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PM / Sleep: Fix race between CPU hotplug and freezer

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The CPU hotplug notifications sent out by the _cpu_up() and _cpu_down()
functions depend on the value of the 'tasks_frozen' argument passed to them
(which indicates whether tasks have been frozen or not).
(Examples for such CPU hotplug notifications: CPU_ONLINE, CPU_ONLINE_FROZEN,
CPU_DEAD, CPU_DEAD_FROZEN).

Thus, it is essential that while the callbacks for those notifications are
running, the state of the system with respect to the tasks being frozen or
not remains unchanged, *throughout that duration*. Hence there is a need for
synchronizing the CPU hotplug code with the freezer subsystem.

Since the freezer is involved only in the Suspend/Hibernate call paths, this
patch hooks the CPU hotplug code to the suspend/hibernate notifiers
PM_[SUSPEND|HIBERNATE]_PREPARE and PM_POST_[SUSPEND|HIBERNATE] to prevent
the race between CPU hotplug and freezer, thus ensuring that CPU hotplug
notifications will always be run with the state of the system really being
what the notifications indicate, _throughout_ their execution time.

Signed-off-by: Srivatsa S. Bhat <srivatsa.bhat@linux.vnet.ibm.com>
Signed-off-by: Rafael J. Wysocki <rjw@sisk.pl>
This commit is contained in:
Srivatsa S. Bhat 2011-11-03 00:59:25 +01:00 committed by Rafael J. Wysocki
parent ddf6ce45a7
commit 79cfbdfa87
1 changed files with 74 additions and 0 deletions
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74
kernel/cpu.c
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@ -15,6 +15,7 @@
#include <linux/stop_machine.h>
#include <linux/mutex.h>
#include <linux/gfp.h>
#include <linux/suspend.h>
#ifdef CONFIG_SMP
/* Serializes the updates to cpu_online_mask, cpu_present_mask */
@ -476,6 +477,79 @@ static int alloc_frozen_cpus(void)
return 0;
}
core_initcall(alloc_frozen_cpus);
/*
* Prevent regular CPU hotplug from racing with the freezer, by disabling CPU
* hotplug when tasks are about to be frozen. Also, don't allow the freezer
* to continue until any currently running CPU hotplug operation gets
* completed.
* To modify the 'cpu_hotplug_disabled' flag, we need to acquire the
* 'cpu_add_remove_lock'. And this same lock is also taken by the regular
* CPU hotplug path and released only after it is complete. Thus, we
* (and hence the freezer) will block here until any currently running CPU
* hotplug operation gets completed.
*/
void cpu_hotplug_disable_before_freeze(void)
{
cpu_maps_update_begin();
cpu_hotplug_disabled = 1;
cpu_maps_update_done();
}
/*
* When tasks have been thawed, re-enable regular CPU hotplug (which had been
* disabled while beginning to freeze tasks).
*/
void cpu_hotplug_enable_after_thaw(void)
{
cpu_maps_update_begin();
cpu_hotplug_disabled = 0;
cpu_maps_update_done();
}
/*
* When callbacks for CPU hotplug notifications are being executed, we must
* ensure that the state of the system with respect to the tasks being frozen
* or not, as reported by the notification, remains unchanged *throughout the
* duration* of the execution of the callbacks.
* Hence we need to prevent the freezer from racing with regular CPU hotplug.
*
* This synchronization is implemented by mutually excluding regular CPU
* hotplug and Suspend/Hibernate call paths by hooking onto the Suspend/
* Hibernate notifications.
*/
static int
cpu_hotplug_pm_callback(struct notifier_block *nb,
unsigned long action, void *ptr)
{
switch (action) {
case PM_SUSPEND_PREPARE:
case PM_HIBERNATION_PREPARE:
cpu_hotplug_disable_before_freeze();
break;
case PM_POST_SUSPEND:
case PM_POST_HIBERNATION:
cpu_hotplug_enable_after_thaw();
break;
default:
return NOTIFY_DONE;
}
return NOTIFY_OK;
}
int cpu_hotplug_pm_sync_init(void)
{
pm_notifier(cpu_hotplug_pm_callback, 0);
return 0;
}
core_initcall(cpu_hotplug_pm_sync_init);
#endif /* CONFIG_PM_SLEEP_SMP */
/**