Hi Waiman,

I've received reports of lockups happening in kernels including
commit d257cc8cb8d5 ("locking/rwsem: Make handoff bit handling more
consistent"). The exact symptoms vary but usually it's either a soft lockup
(older kernel with a backport), the task hanging and never exiting or the
machine becomes generally unresponsive and ssh is broken.  The problem
started in 5.16 and reliably bisected to commit d257cc8cb8d5. Reverting
the patch in 5.16, 5.17 and 5.18 finish the test successfully but I didn't
test a revert on 5.19-rc2 because of other changes layered on top.

The reproducer is simple -- start pairs of CPU hogs pinned to a CPU with
different SCHED_RR priorities that run for a few seconds. It does not
hit every time but usually happens within 10 attempts. On 5.16 at least,
the tasks failed to exit and kept retrying to exit using the following path

[<0>] rwsem_down_write_slowpath+0x2ad/0x580
[<0>] unlink_file_vma+0x2c/0x50
[<0>] free_pgtables+0xbe/0x110
[<0>] exit_mmap+0xc1/0x220
[<0>] mmput+0x52/0x110
[<0>] do_exit+0x2ec/0xb00
[<0>] do_group_exit+0x2d/0x90
[<0>] get_signal+0xb6/0x920
[<0>] arch_do_signal_or_restart+0xba/0x700
[<0>] exit_to_user_mode_prepare+0xb7/0x230
[<0>] irqentry_exit_to_user_mode+0x5/0x20
[<0>] asm_sysvec_apic_timer_interrupt+0x12/0x20
[<0>] preempt_schedule_thunk+0x16/0x18
[<0>] rwsem_down_write_slowpath+0x2ad/0x580
[<0>] unlink_file_vma+0x2c/0x50
[<0>] free_pgtables+0xbe/0x110
[<0>] exit_mmap+0xc1/0x220
[<0>] mmput+0x52/0x110
[<0>] do_exit+0x2ec/0xb00
[<0>] do_group_exit+0x2d/0x90
[<0>] get_signal+0xb6/0x920
[<0>] arch_do_signal_or_restart+0xba/0x700
[<0>] exit_to_user_mode_prepare+0xb7/0x230
[<0>] irqentry_exit_to_user_mode+0x5/0x20
[<0>] asm_sysvec_apic_timer_interrupt+0x12/0x20

The C file and shell script to run it are attached.

-- 
Mel Gorman
SUSE Labs