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Subject[PATCH RT 24/25][RFC 3.0.23-rt39-rc1] cpu: Make hotplug.lock a "sleeping" spinlock on RT
From: Steven Rostedt <rostedt@goodmis.org>

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 files changed, 26 insertions(+), 9 deletions(-)
diff --git a/kernel/cpu.c b/kernel/cpu.c
index b2de274..21c8380 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.
@@ -54,10 +59,22 @@ 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;
@@ -87,8 +104,8 @@ retry:
return;
}
preempt_enable();
- mutex_lock(&cpu_hotplug.lock);
- mutex_unlock(&cpu_hotplug.lock);
+ hotplug_lock();
+ hotplug_unlock();
preempt_disable();
goto retry;
}
@@ -161,9 +178,9 @@ 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 +189,10 @@ 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 +224,11 @@ 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 +236,7 @@ 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 */
--
1.7.8.3



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