[PATCH v5] CPU hotplug, Freezer: Fix race between CPU hotplug and freezer

[PATCH v5] CPU hotplug, Freezer: Fix race between CPU hotplug and freezer

[Srivatsa S. Bhat]

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.

v5: * Removed the new freezer notifiers since freezer is used only in suspend
      and hibernate call paths. Used the suspend/hibernate notifier events
      to synchronize CPU hotplug and freezer.

v4: * Retained the value 0 for the 'tasks_frozen' argument, while calling
      _cpu_up() and _cpu_down().
      Removed the unnecessary PM_POST_FREEZE and PM_THAW_PREPARE notifications.

v3: * Added synchronization between CPU hotplug and the freezer subsystem
      without introducing any new locks in the CPU hotplug call path.

v2: * Removed the atomic_t declaration of tasks_frozen flag and the
      atomic_[set|read] functions since they were unnecessary.
    * Updated the changelog to give an example scenario where things could go
      wrong due to the bug in the CPU hotplug call path.

References:
v1 -> http://thread.gmane.org/gmane.linux.kernel/1198312/
v2 -> http://thread.gmane.org/gmane.linux.kernel/1198312/focus=1199087
v3 -> http://thread.gmane.org/gmane.linux.documentation/3472
v4 -> http://thread.gmane.org/gmane.linux.documentation/3485

Signed-off-by: Srivatsa S. Bhat <srivatsa.bhat@linux.vnet.ibm.com>
---

 kernel/cpu.c |   74 ++++++++++++++++++++++++++++++++++++++++++++++++++++++++++
 1 files changed, 74 insertions(+), 0 deletions(-)

diff --git a/kernel/cpu.c b/kernel/cpu.c
index 12b7458..aa39dd7 100644
--- a/kernel/cpu.c
+++ b/kernel/cpu.c
@@ -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 */
 
 /**

Re: [PATCH v5] CPU hotplug, Freezer: Fix race between CPU hotplug and freezer

[Rafael J. Wysocki]

On Monday, October 31, 2011, Srivatsa S. Bhat wrote:
> 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.
> 
> v5: * Removed the new freezer notifiers since freezer is used only in suspend
>       and hibernate call paths. Used the suspend/hibernate notifier events
>       to synchronize CPU hotplug and freezer.
> 
> v4: * Retained the value 0 for the 'tasks_frozen' argument, while calling
>       _cpu_up() and _cpu_down().
>       Removed the unnecessary PM_POST_FREEZE and PM_THAW_PREPARE notifications.
> 
> v3: * Added synchronization between CPU hotplug and the freezer subsystem
>       without introducing any new locks in the CPU hotplug call path.
> 
> v2: * Removed the atomic_t declaration of tasks_frozen flag and the
>       atomic_[set|read] functions since they were unnecessary.
>     * Updated the changelog to give an example scenario where things could go
>       wrong due to the bug in the CPU hotplug call path.
> 
> References:
> v1 -> http://thread.gmane.org/gmane.linux.kernel/1198312/
> v2 -> http://thread.gmane.org/gmane.linux.kernel/1198312/focus=1199087
> v3 -> http://thread.gmane.org/gmane.linux.documentation/3472
> v4 -> http://thread.gmane.org/gmane.linux.documentation/3485
> 
> Signed-off-by: Srivatsa S. Bhat <srivatsa.bhat@linux.vnet.ibm.com>

Applied to linux-pm/linux-next.

Thanks,
Rafael


> ---
> 
>  kernel/cpu.c |   74 ++++++++++++++++++++++++++++++++++++++++++++++++++++++++++
>  1 files changed, 74 insertions(+), 0 deletions(-)
> 
> diff --git a/kernel/cpu.c b/kernel/cpu.c
> index 12b7458..aa39dd7 100644
> --- a/kernel/cpu.c
> +++ b/kernel/cpu.c
> @@ -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 */
>  
>  /**
> 
> 
>