get_online_cpus() && workqueues

[Oleg Nesterov]

Gautham, Srivatsa, seriously, can't we uglify cpu.c a little bit to solve
the problem. Please see the illustration patch below. It looks complicated,
but in fact it is quite trivial.

In short: work_struct can't use get_online_cpus() due to deadlock with the
CPU_DEAD phase.

Can't we add another nested lock which is dropped right after __cpu_die()?
(in fact I think it could be dropped after __stop_machine_run).

The new read-lock is get_online_map() (just a random name for now). The only
difference wrt get_online_cpus() is that it doesn't protect against CPU_DEAD,
but most users of get_online_cpus() doesn't need this, they only need a
stable cpu_online_map and sometimes they need to be sure that some per-cpu
object (say, cpu_workqueue_struct->thread) can't be destroyed under this
lock.

get_online_map() seem to fit for this, and can be used from work->func().
(actually, I think most users of use get_online_cpus() could use the new
helper instead, but this doen't matter).

Heiko, what do you think? Is it suitable for arch_reinit_sched_domains()?

Oleg.

--- 25/kernel/cpu.c~HP_LOCK	2008-02-16 18:36:37.000000000 +0300
+++ 25/kernel/cpu.c	2008-04-26 18:14:25.000000000 +0400
@@ -25,7 +25,7 @@ static __cpuinitdata RAW_NOTIFIER_HEAD(c
  */
 static int cpu_hotplug_disabled;
 
-static struct {
+static struct cpu_lock {
 	struct task_struct *active_writer;
 	struct mutex lock; /* Synchronizes accesses to refcount, */
 	/*
@@ -33,41 +33,65 @@ static struct {
 	 * an ongoing cpu hotplug operation.
 	 */
 	int refcount;
-} cpu_hotplug;
+} cpu_hotplug, online_map;
+
+static inline void __cpu_hotplug_init(struct cpu_lock *cpu_lock)
+{
+	cpu_lock->active_writer = NULL;
+	mutex_init(&cpu_lock->lock);
+	cpu_lock->refcount = 0;
+}
 
 void __init cpu_hotplug_init(void)
 {
-	cpu_hotplug.active_writer = NULL;
-	mutex_init(&cpu_hotplug.lock);
-	cpu_hotplug.refcount = 0;
+	__cpu_hotplug_init(&cpu_hotplug);
+	__cpu_hotplug_init(&online_map);
 }
 
 #ifdef CONFIG_HOTPLUG_CPU
 
-void get_online_cpus(void)
+void cpu_read_lock(struct cpu_lock *cpu_lock)
 {
 	might_sleep();
-	if (cpu_hotplug.active_writer == current)
+	if (cpu_lock->active_writer == current)
 		return;
-	mutex_lock(&cpu_hotplug.lock);
-	cpu_hotplug.refcount++;
-	mutex_unlock(&cpu_hotplug.lock);
+	mutex_lock(&cpu_lock->lock);
+	cpu_lock->refcount++;
+	mutex_unlock(&cpu_lock->lock);
+}
 
+void get_online_cpus(void)
+{
+	cpu_read_lock(&cpu_hotplug);
 }
 EXPORT_SYMBOL_GPL(get_online_cpus);
 
-void put_online_cpus(void)
+void get_online_map(void)
 {
-	if (cpu_hotplug.active_writer == current)
+	cpu_read_lock(&online_map);
+}
+
+void cpu_read_unlock(struct cpu_lock *cpu_lock)
+{
+	if (cpu_lock->active_writer == current)
 		return;
-	mutex_lock(&cpu_hotplug.lock);
-	if (!--cpu_hotplug.refcount && unlikely(cpu_hotplug.active_writer))
-		wake_up_process(cpu_hotplug.active_writer);
-	mutex_unlock(&cpu_hotplug.lock);
+	mutex_lock(&cpu_lock->lock);
+	if (!--cpu_lock->refcount && unlikely(cpu_lock->active_writer))
+		wake_up_process(cpu_lock->active_writer);
+	mutex_unlock(&cpu_lock->lock);
+}
 
+void put_online_cpus(void)
+{
+	cpu_read_unlock(&cpu_hotplug);
 }
 EXPORT_SYMBOL_GPL(put_online_cpus);
 
