This patch implements a Refcount + Waitqueue based model for
cpu-hotplug.
A thread which wants to prevent cpu-hotplug, will now 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 <[email protected]>
---
include/linux/cpu.h | 2 +
init/main.c | 1
kernel/cpu.c | 91 ++++++++++++++++++++++++++++++++++++----------------
3 files changed, 67 insertions(+), 27 deletions(-)
Index: linux-2.6.23/init/main.c
===================================================================
--- linux-2.6.23.orig/init/main.c
+++ linux-2.6.23/init/main.c
@@ -614,6 +614,7 @@ asmlinkage void __init start_kernel(void
vfs_caches_init_early();
cpuset_init_early();
mem_init();
+ cpu_hotplug_init();
kmem_cache_init();
setup_per_cpu_pageset();
numa_policy_init();
Index: linux-2.6.23/include/linux/cpu.h
===================================================================
--- linux-2.6.23.orig/include/linux/cpu.h
+++ linux-2.6.23/include/linux/cpu.h
@@ -97,6 +97,7 @@ static inline void cpuhotplug_mutex_unlo
mutex_unlock(cpu_hp_mutex);
}
+extern void cpu_hotplug_init(void);
extern void lock_cpu_hotplug(void);
extern void unlock_cpu_hotplug(void);
#define hotcpu_notifier(fn, pri) { \
@@ -116,6 +117,7 @@ static inline void cpuhotplug_mutex_lock
static inline void cpuhotplug_mutex_unlock(struct mutex *cpu_hp_mutex)
{ }
+#define cpu_hotplug_init() do { } while (0)
#define lock_cpu_hotplug() do { } while (0)
#define unlock_cpu_hotplug() do { } while (0)
#define hotcpu_notifier(fn, pri) do { (void)(fn); } while (0)
Index: linux-2.6.23/kernel/cpu.c
===================================================================
--- linux-2.6.23.orig/kernel/cpu.c
+++ linux-2.6.23/kernel/cpu.c
@@ -17,7 +17,6 @@
/* This protects CPUs going up and down... */
static DEFINE_MUTEX(cpu_add_remove_lock);
-static DEFINE_MUTEX(cpu_bitmask_lock);
static __cpuinitdata RAW_NOTIFIER_HEAD(cpu_chain);
@@ -26,45 +25,83 @@ static __cpuinitdata RAW_NOTIFIER_HEAD(c
*/
static int cpu_hotplug_disabled;
+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;
+ struct completion readers_done;
+} cpu_hotplug;
+
+#define writer_exists() (cpu_hotplug.active_writer != NULL)
+
#ifdef CONFIG_HOTPLUG_CPU
-/* Crappy recursive lock-takers in cpufreq! Complain loudly about idiots */
-static struct task_struct *recursive;
-static int recursive_depth;
+void __init cpu_hotplug_init(void)
+{
+ cpu_hotplug.active_writer = NULL;
+ mutex_init(&cpu_hotplug.lock);
+ cpu_hotplug.refcount = 0;
+ init_completion(&cpu_hotplug.readers_done);
+}
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)
+ complete(&cpu_hotplug.readers_done);
+
+ mutex_unlock(&cpu_hotplug.lock);
+
}
EXPORT_SYMBOL_GPL(unlock_cpu_hotplug);
#endif /* CONFIG_HOTPLUG_CPU */
+/*
+ * 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
+ */
+static void cpu_hotplug_begin(void)
+{
+ mutex_lock(&cpu_hotplug.lock);
+ cpu_hotplug.active_writer = current;
+ while (cpu_hotplug.refcount) {
+ mutex_unlock(&cpu_hotplug.lock);
+ wait_for_completion(&cpu_hotplug.readers_done);
+ mutex_lock(&cpu_hotplug.lock);
+ }
+
+}
+
+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)
{
@@ -147,6 +184,7 @@ static int _cpu_down(unsigned int cpu, i
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 +204,7 @@ static int _cpu_down(unsigned int cpu, i
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 +239,7 @@ out_allowed:
set_cpus_allowed(current, old_allowed);
out_release:
raw_notifier_call_chain(&cpu_chain, CPU_LOCK_RELEASE, hcpu);
+ cpu_hotplug_done();
return err;
}
@@ -231,6 +268,7 @@ static int __cpuinit _cpu_up(unsigned in
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 +281,7 @@ static int __cpuinit _cpu_up(unsigned in
}
/* 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 +294,7 @@ out_notify:
__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;
}
--
Gautham R Shenoy
Linux Technology Center
IBM India.
"Freedom comes with a price tag of responsibility, which is still a bargain,
because Freedom is priceless!"
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