4 files changed, 49 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);
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
diff --git a/kernel/cpu.c b/kernel/cpu.c
index 3619e939182e..d61ba88f34e5 100644
--- a/kernel/cpu.c
+++ b/kernel/cpu.c
@@ -21,6 +21,24 @@ EXPORT_SYMBOL_GPL(cpucontrol);
 
 static struct notifier_block *cpu_chain;
 
+/*
+ * Used to check by callers if they need to acquire the cpucontrol
+ * or not to protect a cpu from being removed. Its sometimes required to
+ * call these functions both for normal operations, and in response to
+ * a cpu being added/removed. If the context of the call is in the same
+ * thread context as a CPU hotplug thread, we dont need to take the lock
+ * since its already protected
+ * check drivers/cpufreq/cpufreq.c for its usage - Ashok Raj
+ */
+
+int current_in_cpu_hotplug(void)
+{
+        return (current->flags & PF_HOTPLUG_CPU);
+}
+
+EXPORT_SYMBOL_GPL(current_in_cpu_hotplug);
+
+
 /* Need to know about CPUs going up/down? */
 int register_cpu_notifier(struct notifier_block *nb)
 {
@@ -94,6 +112,13 @@ int cpu_down(unsigned int cpu)
                 goto out;
         }
 
+        /*
+         * Leave a trace in current->flags indicating we are already in
+         * process of performing CPU hotplug. Callers can check if cpucontrol
+         * is already acquired by current thread, and if so not cause
+         * a dead lock by not acquiring the lock
+         */
+        current->flags |= PF_HOTPLUG_CPU;
         err = notifier_call_chain(&cpu_chain, CPU_DOWN_PREPARE,
                                                 (void *)(long)cpu);
         if (err == NOTIFY_BAD) {
@@ -146,6 +171,7 @@ out_thread:
 out_allowed:
         set_cpus_allowed(current, old_allowed);
 out:
+        current->flags &= ~PF_HOTPLUG_CPU;
         unlock_cpu_hotplug();
         return err;
 }
@@ -163,6 +189,12 @@ int __devinit cpu_up(unsigned int cpu)
                 ret = -EINVAL;
                 goto out;
         }
+
+        /*
+         * Leave a trace in current->flags indicating we are already in
+         * process of performing CPU hotplug.
+         */
+        current->flags |= PF_HOTPLUG_CPU;
         ret = notifier_call_chain(&cpu_chain, CPU_UP_PREPARE, hcpu);
         if (ret == NOTIFY_BAD) {
                 printk("%s: attempt to bring up CPU %u failed\n",
@@ -185,6 +217,7 @@ out_notify:
         if (ret != 0)
                 notifier_call_chain(&cpu_chain, CPU_UP_CANCELED, hcpu);
 out:
+        current->flags &= ~PF_HOTPLUG_CPU;
         up(&cpucontrol);
         return ret;
 }