/0x2c0
[   47.844631]  __local_bh_disable_ip+0x127/0x2c0
[   47.844634]  sock_i_ino+0x22/0x60
[   47.844635]  tcp4_seq_show+0x14f/0x420
[   47.844640]  seq_read+0x27c/0x420
[   47.844643]  proc_reg_read+0x5c/0x80
[   47.844645]  vfs_read+0xd1/0x1d0
[   47.844648]  ksys_read+0x87/0xc0
[   47.844649]  ? lockdep_hardirqs_on+0x78/0x100
[   47.844652]  do_syscall_64+0x33/0x40
[   47.844654]  entry_SYSCALL_64_after_hwframe+0x44/0xa9
[   47.844656] RIP: 0033:0x7fb3f3c23e51
[   47.844658] Code: 7d 81 20 00 f7 d8 64 89 02 48 c7 c0 ff ff ff ff eb ba 0f 1f 80 00 00 00 00 8b 05 1a c3 20 00 48 63 ff 85 c0 75 13 31 c0 0f 05 <48> 3d 00 f0 ff ff 77 57 f3 c3 0f 1f 44 00 00 55 53 48 89 d5 48 89
[   47.844660] RSP: 002b:00007ffd7604f108 EFLAGS: 00000246 ORIG_RAX: 0000000000000000
[   47.844661] RAX: ffffffffffffffda RBX: 0000000000000000 RCX: 00007fb3f3c23e51
[   47.844662] RDX: 0000000000002000 RSI: 000055dbff4da600 RDI: 0000000000000003
[   47.844662] RBP: 0000000000002000 R08: 000055dbff4d9290 R09: 000055dbff4da600
[   47.844663] R10: ffffffffffffffb0 R11: 0000000000000246 R12: 000055dbff4da600
[   47.844664] R13: 000055dbff4ae260 R14: 000055dbff4d92c0 R15: 0000000000000003