From mboxrd@z Thu Jan 1 00:00:00 1970 From: Peter Zijlstra Subject: Re: [PATCH 2/2] cpufreq: Fix a circular lock dependency problem Date: Mon, 23 Jul 2018 21:16:27 +0200 Message-ID: <20180723191627.GJ2494@hirez.programming.kicks-ass.net> References: <1532368179-15263-1-git-send-email-longman@redhat.com> <1532368179-15263-3-git-send-email-longman@redhat.com> Mime-Version: 1.0 Content-Type: text/plain; charset=us-ascii Return-path: Content-Disposition: inline In-Reply-To: <1532368179-15263-3-git-send-email-longman@redhat.com> Sender: linux-kernel-owner@vger.kernel.org To: Waiman Long Cc: "Rafael J. Wysocki" , Viresh Kumar , Thomas Gleixner , Ingo Molnar , linux-kernel@vger.kernel.org, linux-pm@vger.kernel.org, "Paul E. McKenney" , Greg Kroah-Hartman , Konrad Rzeszutek Wilk List-Id: linux-pm@vger.kernel.org On Mon, Jul 23, 2018 at 01:49:39PM -0400, Waiman Long wrote: > drivers/cpufreq/cpufreq.c | 16 +++++++++++++++- > 1 file changed, 15 insertions(+), 1 deletion(-) > > diff --git a/drivers/cpufreq/cpufreq.c b/drivers/cpufreq/cpufreq.c > index b0dfd32..9cf02d7 100644 > --- a/drivers/cpufreq/cpufreq.c > +++ b/drivers/cpufreq/cpufreq.c > @@ -922,8 +922,22 @@ static ssize_t store(struct kobject *kobj, struct attribute *attr, > struct cpufreq_policy *policy = to_policy(kobj); > struct freq_attr *fattr = to_attr(attr); > ssize_t ret = -EINVAL; > + int retries = 3; > > - cpus_read_lock(); > + /* > + * cpus_read_trylock() is used here to work around a circular lock > + * dependency problem with respect to the cpufreq_register_driver(). > + * With a simple retry loop, the chance of not able to get the > + * read lock is extremely small. > + */ > + while (!cpus_read_trylock()) { > + if (retries-- <= 0) > + return -EBUSY; > + /* > + * Sleep for about 50ms and retry again. > + */ > + msleep(50); > + } That's atrocious.