Re: Lockups due to "locking/rwsem: Make handoff bit handling more consistent"

[Waiman Long]

On 6/17/22 09:43, Mel Gorman wrote:
> 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.
Thanks for the reproducer and I will try to reproduce it locally.

It is a known issue that I have receive similar report from an Oracle 
engineer. That is the reason I posted commit 1ee326196c66 
("locking/rwsem: Always try to wake waiters in out_nolock path") that 
was merged in v5.19. I believe it helps but it may not be able to 
eliminate all possible race conditions. To make rwsem behave more like 
before commit d257cc8cb8d5 ("locking/rwsem: Make handoff bit handling 
more consistent"), I posted a follow-up patch

https://lore.kernel.org/lkml/20220427173124.1428050-1-longman@xxxxxxxxxx/

But it hasn't gotten review yet.

I will try your reproducer to see if these patches are able to address 
the lockup problem.

Thanks,
Longman

commit d257cc8cb8d5 ("locking/rwsem: Make handoff bit handling more
consistent").