Comment 0 for bug 1950666

[IMPACT]

Commit 3b9a907223d7 (ipmi: fix sleep-in-atomic in free_user at cleanup SRCU user->release_barrier)
pushes the removal of an ipmi_user into the system's workqueue.

Whenever an ipmi_user struct is about to be removed it is scheduled as a work on
the system's workqueue to guarantee the free operation won't be executed in atomic context.
When the work is executed the free_user_work() function is invoked which frees the ipmi_user.

When ipmi_msghandler module is removed in cleanup_ipmi() function, there is no
check if there are any pending works to be executed.
Therefore, there is a potential race condition :
An ipmi_user is scheduled for removal and shortly to remove ipmi_msghandler module.
If the scheduled work delays execution for any reason and the module is removed
first then when the work is executed the pages of free_user_work() are gone and
the system crashes with the following :

BUG: unable to handle page fault for address: ffffffffc05c3450
#PF: supervisor instruction fetch in kernel mode
#PF: error_code(0x0010) - not-present page
PGD 635420e067 P4D 635420e067 PUD 6354210067 PMD 4711e51067 PTE 0
Oops: 0010 [#1] SMP PTI
CPU: 19 PID: 29646 Comm: kworker/19:1 Kdump: loaded Not tainted 5.4.0-77-generic #86~18.04.1-Ubuntu
Hardware name: Ciara Technologies ORION RS610-G4-DTH4S/MR91-FS1-Y9, BIOS F29 05/23/2019
Workqueue: events 0xffffffffc05c3450
RIP: 0010:0xffffffffc05c3450
Code: Bad RIP value.
RSP: 0018:ffffb721333c3e88 EFLAGS: 00010286
RAX: ffffffffc05c3450 RBX: ffff92a95f56a740 RCX: ffffb7221cfd14e8
RDX: 0000000000000001 RSI: ffff92616040d4b0 RDI: ffffb7221cf404e0
RBP: ffffb721333c3ec0 R08: 000073746e657665 R09: 8080808080808080
R10: ffffb721333c3de0 R11: fefefefefefefeff R12: ffff92a95f570700
R13: ffff92a0a40ece40 R14: ffffb7221cf404e0 R15: 0ffff92a95f57070
FS: 0000000000000000(0000) GS:ffff92a95f540000(0000) knlGS:0000000000000000
CS: 0010 DS: 0000 ES: 0000 CR0: 0000000080050033
CR2: ffffffffc05c3426 CR3: 00000081e9bfc005 CR4: 00000000007606e0
DR0: 0000000000000000 DR1: 0000000000000000 DR2: 0000000000000000
DR3: 0000000000000000 DR6: 00000000fffe0ff0 DR7: 0000000000000400
PKRU: 55555554
Call Trace:
? process_one_work+0x20f/0x400
worker_thread+0x34/0x410
kthread+0x121/0x140
? process_one_work+0x400/0x400
? kthread_park+0x90/0x90
ret_from_fork+0x35/0x40
Modules linked in: xt_REDIRECT xt_owner ipt_rpfilter xt_CT xt_multiport xt_set ip_set_hash_ip veth xt_statistic ipt_REJECT
... megaraid_sas ahci libahci wmi [last unloaded: ipmi_msghandler]
CR2: ffffffffc05c3450

[TEST CASE]

[WHERE PROBLEMS COULD OCCUR]

[OTHER]

Upstream is affected too, working on a patch to address this.