Issue

• The server is getting hung up.

• The server crashes if hung_task_panic is enabled. Otherwise a hard reboot is the only resolution once the server gets hung up completely.

• Blocked task message observed in kernel ring buffer:

[13919.504321] INFO: task fsbspamd:19047 blocked for more than 120 seconds.
[13919.504396] "echo 0 > /proc/sys/kernel/hung_task_timeout_secs" disables this message.
[13919.504433] fsbspamd        D ffff999c7fc1acc0     0 19047   1246 0x00000080
[13919.504458] Call Trace:
[13919.504595]  [<ffffffffb0b85da9>] schedule+0x29/0x70
[13919.504668]  [<ffffffffb0699d55>] fanotify_handle_event+0x265/0x3a0
[13919.504704]  [<ffffffffb0acd65c>] ? udp_send_skb+0xac/0x2b0
[13919.504749]  [<ffffffffb04c7780>] ? wake_up_atomic_t+0x30/0x30
[13919.504754]  [<ffffffffb0696128>] fsnotify+0x388/0x460
[13919.504786]  [<ffffffffb0707f3e>] security_file_open+0x6e/0x70
[13919.504822]  [<ffffffffb064a7b9>] do_dentry_open+0xc9/0x2d0
[13919.504829]  [<ffffffffb0707992>] ? security_inode_permission+0x22/0x30
[13919.504834]  [<ffffffffb064aa5a>] vfs_open+0x5a/0xb0
[13919.504853]  [<ffffffffb06590da>] ? may_open+0x5a/0x120
[13919.504860]  [<ffffffffb065d006>] do_last+0x1f6/0x1340
[13919.504866]  [<ffffffffb065e21d>] path_openat+0xcd/0x5a0
[13919.504872]  [<ffffffffb066046d>] do_filp_open+0x4d/0xb0
[13919.504884]  [<ffffffffb066e512>] ? __alloc_fd+0xc2/0x170
[13919.504890]  [<ffffffffb064c044>] do_sys_open+0x124/0x220
[13919.504896]  [<ffffffffb064c15e>] SyS_open+0x1e/0x20
[13919.504910]  [<ffffffffb0b92ed2>] system_call_fastpath+0x25/0x2a

• Lots of tasks are calling fanotify_handle_event(), waiting for permission event response from userspace, and getting stuck in TASK_UNINTERRUPTIBLE sleep indefinitely.

• At this time, a task, that is responsible for responding with permission event response back to the tasks, itself is waiting for permission event response from userspace as well, which is a deadlock condition caused by userspace bug.