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

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On Fri, Feb 21, 2020 at 07:00:45PM -0500, Daniel Jordan wrote:
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.

I've dropped it, thanks.

--
Thanks,
Sasha



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