Re: [PATCH 4.19 061/191] padata: always acquire cpu_hotplug_lock before pinst->lock

[Daniel Jordan]

On Fri, Feb 21, 2020 at 08:40:34AM +0100, Greg Kroah-Hartman wrote:
> From: Daniel Jordan <daniel.m.jordan@xxxxxxxxxx>
> 
> [ Upstream commit 38228e8848cd7dd86ccb90406af32de0cad24be3 ]
> 
> lockdep complains when padata's paths to update cpumasks via CPU hotplug
> and sysfs are both taken:
> 
>   # echo 0 > /sys/devices/system/cpu/cpu1/online
>   # echo ff > /sys/kernel/pcrypt/pencrypt/parallel_cpumask
> 
>   ======================================================
>   WARNING: possible circular locking dependency detected
>   5.4.0-rc8-padata-cpuhp-v3+ #1 Not tainted
>   ------------------------------------------------------
>   bash/205 is trying to acquire lock:
>   ffffffff8286bcd0 (cpu_hotplug_lock.rw_sem){++++}, at: padata_set_cpumask+0x2b/0x120
> 
>   but task is already holding lock:
>   ffff8880001abfa0 (&pinst->lock){+.+.}, at: padata_set_cpumask+0x26/0x120
> 
>   which lock already depends on the new lock.

I think this patch should be dropped from all stable queues (4.4, 4.9, 4.14,
4.19, 5.4, and 5.5).

The main benefit is to un-break lockdep for testing with future padata changes,
and an actual deadlock seems unlikely.

These stable versions don't fix the ordering in padata_remove_cpu() either
(nothing calls it though).

I tried the other stable padata patch in this cycle ("padata: validate cpumask
without removed CPU during offline"), it passed my tests and should stay in.

thanks,
Daniel