Re: [PATCH] remove rwsem lock from CPUFREQ_GOV_STOP call (secondcall site)

From: Mathieu Desnoyers
Date: Mon Jun 08 2009 - 13:17:55 EST

OK, here is the re-run, with comment and acked-by added.



remove rwsem lock from CPUFREQ_GOV_STOP call (second call site)

commit 42a06f2166f2f6f7bf04f32b4e823eacdceafdc9

Missed a call site for CPUFREQ_GOV_STOP to remove the rwlock taken around the
teardown. To make a long story short, the rwlock write-lock causes a circular
dependency with cancel_delayed_work_sync(), because the timer handler takes the
read lock.

Note that all callers to __cpufreq_set_policy are taking the rwsem. All sysfs
callers (writers) hold the write rwsem at the earliest sysfs calling stage.

However, the rwlock write-lock is not needed upon governor stop.

Change :
- Added comment from Venkyi at lock definition site.

Signed-off-by: Mathieu Desnoyers <mathieu.desnoyers@xxxxxxxxxx>
Acked-by: Venkatesh Pallipadi <venkatesh.pallipadi@xxxxxxxxx>
CC: rjw@xxxxxxx
CC: mingo@xxxxxxx
CC: Shaohua Li <>
CC: Pekka Enberg <penberg@xxxxxxxxxxxxxx>
CC: Dave Young <hidave.darkstar@xxxxxxxxx>
CC: "Rafael J. Wysocki" <rjw@xxxxxxx>
CC: Rusty Russell <rusty@xxxxxxxxxxxxxxx>
CC: trenn@xxxxxxx
CC: sven.wegener@xxxxxxxxxxx
CC: cpufreq@xxxxxxxxxxxxxxx
drivers/cpufreq/cpufreq.c | 13 ++++++++++++-
1 file changed, 12 insertions(+), 1 deletion(-)

Index: linux-2.6-lttng/drivers/cpufreq/cpufreq.c
--- linux-2.6-lttng.orig/drivers/cpufreq/cpufreq.c 2009-06-08 12:47:22.000000000 -0400
+++ linux-2.6-lttng/drivers/cpufreq/cpufreq.c 2009-06-08 12:48:38.000000000 -0400
@@ -61,6 +61,8 @@ static DEFINE_SPINLOCK(cpufreq_driver_lo
* are concerned with are online after they get the lock.
* - Governor routines that can be called in cpufreq hotplug path should not
* take this sem as top level hotplug notifier handler takes this.
+ * - Lock should not be held across
+ * __cpufreq_governor(data, CPUFREQ_GOV_STOP);
static DEFINE_PER_CPU(int, policy_cpu);
static DEFINE_PER_CPU(struct rw_semaphore, cpu_policy_rwsem);
@@ -1697,8 +1699,17 @@ static int __cpufreq_set_policy(struct c
dprintk("governor switch\n");

/* end old governor */
- if (data->governor)
+ if (data->governor) {
+ /*
+ * Need to release the rwsem around governor
+ * stop due to lock dependency between
+ * cancel_delayed_work_sync and the read lock
+ * taken in the delayed work handler.
+ */
+ unlock_policy_rwsem_write(data->cpu);
__cpufreq_governor(data, CPUFREQ_GOV_STOP);
+ lock_policy_rwsem_write(data->cpu);
+ }

/* start new governor */
data->governor = policy->governor;

Mathieu Desnoyers
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