linux-kernel - [PATCH v2 0/2] cpufreq: Fix a circular lock dependency problem

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Message-Id: <1532456765-17576-1-git-send-email-longman@redhat.com>
Date:   Tue, 24 Jul 2018 14:26:03 -0400
From:   Waiman Long <longman@...hat.com>
To:     "Rafael J. Wysocki" <rjw@...ysocki.net>,
        Viresh Kumar <viresh.kumar@...aro.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Peter Zijlstra <peterz@...radead.org>,
        Ingo Molnar <mingo@...nel.org>
Cc:     linux-kernel@...r.kernel.org, linux-pm@...r.kernel.org,
        "Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Konrad Rzeszutek Wilk <konrad.wilk@...cle.com>,
        Waiman Long <longman@...hat.com>
Subject: [PATCH v2 0/2] cpufreq: Fix a circular lock dependency problem

 v2:
  - Remove the retry loop in the store method.

This patchset works around a circular lock dependency issue in the
cpufreq driver reported by lockdep. The two locks involved are the
cpu_hotplup_lock and the reference count of a sysfs file.

The cpufreq_register_driver() function uses the lock sequence:

  cpus_read_lock --> kn->count

Whereas the cpufreq sysfs store method uses the sequence:

  kn->count --> cpus_read_lock

This is not really an issue as a shared lock is used on the
cpu_hotplup_lock. However, the lockdep code isn't able to handle
shared locking. So one way to work around this is to define a
cpus_read_trylock() function and uses it in the store method instead.

Waiman Long (2):
  cpu/hotplug: Add a cpus_read_trylock() function
  cpufreq: Fix a circular lock dependency problem

 drivers/cpufreq/cpufreq.c | 7 ++++++-
 include/linux/cpu.h       | 2 ++
 kernel/cpu.c              | 6 ++++++
 3 files changed, 14 insertions(+), 1 deletion(-)

-- 
1.8.3.1