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>
---
 kernel/cpu.c |   35 ++++++++++++++++++++++++++---------
 1 file changed, 26 insertions(+), 9 deletions(-)

--- a/kernel/cpu.c
+++ b/kernel/cpu.c
@@ -51,7 +51,12 @@ 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.
@@ -59,10 +64,22 @@ static struct {
 	int refcount;
 } cpu_hotplug = {
 	.active_writer = NULL,
+#ifdef CONFIG_PREEMPT_RT_FULL
+	.lock = __SPIN_LOCK_UNLOCKED(cpu_hotplug.lock),
+#else
 	.lock = __MUTEX_INITIALIZER(cpu_hotplug.lock),
+#endif
 	.refcount = 0,
 };
 
+#ifdef CONFIG_PREEMPT_RT_FULL
+# define hotplug_lock() rt_spin_lock(&cpu_hotplug.lock)
+# define hotplug_unlock() rt_spin_unlock(&cpu_hotplug.lock)
+#else
+# define hotplug_lock() mutex_lock(&cpu_hotplug.lock)
+# define hotplug_unlock() mutex_unlock(&cpu_hotplug.lock)
+#endif
+
 struct hotplug_pcp {
 	struct task_struct *unplug;
 	int refcount;
@@ -92,8 +109,8 @@ retry:
 		return;
 	}
 	preempt_enable();
-	mutex_lock(&cpu_hotplug.lock);
-	mutex_unlock(&cpu_hotplug.lock);
+	hotplug_lock();
+	hotplug_unlock();
 	preempt_disable();
 	goto retry;
 }
@@ -165,9 +182,9 @@ 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);
@@ -176,14 +193,14 @@ void put_online_cpus(void)
 {
 	if (cpu_hotplug.active_writer == current)
 		return;
-	mutex_lock(&cpu_hotplug.lock);
 
+	hotplug_lock();
 	if (WARN_ON(!cpu_hotplug.refcount))
 		cpu_hotplug.refcount++; /* try to fix things up */
 
 	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);
@@ -215,11 +232,11 @@ 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();
 	}
 }
@@ -227,7 +244,7 @@ static void cpu_hotplug_begin(void)
 static void cpu_hotplug_done(void)
 {
 	cpu_hotplug.active_writer = NULL;
-	mutex_unlock(&cpu_hotplug.lock);
+	hotplug_unlock();
 }
 
 /*