cpu-hotplug: refcount based cpu hotplug
This patch implements a Refcount + Waitqueue based model for
cpu-hotplug.
Now, a thread which wants to prevent cpu-hotplug, will bump up a global
refcount and the thread which wants to perform a cpu-hotplug operation
will block till the global refcount goes to zero.
The readers, if any, during an ongoing cpu-hotplug operation are blocked
until the cpu-hotplug operation is over.
Signed-off-by: Gautham R Shenoy <ego@in.ibm.com>
Signed-off-by: Paul Jackson <pj@sgi.com> [For !CONFIG_HOTPLUG_CPU ]
Signed-off-by: Ingo Molnar <mingo@elte.hu>
diff --git a/kernel/cpu.c b/kernel/cpu.c
index 6b3a0c1..656dc3f 100644
--- a/kernel/cpu.c
+++ b/kernel/cpu.c
@@ -15,9 +15,8 @@
#include <linux/stop_machine.h>
#include <linux/mutex.h>
-/* This protects CPUs going up and down... */
+/* Serializes the updates to cpu_online_map, cpu_present_map */
static DEFINE_MUTEX(cpu_add_remove_lock);
-static DEFINE_MUTEX(cpu_bitmask_lock);
static __cpuinitdata RAW_NOTIFIER_HEAD(cpu_chain);
@@ -26,52 +25,123 @@
*/
static int cpu_hotplug_disabled;
-#ifdef CONFIG_HOTPLUG_CPU
+static struct {
+ struct task_struct *active_writer;
+ struct mutex lock; /* Synchronizes accesses to refcount, */
+ /*
+ * Also blocks the new readers during
+ * an ongoing cpu hotplug operation.
+ */
+ int refcount;
+ wait_queue_head_t writer_queue;
+} cpu_hotplug;
-/* Crappy recursive lock-takers in cpufreq! Complain loudly about idiots */
-static struct task_struct *recursive;
-static int recursive_depth;
+#define writer_exists() (cpu_hotplug.active_writer != NULL)
+
+void __init cpu_hotplug_init(void)
+{
+ cpu_hotplug.active_writer = NULL;
+ mutex_init(&cpu_hotplug.lock);
+ cpu_hotplug.refcount = 0;
+ init_waitqueue_head(&cpu_hotplug.writer_queue);
+}
+
+#ifdef CONFIG_HOTPLUG_CPU
void lock_cpu_hotplug(void)
{
- struct task_struct *tsk = current;
-
- if (tsk == recursive) {
- static int warnings = 10;
- if (warnings) {
- printk(KERN_ERR "Lukewarm IQ detected in hotplug locking\n");
- WARN_ON(1);
- warnings--;
- }
- recursive_depth++;
+ might_sleep();
+ if (cpu_hotplug.active_writer == current)
return;
- }
- mutex_lock(&cpu_bitmask_lock);
- recursive = tsk;
+ mutex_lock(&cpu_hotplug.lock);
+ cpu_hotplug.refcount++;
+ mutex_unlock(&cpu_hotplug.lock);
+
}
EXPORT_SYMBOL_GPL(lock_cpu_hotplug);
void unlock_cpu_hotplug(void)
{
- WARN_ON(recursive != current);
- if (recursive_depth) {
- recursive_depth--;
+ if (cpu_hotplug.active_writer == current)
return;
- }
- recursive = NULL;
- mutex_unlock(&cpu_bitmask_lock);
+ mutex_lock(&cpu_hotplug.lock);
+ cpu_hotplug.refcount--;
+
+ if (unlikely(writer_exists()) && !cpu_hotplug.refcount)
+ wake_up(&cpu_hotplug.writer_queue);
+
+ mutex_unlock(&cpu_hotplug.lock);
+
}
EXPORT_SYMBOL_GPL(unlock_cpu_hotplug);
#endif /* CONFIG_HOTPLUG_CPU */
+/*
+ * The following two API's must be used when attempting
+ * to serialize the updates to cpu_online_map, cpu_present_map.
+ */
+void cpu_maps_update_begin(void)
+{
+ mutex_lock(&cpu_add_remove_lock);
+}
+
+void cpu_maps_update_done(void)
+{
+ mutex_unlock(&cpu_add_remove_lock);
+}
+
+/*
+ * This ensures that the hotplug operation can begin only when the
+ * refcount goes to zero.
+ *
+ * Note that during a cpu-hotplug operation, the new readers, if any,
+ * will be blocked by the cpu_hotplug.lock
+ *
+ * Since cpu_maps_update_begin 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:
+ * - Refcount goes to zero, last reader wakes up the sleeping
+ * writer.
+ * - Last reader unlocks the cpu_hotplug.lock.
+ * - A new reader arrives at this moment, bumps up the refcount.
+ * - The writer acquires the cpu_hotplug.lock finds the refcount
+ * non zero and goes to sleep again.
+ *
+ * However, this is very difficult to achieve in practice since
+ * lock_cpu_hotplug() not an api which is called all that often.
