[PATCH] clean up lock_cpu_hotplug() in cpufreq

There are some callers in cpufreq hotplug notify path that the lowest
function calls lock_cpu_hotplug().  The lock is already held during
cpu_up() and cpu_down() calls when the notify calls are broadcast to
registered clients.

Ideally if possible, we could disable_preempt() at the highest caller and
make sure we dont sleep in the path down in cpufreq->driver_target() calls
but the calls are so intertwined and cumbersome to cleanup.

Hence we consistently use lock_cpu_hotplug() and unlock_cpu_hotplug() in
all places.

 - Removed export of cpucontrol semaphore and made it static.
 - removed explicit uses of up/down with lock_cpu_hotplug()
   so we can keep track of the the callers in same thread context and
   just keep refcounts without calling a down() that causes a deadlock.
 - Removed current_in_hotplug() uses
 - Removed PF_HOTPLUG_CPU in sched.h introduced for the current_in_hotplug()
   temporary workaround.

Tested with insmod of cpufreq_stat.ko, and logical online/offline
to make sure we dont have any hang situations.

Signed-off-by: Ashok Raj <ashok.raj@intel.com>
Cc: Zwane Mwaikambo <zwane@linuxpower.ca>
Cc: Shaohua Li <shaohua.li@intel.com>
Cc: "Siddha, Suresh B" <suresh.b.siddha@intel.com>
Signed-off-by: Andrew Morton <akpm@osdl.org>
Signed-off-by: Linus Torvalds <torvalds@osdl.org>

drivers/cpufreq/cpufreq.c

@@ -1113,21 +1113,13 @@ int __cpufreq_driver_target(struct cpufreq_policy *policy,
 {
 	int retval = -EINVAL;
 
-	/*
-	 * 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.
-	 */
-	if (!current_in_cpu_hotplug())
-		lock_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);
 
-	if (!current_in_cpu_hotplug())
-		unlock_cpu_hotplug();
+	unlock_cpu_hotplug();
 
 	return retval;
 }

include/linux/cpu.h

@@ -65,10 +65,9 @@ extern struct sysdev_class cpu_sysdev_class;
 
 #ifdef CONFIG_HOTPLUG_CPU
 /* Stop CPUs going up and down. */
-extern struct semaphore cpucontrol;
-#define lock_cpu_hotplug()	down(&cpucontrol)
-#define unlock_cpu_hotplug()	up(&cpucontrol)
-#define lock_cpu_hotplug_interruptible() down_interruptible(&cpucontrol)
+extern void lock_cpu_hotplug(void);
+extern void unlock_cpu_hotplug(void);
+extern int lock_cpu_hotplug_interruptible(void);
 #define hotcpu_notifier(fn, pri) {				\
 	static struct notifier_block fn##_nb =			\
 		{ .notifier_call = fn, .priority = pri };	\

include/linux/sched.h

@@ -908,7 +908,6 @@ 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

kernel/cpu.c

@@ -16,47 +16,76 @@
 #include <asm/semaphore.h>
 
 /* This protects CPUs going up and down... */
-DECLARE_MUTEX(cpucontrol);
-EXPORT_SYMBOL_GPL(cpucontrol);
+static DECLARE_MUTEX(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
- */
+#ifdef CONFIG_HOTPLUG_CPU
+static struct task_struct *lock_cpu_hotplug_owner;
+static int lock_cpu_hotplug_depth;
 
-int current_in_cpu_hotplug(void)
+static int __lock_cpu_hotplug(int interruptible)
 {
-	return (current->flags & PF_HOTPLUG_CPU);
+	int ret = 0;
+
+	if (lock_cpu_hotplug_owner != current) {
+		if (interruptible)
+			ret = down_interruptible(&cpucontrol);
+		else
+			down(&cpucontrol);
+	}
+
+	/*
+	 * Set only if we succeed in locking
+	 */
+	if (!ret) {
+		lock_cpu_hotplug_depth++;
+		lock_cpu_hotplug_owner = current;
+	}
+
+	return ret;
 }
 
-EXPORT_SYMBOL_GPL(current_in_cpu_hotplug);
+void lock_cpu_hotplug(void)
+{
+	__lock_cpu_hotplug(0);
+}
+EXPORT_SYMBOL_GPL(lock_cpu_hotplug);
 
+void unlock_cpu_hotplug(void)
+{
+	if (--lock_cpu_hotplug_depth == 0) {
+		lock_cpu_hotplug_owner = NULL;
+		up(&cpucontrol);
+	}
+}
+EXPORT_SYMBOL_GPL(unlock_cpu_hotplug);
+
+int lock_cpu_hotplug_interruptible(void)
+{
+	return __lock_cpu_hotplug(1);
+}
+EXPORT_SYMBOL_GPL(lock_cpu_hotplug_interruptible);
+#endif	/* CONFIG_HOTPLUG_CPU */
 
 /* Need to know about CPUs going up/down? */
 int register_cpu_notifier(struct notifier_block *nb)
 {
 	int ret;
 
-	if ((ret = down_interruptible(&cpucontrol)) != 0)
+	if ((ret = lock_cpu_hotplug_interruptible()) != 0)
 		return ret;
 	ret = notifier_chain_register(&cpu_chain, nb);
-	up(&cpucontrol);
+	unlock_cpu_hotplug();
 	return ret;
 }
 EXPORT_SYMBOL(register_cpu_notifier);
 
 void unregister_cpu_notifier(struct notifier_block *nb)
 {
-	down(&cpucontrol);
+	lock_cpu_hotplug();
 	notifier_chain_unregister(&cpu_chain, nb);
-	up(&cpucontrol);
+	unlock_cpu_hotplug();
 }
 EXPORT_SYMBOL(unregister_cpu_notifier);
 
@@ -112,13 +141,6 @@ 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) {
@@ -171,7 +193,6 @@ int cpu_down(unsigned int cpu)
 out_allowed:
 	set_cpus_allowed(current, old_allowed);
 out:
-	current->flags &= ~PF_HOTPLUG_CPU;
 	unlock_cpu_hotplug();
 	return err;
 }
@@ -182,19 +203,14 @@ int __devinit cpu_up(unsigned int cpu)
 	int ret;
 	void *hcpu = (void *)(long)cpu;
 
-	if ((ret = down_interruptible(&cpucontrol)) != 0)
+	if ((ret = lock_cpu_hotplug_interruptible()) != 0)
 		return ret;
 
 	if (cpu_online(cpu) || !cpu_present(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",
@@ -217,7 +233,6 @@ int __devinit cpu_up(unsigned int cpu)
 	if (ret != 0)
 		notifier_call_chain(&cpu_chain, CPU_UP_CANCELED, hcpu);
 out:
-	current->flags &= ~PF_HOTPLUG_CPU;
-	up(&cpucontrol);
+	unlock_cpu_hotplug();
 	return ret;
 }