soft lockup in fanotify_read

[fruggeri@arista.com (Francesco Ruggeri)]

We are getting this soft lockup in fanotify_read.
The reason is that this code does not seem to scale to cases where there
are big bursts of events generated by fanotify_handle_event.
fanotify_read acquires group->notification_lock for each event.
fanotify_handle_event uses the lock to add one event, which also involves
fanotify_merge, which scans the whole list trying to find an event to
merge the new one with.
In our case fanotify_read is invoked with a buffer big enough for 200
events, and what happens is that every time fanotify_read dequeues an
event and releases the lock, fanotify_handle_event adds several more,
scanning a longer and longer list. This causes fanotify_read to wait
longer and longer for the lock, and the soft lockup happens before
fanotify_read can reach 200 events.
Is it intentional for fanotify_read to acquire the lock for each event,
rather than batching together a user buffer worth of events?

Thanks,
Francesco Ruggeri

[5752801.578813] watchdog: BUG: soft lockup - CPU#15 stuck for 22s! [fstrace:23105]
[5752801.586804] Modules linked in: ...
[5752801.586871] CPU: 15 PID: 23105 Comm: fstrace Tainted: G        W  O L    4.19.112-16802951.AroraKernel419.el7.x86_64 #1
[5752801.586872] Hardware name: Supermicro SYS-6029TP-HTR/X11DPT-PS, BIOS 3.1 04/30/2019
[5752801.586879] RIP: 0010:queued_spin_lock_slowpath+0x141/0x17c
[5752801.586881] Code: 5e c1 e9 12 83 e0 03 ff c9 48 c1 e0 04 48 63 c9 48 05 c0 14 02 00 48 03 04 cd 20 e7 e9 81 48 89 10 8b 42 08 85 c0 75 04 f3 90 <eb> f5 48 8b 0a 48 85 c9 74 03 0f 0d 09 8b 07 66 85 c0 74 93 f3 90
[5752801.586882] RSP: 0018:ffffc90027c5fd18 EFLAGS: 00000246 ORIG_RAX: ffffffffffffff13
[5752801.586883] RAX: 0000000000000000 RBX: ffff8883a9078ce8 RCX: 000000000000001e
[5752801.586884] RDX: ffff88df7f6e14c0 RSI: 0000000000400000 RDI: ffff88b06306e30c
[5752801.586885] RBP: ffffc90027c5fd18 R08: 0000000000000001 R09: ffffffff811e8b7e
[5752801.586886] R10: ffffc90027c5fce0 R11: 0000000000000000 R12: ffff88b06306e300
[5752801.586887] R13: 00000000ff935638 R14: ffff88b4af422600 R15: 0000000000000018
[5752801.586888] FS:  0000000000000000(0000) GS:ffff88df7f6c0000(0063) knlGS:0000000009c6a880
[5752801.586889] CS:  0010 DS: 002b ES: 002b CR0: 0000000080050033
[5752801.586890] CR2: 0000000009430914 CR3: 000000035bbef006 CR4: 00000000007606e0
[5752801.586891] DR0: 000000000001864c DR1: 0000000000000000 DR2: 0000000000000000
[5752801.586891] DR3: 0000000000000000 DR6: 00000000fffe0ff0 DR7: 0000000000000600
[5752801.586892] PKRU: 55555554
[5752801.586892] Call Trace:
[5752801.586898]  do_raw_spin_lock+0x1a/0x1c
[5752801.586901]  _raw_spin_lock+0xe/0x10
[5752801.586905]  fanotify_read+0xa3/0x32d
[5752801.586912]  ? __wake_up_sync+0x12/0x12
[5752801.586916]  copy_oldmem_page+0xa9/0xa9
[5752801.586920]  ? fsnotify_perm+0x60/0x6c
[5752801.586921]  ? fsnotify_perm+0x60/0x6c
[5752801.586923]  ? security_file_permission+0x37/0x3e
[5752801.586926]  vfs_read+0xa4/0xdc
[5752801.586928]  ksys_read+0x64/0xab
[5752801.586930]  __ia32_sys_read+0x18/0x1a
[5752801.586933]  do_fast_syscall_32+0xaf/0xf6
[5752801.586935]  entry_SYSENTER_compat+0x6b/0x7a
[5752801.586936] RIP: 0023:0xf7f64c29
[5752801.586938] Code: 5b 5d c3 8b 04 24 c3 8b 14 24 c3 8b 1c 24 c3 8b 34 24 c3 90 90 90 90 90 90 90 90 90 90 90 90 90 90 51 52 55 89 e5 0f 34 cd 80 <5d> 5a 59 c3 90 90 90 90 eb 0d 90 90 90 90 90 90 90 90 90 90 90 90
[5752801.586939] RSP: 002b:00000000ff933828 EFLAGS: 00000246 ORIG_RAX: 0000000000000003
[5752801.586940] RAX: ffffffffffffffda RBX: 0000000000000004 RCX: 00000000ff9348d0
[5752801.586941] RDX: 00000000000012c0 RSI: 0000000009c72da8 RDI: 00000000ff9348d0
[5752801.586941] RBP: 00000000ff935ba8 R08: 0000000000000000 R09: 0000000000000000
[5752801.586942] R10: 0000000000000000 R11: 0000000000000000 R12: 0000000000000000
[5752801.586942] R13: 0000000000000000 R14: 0000000000000000 R15: 0000000000000000