Commit 79cfbdfa authored by Srivatsa S. Bhat's avatar Srivatsa S. Bhat Committed by Rafael J. Wysocki
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PM / Sleep: Fix race between CPU hotplug and freezer 



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: default avatarSrivatsa S. Bhat <srivatsa.bhat@linux.vnet.ibm.com>
Signed-off-by: default avatarRafael J. Wysocki <rjw@sisk.pl>
parent ddf6ce45

kernel/cpu.c

+74 −0
Original line number Diff line number Diff line
@@ -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 */



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