[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>

2 files changed, 6 insertions, 0 deletions

diff --git a/include/linux/cpu.h b/include/linux/cpu.h
index 1f7b2c097503..43c44530ef9d 100644
--- a/include/linux/cpu.h
+++ b/include/linux/cpu.h
@@ -42,6 +42,7 @@ struct notifier_block;
 /* Need to know about CPUs going up/down? */
 extern int register_cpu_notifier(struct notifier_block *nb);
 extern void unregister_cpu_notifier(struct notifier_block *nb);
+extern int current_in_cpu_hotplug(void);
 
 int cpu_up(unsigned int cpu);
 
@@ -54,6 +55,10 @@ static inline int register_cpu_notifier(struct notifier_block *nb)
 static inline void unregister_cpu_notifier(struct notifier_block *nb)
 {
 }
+static inline int current_in_cpu_hotplug(void)
+{
+        return 0;
+}
 
 #endif /* CONFIG_SMP */
 extern struct sysdev_class cpu_sysdev_class;
diff --git a/include/linux/sched.h b/include/linux/sched.h
index 03b68a7b4b82..2bbf968b23d9 100644
--- a/include/linux/sched.h
+++ b/include/linux/sched.h
@@ -909,6 +909,7 @@ do { if (atomic_dec_and_test(&(tsk)->usage)) __put_task_struct(tsk); } while(0)
 #define PF_SYNCWRITE    0x00200000      /* I am doing a sync write */
 #define PF_BORROWED_MM  0x00400000      /* I am a kthread doing use_mm */
 #define PF_RANDOMIZE    0x00800000      /* randomize virtual address space */
+#define PF_HOTPLUG_CPU  0x01000000      /* Currently performing CPU hotplug */
 
 /*
  * Only the _current_ task can read/write to tsk->flags, but other