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Subject[PATCH 2/4] Define and use new events,CPU_LOCK_ACQUIRE and CPU_LOCK_RELEASE.
This is an attempt to provide an alternate mechanism for postponing
a hotplug event instead of using a global mechanism like lock_cpu_hotplug.

The proposal is to add two new events namely CPU_LOCK_ACQUIRE and
CPU_LOCK_RELEASE. The notification for these two events would be sent
out before and after a cpu_hotplug event respectively.

During the CPU_LOCK_ACQUIRE event, a cpu-hotplug-aware subsystem is
supposed to acquire any per-subsystem hotcpu mutex ( Eg. workqueue_mutex
in kernel/workqueue.c ).

During the CPU_LOCK_RELEASE release event the cpu-hotplug-aware subsystem
is supposed to release the per-subsystem hotcpu mutex.

The reasons for defining new events as opposed to reusing the existing events
like CPU_UP_PREPARE/CPU_UP_FAILED/CPU_ONLINE for locking/unlocking of
per-subsystem hotcpu mutexes are as follow:

- CPU_LOCK_ACQUIRE: All hotcpu mutexes are taken before subsystems
start handling pre-hotplug events like CPU_UP_PREPARE/CPU_DOWN_PREPARE
etc, thus ensuring a clean handling of these events.

- CPU_LOCK_RELEASE: The hotcpu mutexes will be released only after
all subsystems have handled post-hotplug events like CPU_DOWN_FAILED,
CPU_DEAD,CPU_ONLINE etc thereby ensuring that there are no subsequent
clashes amongst the interdependent subsystems after a cpu hotplugs.

This patch also uses __raw_notifier_call chain in _cpu_up to take care
of the dependency between the two consequetive calls to

Signed-off-by: Gautham R Shenoy <>

include/linux/notifier.h | 2 ++
kernel/cpu.c | 15 +++++++++++----
2 files changed, 13 insertions(+), 4 deletions(-)

Index: hotplug/kernel/cpu.c
--- hotplug.orig/kernel/cpu.c
+++ hotplug/kernel/cpu.c
@@ -132,6 +132,8 @@ static int _cpu_down(unsigned int cpu)
if (!cpu_online(cpu))
return -EINVAL;

+ raw_notifier_call_chain(&cpu_chain, CPU_LOCK_ACQUIRE,
+ (void *)(long)cpu);
err = raw_notifier_call_chain(&cpu_chain, CPU_DOWN_PREPARE,
(void *)(long)cpu);
if (err == NOTIFY_BAD) {
@@ -185,6 +187,8 @@ out_thread:
err = kthread_stop(p);
set_cpus_allowed(current, old_allowed);
+ raw_notifier_call_chain(&cpu_chain, CPU_LOCK_RELEASE,
+ (void *)(long)cpu);
return err;

@@ -206,13 +210,15 @@ int cpu_down(unsigned int cpu)
/* Requires cpu_add_remove_lock to be held */
static int __devinit _cpu_up(unsigned int cpu)
- int ret;
+ int ret, nr_calls = 0;
void *hcpu = (void *)(long)cpu;

if (cpu_online(cpu) || !cpu_present(cpu))
return -EINVAL;

- ret = raw_notifier_call_chain(&cpu_chain, CPU_UP_PREPARE, hcpu);
+ raw_notifier_call_chain(&cpu_chain, CPU_LOCK_ACQUIRE, hcpu);
+ ret = __raw_notifier_call_chain(&cpu_chain, CPU_UP_PREPARE, hcpu,
+ -1, &nr_calls);
if (ret == NOTIFY_BAD) {
printk("%s: attempt to bring up CPU %u failed\n",
__FUNCTION__, cpu);
@@ -233,8 +239,9 @@ static int __devinit _cpu_up(unsigned in

if (ret != 0)
- raw_notifier_call_chain(&cpu_chain,
+ __raw_notifier_call_chain(&cpu_chain,
+ CPU_UP_CANCELED, hcpu, nr_calls, NULL);
+ raw_notifier_call_chain(&cpu_chain, CPU_LOCK_RELEASE, hcpu);

return ret;
Index: hotplug/include/linux/notifier.h
--- hotplug.orig/include/linux/notifier.h
+++ hotplug/include/linux/notifier.h
@@ -194,6 +194,8 @@ extern int __srcu_notifier_call_chain(st
#define CPU_DOWN_PREPARE 0x0005 /* CPU (unsigned)v going down */
#define CPU_DOWN_FAILED 0x0006 /* CPU (unsigned)v NOT going down */
#define CPU_DEAD 0x0007 /* CPU (unsigned)v dead */
+#define CPU_LOCK_ACQUIRE 0x0008 /* Acquire all hotcpu locks */
+#define CPU_LOCK_RELEASE 0x0009 /* Release all hotcpu locks */

#endif /* __KERNEL__ */
#endif /* _LINUX_NOTIFIER_H */
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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