cpufreq: interactive: fix racy timer stopping

When stopping the governor, del_timer_sync() can race against an
invocation of the idle notifier callback, which has the potential
to reactivate the timer.

To fix this issue, a read-write semaphore is used. Multiple readers are
allowed as long as pcpu->governor_enabled is true.  However it can be
moved to false only after taking a write semaphore which would wait for
any on-going asynchronous activities to complete and prevent any more of
those activities to be initiated.

[toddpoynor@google.com: cosmetic and commit text changes]
Change-Id: Ib51165a735d73dcf964a06754c48bdc1913e13d0
Signed-off-by: Nicolas Pitre <nicolas.pitre@linaro.org>

1 files changed, 28 insertions, 10 deletions

diff --git a/drivers/cpufreq/cpufreq_interactive.c b/drivers/cpufreq/cpufreq_interactive.c
index f8e9ee9f713..74f56093d2f 100644
--- a/drivers/cpufreq/cpufreq_interactive.c
+++ b/drivers/cpufreq/cpufreq_interactive.c
@@ -21,7 +21,7 @@
 #include <linux/cpufreq.h>
 #include <linux/module.h>
 #include <linux/moduleparam.h>
-#include <linux/mutex.h>
+#include <linux/rwsem.h>
 #include <linux/sched.h>
 #include <linux/sched/rt.h>
 #include <linux/tick.h>
@@ -29,7 +29,6 @@
 #include <linux/timer.h>
 #include <linux/workqueue.h>
 #include <linux/kthread.h>
-#include <linux/mutex.h>
 #include <linux/slab.h>
 #include <asm/cputime.h>
 
@@ -52,6 +51,7 @@ struct cpufreq_interactive_cpuinfo {
         unsigned int floor_freq;
         u64 floor_validate_time;
         u64 hispeed_validate_time;
+        struct rw_semaphore enable_sem;
         int governor_enabled;
 };
 
@@ -279,8 +279,8 @@ static void cpufreq_interactive_timer(unsigned long data)
         unsigned long flags;
         bool boosted;
 
-        smp_rmb();
-
+        if (!down_read_trylock(&pcpu->enable_sem))
+                return;
         if (!pcpu->governor_enabled)
                 goto exit;
 
@@ -387,6 +387,7 @@ rearm:
                 cpufreq_interactive_timer_resched(pcpu);
 
 exit:
+        up_read(&pcpu->enable_sem);
         return;
 }
 
@@ -396,8 +397,12 @@ static void cpufreq_interactive_idle_start(void)
                 &per_cpu(cpuinfo, smp_processor_id());
         int pending;
 
-        if (!pcpu->governor_enabled)
+        if (!down_read_trylock(&pcpu->enable_sem))
+                return;
+        if (!pcpu->governor_enabled) {
+                up_read(&pcpu->enable_sem);
                 return;
+        }
 
         pending = timer_pending(&pcpu->cpu_timer);
 
@@ -414,6 +419,7 @@ static void cpufreq_interactive_idle_start(void)
                         cpufreq_interactive_timer_resched(pcpu);
         }
 
+        up_read(&pcpu->enable_sem);
 }
 
 static void cpufreq_interactive_idle_end(void)
@@ -421,8 +427,12 @@ static void cpufreq_interactive_idle_end(void)
         struct cpufreq_interactive_cpuinfo *pcpu =
                 &per_cpu(cpuinfo, smp_processor_id());
 
-        if (!pcpu->governor_enabled)
+        if (!down_read_trylock(&pcpu->enable_sem))
+                return;
+        if (!pcpu->governor_enabled) {
+                up_read(&pcpu->enable_sem);
                 return;
+        }
 
         /* Arm the timer for 1-2 ticks later if not already. */
         if (!timer_pending(&pcpu->cpu_timer)) {
@@ -432,6 +442,8 @@ static void cpufreq_interactive_idle_end(void)
                 del_timer(&pcpu->cpu_slack_timer);
                 cpufreq_interactive_timer(smp_processor_id());
         }
+
+        up_read(&pcpu->enable_sem);
 }
 
 static int cpufreq_interactive_speedchange_task(void *data)
@@ -466,10 +478,12 @@ static int cpufreq_interactive_speedchange_task(void *data)
                         unsigned int max_freq = 0;
 
                         pcpu = &per_cpu(cpuinfo, cpu);
-                        smp_rmb();
-
-                        if (!pcpu->governor_enabled)
+                        if (!down_read_trylock(&pcpu->enable_sem))
+                                continue;
+                        if (!pcpu->governor_enabled) {
+                                up_read(&pcpu->enable_sem);
                                 continue;
+                        }
 
                         for_each_cpu(j, pcpu->policy->cpus) {
                                 struct cpufreq_interactive_cpuinfo *pjcpu =
@@ -486,6 +500,8 @@ static int cpufreq_interactive_speedchange_task(void *data)
                         trace_cpufreq_interactive_setspeed(cpu,
                                                      pcpu->target_freq,
                                                      pcpu->policy->cur);
+
+                        up_read(&pcpu->enable_sem);
                 }
         }
 
@@ -936,10 +952,11 @@ static int cpufreq_governor_interactive(struct cpufreq_policy *policy,
         case CPUFREQ_GOV_STOP:
                 for_each_cpu(j, policy->cpus) {
                         pcpu = &per_cpu(cpuinfo, j);
+                        down_write(&pcpu->enable_sem);
                         pcpu->governor_enabled = 0;
-                        smp_wmb();
                         del_timer_sync(&pcpu->cpu_timer);
                         del_timer_sync(&pcpu->cpu_slack_timer);
+                        up_write(&pcpu->enable_sem);
                 }
 
                 if (atomic_dec_return(&active_count) > 0)
@@ -989,6 +1006,7 @@ static int __init cpufreq_interactive_init(void)
                 init_timer(&pcpu->cpu_slack_timer);
                 pcpu->cpu_slack_timer.function = cpufreq_interactive_nop_timer;
                 spin_lock_init(&pcpu->load_lock);
+                init_rwsem(&pcpu->enable_sem);
         }
 
         spin_lock_init(&target_loads_lock);