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From:   "Joel Fernandes (Google)" <joelaf@google.com>
To:     linux-kernel@vger.kernel.org
Cc:     "Joel Fernandes (Google)" <joel@joelfernandes.org>,
        "Rafael J . Wysocki" <rafael.j.wysocki@intel.com>,
        Peter Zijlstra <peterz@infradead.org>,
        Ingo Molnar <mingo@redhat.com>,
        Patrick Bellasi <patrick.bellasi@arm.com>,
        Juri Lelli <juri.lelli@redhat.com>,
        Luca Abeni <luca.abeni@santannapisa.it>,
        Todd Kjos <tkjos@google.com>, claudio@evidence.eu.com,
        kernel-team@android.com, linux-pm@vger.kernel.org
Subject: [PATCH RFC] schedutil: Address the r/w ordering race in kthread
Date:   Tue, 22 May 2018 16:50:28 -0700
Message-Id: <20180522235028.80564-1-joel@joelfernandes.org>
Sender: linux-kernel-owner@vger.kernel.org
Precedence: bulk

Currently there is a race in schedutil code for slow-switch single-CPU
systems. Fix it by enforcing ordering the write to work_in_progress to
happen before the read of next_freq.

Kthread                                       Sched update

sugov_work()				      sugov_update_single()

      lock();
      // The CPU is free to rearrange below
      // two in any order, so it may clear
      // the flag first and then read next
      // freq. Lets assume it does.
      work_in_progress = false

                                               if (work_in_progress)
                                                     return;

                                               sg_policy->next_freq = 0;
      freq = sg_policy->next_freq;
                                               sg_policy->next_freq = real-freq;
      unlock();

Reported-by: Viresh Kumar <viresh.kumar@linaro.org>
CC: Rafael J. Wysocki <rafael.j.wysocki@intel.com>
CC: Peter Zijlstra <peterz@infradead.org>
CC: Ingo Molnar <mingo@redhat.com>
CC: Patrick Bellasi <patrick.bellasi@arm.com>
CC: Juri Lelli <juri.lelli@redhat.com>
Cc: Luca Abeni <luca.abeni@santannapisa.it>
CC: Todd Kjos <tkjos@google.com>
CC: claudio@evidence.eu.com
CC: kernel-team@android.com
CC: linux-pm@vger.kernel.org
Signed-off-by: Joel Fernandes (Google) <joel@joelfernandes.org>
---
I split this into separate patch, because this race can also happen in
mainline.

 kernel/sched/cpufreq_schedutil.c | 7 +++++++
 1 file changed, 7 insertions(+)

diff --git a/kernel/sched/cpufreq_schedutil.c b/kernel/sched/cpufreq_schedutil.c
index 5c482ec38610..ce7749da7a44 100644
--- a/kernel/sched/cpufreq_schedutil.c
+++ b/kernel/sched/cpufreq_schedutil.c
@@ -401,6 +401,13 @@ static void sugov_work(struct kthread_work *work)
 	 */
 	raw_spin_lock_irqsave(&sg_policy->update_lock, flags);
 	freq = sg_policy->next_freq;
+
+	/*
+	 * sugov_update_single can access work_in_progress without update_lock,
+	 * make sure next_freq is read before work_in_progress is set.
+	 */
+	smp_mb();
+
 	sg_policy->work_in_progress = false;
 	raw_spin_unlock_irqrestore(&sg_policy->update_lock, flags);
 
-- 
2.17.0.441.gb46fe60e1d-goog