+ *
+ */
+static void cpu_hotplug_begin(void)
+{
+ DECLARE_WAITQUEUE(wait, current);
+
+ mutex_lock(&cpu_hotplug.lock);
+
+ cpu_hotplug.active_writer = current;
+ add_wait_queue_exclusive(&cpu_hotplug.writer_queue, &wait);
+ while (cpu_hotplug.refcount) {
+ set_current_state(TASK_UNINTERRUPTIBLE);
+ mutex_unlock(&cpu_hotplug.lock);
+ schedule();
+ mutex_lock(&cpu_hotplug.lock);
+ }
+ remove_wait_queue_locked(&cpu_hotplug.writer_queue, &wait);
+}
+
+static void cpu_hotplug_done(void)
+{
+ cpu_hotplug.active_writer = NULL;
+ mutex_unlock(&cpu_hotplug.lock);
+}
/* Need to know about CPUs going up/down? */
int __cpuinit register_cpu_notifier(struct notifier_block *nb)
{
int ret;
- mutex_lock(&cpu_add_remove_lock);
+ cpu_maps_update_begin();
ret = raw_notifier_chain_register(&cpu_chain, nb);
- mutex_unlock(&cpu_add_remove_lock);
+ cpu_maps_update_done();
return ret;
}
@@ -81,9 +151,9 @@
void unregister_cpu_notifier(struct notifier_block *nb)
{
- mutex_lock(&cpu_add_remove_lock);
+ cpu_maps_update_begin();
raw_notifier_chain_unregister(&cpu_chain, nb);
- mutex_unlock(&cpu_add_remove_lock);
+ cpu_maps_update_done();
}
EXPORT_SYMBOL(unregister_cpu_notifier);
@@ -147,6 +217,7 @@
if (!cpu_online(cpu))
return -EINVAL;
+ cpu_hotplug_begin();
raw_notifier_call_chain(&cpu_chain, CPU_LOCK_ACQUIRE, hcpu);
err = __raw_notifier_call_chain(&cpu_chain, CPU_DOWN_PREPARE | mod,
hcpu, -1, &nr_calls);
@@ -166,9 +237,7 @@
cpu_clear(cpu, tmp);
set_cpus_allowed(current, tmp);
- mutex_lock(&cpu_bitmask_lock);
p = __stop_machine_run(take_cpu_down, &tcd_param, cpu);
- mutex_unlock(&cpu_bitmask_lock);
if (IS_ERR(p) || cpu_online(cpu)) {
/* CPU didn't die: tell everyone. Can't complain. */
@@ -203,6 +272,7 @@
set_cpus_allowed(current, old_allowed);
out_release:
raw_notifier_call_chain(&cpu_chain, CPU_LOCK_RELEASE, hcpu);
+ cpu_hotplug_done();
return err;
}
@@ -210,13 +280,13 @@
{
int err = 0;
- mutex_lock(&cpu_add_remove_lock);
+ cpu_maps_update_begin();
if (cpu_hotplug_disabled)
err = -EBUSY;
else
err = _cpu_down(cpu, 0);
- mutex_unlock(&cpu_add_remove_lock);
+ cpu_maps_update_done();
return err;
}
#endif /*CONFIG_HOTPLUG_CPU*/
@@ -231,6 +301,7 @@
if (cpu_online(cpu) || !cpu_present(cpu))
return -EINVAL;
+ cpu_hotplug_begin();
raw_notifier_call_chain(&cpu_chain, CPU_LOCK_ACQUIRE, hcpu);
ret = __raw_notifier_call_chain(&cpu_chain, CPU_UP_PREPARE | mod, hcpu,
-1, &nr_calls);
@@ -243,9 +314,7 @@
}
/* Arch-specific enabling code. */
- mutex_lock(&cpu_bitmask_lock);
ret = __cpu_up(cpu);
- mutex_unlock(&cpu_bitmask_lock);
if (ret != 0)
goto out_notify;
BUG_ON(!cpu_online(cpu));
@@ -258,6 +327,7 @@
__raw_notifier_call_chain(&cpu_chain,
CPU_UP_CANCELED | mod, hcpu, nr_calls, NULL);
raw_notifier_call_chain(&cpu_chain, CPU_LOCK_RELEASE, hcpu);
+ cpu_hotplug_done();
return ret;
}
@@ -275,13 +345,13 @@
return -EINVAL;
}
- mutex_lock(&cpu_add_remove_lock);
+ cpu_maps_update_begin();
if (cpu_hotplug_disabled)
err = -EBUSY;
else
err = _cpu_up(cpu, 0);
- mutex_unlock(&cpu_add_remove_lock);
+ cpu_maps_update_done();
return err;
}
@@ -292,7 +362,7 @@
{
int cpu, first_cpu, error = 0;
- mutex_lock(&cpu_add_remove_lock);
+ cpu_maps_update_begin();
first_cpu = first_cpu(cpu_online_map);
/* We take down all of the non-boot CPUs in one shot to avoid races
* with the userspace trying to use the CPU hotplug at the same time
@@ -319,7 +389,7 @@
} else {
printk(KERN_ERR "Non-boot CPUs are not disabled\n");
}
- mutex_unlock(&cpu_add_remove_lock);
+ cpu_maps_update_done();
return error;
}
@@ -328,7 +398,7 @@
int cpu, error;
/* Allow everyone to use the CPU hotplug again */
- mutex_lock(&cpu_add_remove_lock);
+ cpu_maps_update_begin();
cpu_hotplug_disabled = 0;
if (cpus_empty(frozen_cpus))
goto out;
@@ -344,6 +414,6 @@
}
cpus_clear(frozen_cpus);
out:
- mutex_unlock(&cpu_add_remove_lock);
+ cpu_maps_update_done();
}
#endif /* CONFIG_PM_SLEEP_SMP */
