~ubuntu-branches/debian/wheezy/linux-2.6/wheezy

Subject: cpu: Make hotplug.lock a "sleeping" spinlock on RT

From: Steven Rostedt <rostedt@goodmis.org>

Date: Fri, 02 Mar 2012 10:36:57 -0500

Tasks can block on hotplug.lock in pin_current_cpu(), but their state

might be != RUNNING. So the mutex wakeup will set the state

unconditionally to RUNNING. That might cause spurious unexpected

wakeups. We could provide a state preserving mutex_lock() function,

but this is semantically backwards. So instead we convert the

hotplug.lock() to a spinlock for RT, which has the state preserving

semantics already.

Signed-off-by: Steven Rostedt <rostedt@goodmis.org>

Cc: Carsten Emde <C.Emde@osadl.org>

Cc: John Kacur <jkacur@redhat.com>

Cc: Peter Zijlstra <peterz@infradead.org>

Cc: Clark Williams <clark.williams@gmail.com>

Cc: stable-rt@vger.kernel.org

Link: http://lkml.kernel.org/r/1330702617.25686.265.camel@gandalf.stny.rr.com

Signed-off-by: Thomas Gleixner <tglx@linutronix.de>

diff --git a/kernel/cpu.c b/kernel/cpu.c

index fa40834..c25b5ff 100644

--- a/kernel/cpu.c

+++ b/kernel/cpu.c

@@ -46,7 +46,12 @@ static int cpu_hotplug_disabled;

 static struct {

 	struct task_struct *active_writer;

+#ifdef CONFIG_PREEMPT_RT_FULL

+	/* Makes the lock keep the task's state */

+	spinlock_t lock;

+#else

 	struct mutex lock; /* Synchronizes accesses to refcount, */

+#endif

 	/*

 	 * Also blocks the new readers during

 	 * an ongoing cpu hotplug operation.

@@ -58,6 +63,14 @@ static struct {

 	.refcount = 0,

 };

+#ifdef CONFIG_PREEMPT_RT_FULL

+# define hotplug_lock() spin_lock(&cpu_hotplug.lock)

+# define hotplug_unlock() spin_unlock(&cpu_hotplug.lock)

+#else

+# define hotplug_lock() mutex_lock(&cpu_hotplug.lock)

+# define hotplug_lock() mutex_unlock(&cpu_hotplug.lock)

+#endif

+

 struct hotplug_pcp {

 	struct task_struct *unplug;

 	int refcount;

@@ -87,8 +100,8 @@ retry:

 		return;

 	}

 	preempt_enable();

-	mutex_lock(&cpu_hotplug.lock);

-	mutex_unlock(&cpu_hotplug.lock);

+	hotplug_lock();

+	hotplug_unlock();

 	preempt_disable();

 	goto retry;

 }

@@ -161,9 +174,9 @@ void get_online_cpus(void)

 	might_sleep();

 	if (cpu_hotplug.active_writer == current)

 		return;

-	mutex_lock(&cpu_hotplug.lock);

+	hotplug_lock();

 	cpu_hotplug.refcount++;

-	mutex_unlock(&cpu_hotplug.lock);

+	hotplug_unlock();

 }

 EXPORT_SYMBOL_GPL(get_online_cpus);

@@ -172,10 +185,10 @@ void put_online_cpus(void)

 {

 	if (cpu_hotplug.active_writer == current)

 		return;

-	mutex_lock(&cpu_hotplug.lock);

+	hotplug_lock();

 	if (!--cpu_hotplug.refcount && unlikely(cpu_hotplug.active_writer))

 		wake_up_process(cpu_hotplug.active_writer);

-	mutex_unlock(&cpu_hotplug.lock);

+	hotplug_unlock();

 }

 EXPORT_SYMBOL_GPL(put_online_cpus);

@@ -207,11 +220,11 @@ static void cpu_hotplug_begin(void)

 	cpu_hotplug.active_writer = current;

 	for (;;) {

-		mutex_lock(&cpu_hotplug.lock);

+		hotplug_lock();

 		if (likely(!cpu_hotplug.refcount))

 			break;

 		__set_current_state(TASK_UNINTERRUPTIBLE);

-		mutex_unlock(&cpu_hotplug.lock);

+		hotplug_unlock();

 		schedule();

 	}

 }

@@ -219,7 +232,7 @@ static void cpu_hotplug_begin(void)

 static void cpu_hotplug_done(void)

 {

 	cpu_hotplug.active_writer = NULL;

-	mutex_unlock(&cpu_hotplug.lock);

+	hotplug_unlock();

 }

 #else /* #if CONFIG_HOTPLUG_CPU */