[PATCH] cpu hotplug: fix locking in cpufreq drivers

When calling target drivers to set frequency, we take cpucontrol lock.
When we modified the code to accomodate CPU hotplug, there was an attempt
to take a double lock of cpucontrol leading to a deadlock.  Since the
current thread context is already holding the cpucontrol lock, we dont need
to make another attempt to acquire it.

Now we leave a trace in current->flags indicating current thread already is
under cpucontrol lock held, so we dont attempt to do this another time.

Thanks to Andrew Morton for the beating:-)

From: Brice Goglin <Brice.Goglin@ens-lyon.org>

  Build fix

(akpm: this patch is still unpleasant.  Ashok continues to look for a cleaner
solution, doesn't he?  ;))

Signed-off-by: Ashok Raj <ashok.raj@intel.com>
Signed-off-by: Brice Goglin <Brice.Goglin@ens-lyon.org>
Signed-off-by: Andrew Morton <akpm@osdl.org>
Signed-off-by: Linus Torvalds <torvalds@osdl.org>

1 files changed, 10 insertions, 14 deletions

diff --git a/drivers/cpufreq/cpufreq.c b/drivers/cpufreq/cpufreq.c
index 25acf478c9e8..23a63207d747 100644
--- a/drivers/cpufreq/cpufreq.c
+++ b/drivers/cpufreq/cpufreq.c
@@ -38,7 +38,6 @@ static struct cpufreq_driver   	*cpufreq_driver;
 static struct cpufreq_policy    *cpufreq_cpu_data[NR_CPUS];
 static DEFINE_SPINLOCK(cpufreq_driver_lock);
 
-
 /* internal prototypes */
 static int __cpufreq_governor(struct cpufreq_policy *policy, unsigned int event);
 static void handle_update(void *data);
@@ -1115,24 +1114,21 @@ int __cpufreq_driver_target(struct cpufreq_policy *policy,
         int retval = -EINVAL;
 
         /*
-         * Converted the lock_cpu_hotplug to preempt_disable()
-         * and preempt_enable(). This is a bit kludgy and relies on how cpu
-         * hotplug works. All we need is a guarantee that cpu hotplug won't make
-         * progress on any cpu. Once we do preempt_disable(), this would ensure
-         * that hotplug threads don't get onto this cpu, thereby delaying
-         * the cpu remove process.
-         *
-         * We removed the lock_cpu_hotplug since we need to call this function
-         * via cpu hotplug callbacks, which result in locking the cpu hotplug
-         * thread itself. Agree this is not very clean, cpufreq community
-         * could improve this if required. - Ashok Raj <ashok.raj@intel.com>
+         * If we are already in context of hotplug thread, we dont need to
+         * acquire the hotplug lock. Otherwise acquire cpucontrol to prevent
+         * hotplug from removing this cpu that we are working on.
          */
-        preempt_disable();
+        if (!current_in_cpu_hotplug())
+                lock_cpu_hotplug();
+
         dprintk("target for CPU %u: %u kHz, relation %u\n", policy->cpu,
                 target_freq, relation);
         if (cpu_online(policy->cpu) && cpufreq_driver->target)
                 retval = cpufreq_driver->target(policy, target_freq, relation);
-        preempt_enable();
+
+        if (!current_in_cpu_hotplug())
+                unlock_cpu_hotplug();
+
         return retval;
 }
 EXPORT_SYMBOL_GPL(__cpufreq_driver_target);