+void put_online_map(void)
+{
+	cpu_read_unlock(&online_map);
+}
+
 #endif	/* CONFIG_HOTPLUG_CPU */
 
 /*
@@ -91,7 +115,7 @@ void cpu_maps_update_done(void)
  * Note that during a cpu-hotplug operation, the new readers, if any,
  * will be blocked by the cpu_hotplug.lock
  *
- * Since cpu_hotplug_begin() is always called after invoking
+ * Since cpu_write_lock() is always called after invoking
  * cpu_maps_update_begin(), we can be sure that only one writer is active.
  *
  * Note that theoretically, there is a possibility of a livelock:
@@ -106,25 +130,26 @@ void cpu_maps_update_done(void)
  * get_online_cpus() not an api which is called all that often.
  *
  */
-static void cpu_hotplug_begin(void)
+static void cpu_write_lock(struct cpu_lock *cpu_lock)
 {
-	cpu_hotplug.active_writer = current;
+	cpu_lock->active_writer = current;
 
 	for (;;) {
-		mutex_lock(&cpu_hotplug.lock);
-		if (likely(!cpu_hotplug.refcount))
+		mutex_lock(&cpu_lock->lock);
+		if (likely(!cpu_lock->refcount))
 			break;
 		__set_current_state(TASK_UNINTERRUPTIBLE);
-		mutex_unlock(&cpu_hotplug.lock);
+		mutex_unlock(&cpu_lock->lock);
 		schedule();
 	}
 }
 
-static void cpu_hotplug_done(void)
+static void cpu_write_unlock(struct cpu_lock *cpu_lock)
 {
-	cpu_hotplug.active_writer = NULL;
-	mutex_unlock(&cpu_hotplug.lock);
+	cpu_lock->active_writer = NULL;
+	mutex_unlock(&cpu_lock->lock);
 }
+
 /* Need to know about CPUs going up/down? */
 int __cpuinit register_cpu_notifier(struct notifier_block *nb)
 {
@@ -207,7 +232,8 @@ static int _cpu_down(unsigned int cpu, i
 	if (!cpu_online(cpu))
 		return -EINVAL;
 
-	cpu_hotplug_begin();
+	cpu_write_lock(&cpu_hotplug);
+	cpu_write_lock(&online_map);
 	err = __raw_notifier_call_chain(&cpu_chain, CPU_DOWN_PREPARE | mod,
 					hcpu, -1, &nr_calls);
 	if (err == NOTIFY_BAD) {
@@ -238,6 +264,7 @@ static int _cpu_down(unsigned int cpu, i
 			err = PTR_ERR(p);
 			goto out_allowed;
 		}
+		err = -EAGAIN;
 		goto out_thread;
 	}
 
@@ -247,6 +274,7 @@ static int _cpu_down(unsigned int cpu, i
 
 	/* This actually kills the CPU. */
 	__cpu_die(cpu);
+	cpu_write_unlock(&online_map);
 
 	/* CPU is completely dead: tell everyone.  Too late to complain. */
 	if (raw_notifier_call_chain(&cpu_chain, CPU_DEAD | mod,
@@ -260,7 +288,9 @@ out_thread:
 out_allowed:
 	set_cpus_allowed(current, old_allowed);
 out_release:
-	cpu_hotplug_done();
+	if (err)
+		cpu_write_unlock(&online_map);
+	cpu_write_unlock(&cpu_hotplug);
 	return err;
 }
 
@@ -289,7 +319,8 @@ static int __cpuinit _cpu_up(unsigned in
 	if (cpu_online(cpu) || !cpu_present(cpu))
 		return -EINVAL;
 
-	cpu_hotplug_begin();
+	cpu_write_lock(&cpu_hotplug);
+	cpu_write_lock(&online_map);
 	ret = __raw_notifier_call_chain(&cpu_chain, CPU_UP_PREPARE | mod, hcpu,
 							-1, &nr_calls);
 	if (ret == NOTIFY_BAD) {
@@ -313,7 +344,8 @@ out_notify:
 	if (ret != 0)
 		__raw_notifier_call_chain(&cpu_chain,
 				CPU_UP_CANCELED | mod, hcpu, nr_calls, NULL);
-	cpu_hotplug_done();
+	cpu_write_unlock(&online_map);
+	cpu_write_unlock(&cpu_hotplug);
 
 	return ret;
 }

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