PM / reboot: Eliminate race between reboot and suspend

At present, "systemctl suspend" and "shutdown" can run in parrallel. A
system can suspend after devices_shutdown(), and resume. Then the shutdown
task goes on to power off. This causes many devices are not really shut
off. Hence replacing reboot_mutex with system_transition_mutex (renamed
from pm_mutex) to achieve the exclusion. The renaming of pm_mutex as
system_transition_mutex can be better to reflect the purpose of the mutex.

Signed-off-by: Pingfan Liu <[email protected]>
Acked-by: Pavel Machek <[email protected]>
Signed-off-by: Rafael J. Wysocki <[email protected]>

Documentation/power/freezing-of-tasks.txt

@@ -204,26 +204,26 @@ VI. Are there any precautions to be taken to prevent freezing failures?
 
 Yes, there are.
 
-First of all, grabbing the 'pm_mutex' lock to mutually exclude a piece of code
+First of all, grabbing the 'system_transition_mutex' lock to mutually exclude a piece of code
 from system-wide sleep such as suspend/hibernation is not encouraged.
 If possible, that piece of code must instead hook onto the suspend/hibernation
 notifiers to achieve mutual exclusion. Look at the CPU-Hotplug code
 (kernel/cpu.c) for an example.
 
-However, if that is not feasible, and grabbing 'pm_mutex' is deemed necessary,
-it is strongly discouraged to directly call mutex_[un]lock(&pm_mutex) since
+However, if that is not feasible, and grabbing 'system_transition_mutex' is deemed necessary,
+it is strongly discouraged to directly call mutex_[un]lock(&system_transition_mutex) since
 that could lead to freezing failures, because if the suspend/hibernate code
-successfully acquired the 'pm_mutex' lock, and hence that other entity failed
+successfully acquired the 'system_transition_mutex' lock, and hence that other entity failed
 to acquire the lock, then that task would get blocked in TASK_UNINTERRUPTIBLE
 state. As a consequence, the freezer would not be able to freeze that task,
 leading to freezing failure.
 
 However, the [un]lock_system_sleep() APIs are safe to use in this scenario,
 since they ask the freezer to skip freezing this task, since it is anyway
-"frozen enough" as it is blocked on 'pm_mutex', which will be released
+"frozen enough" as it is blocked on 'system_transition_mutex', which will be released
 only after the entire suspend/hibernation sequence is complete.
 So, to summarize, use [un]lock_system_sleep() instead of directly using
-mutex_[un]lock(&pm_mutex). That would prevent freezing failures.
+mutex_[un]lock(&system_transition_mutex). That would prevent freezing failures.
 
 V. Miscellaneous
 /sys/power/pm_freeze_timeout controls how long it will cost at most to freeze

Documentation/power/suspend-and-cpuhotplug.txt

@@ -32,7 +32,7 @@ More details follow:
                                     sysfs file
                                         |
                                         v
-                               Acquire pm_mutex lock
+                               Acquire system_transition_mutex lock
                                         |
                                         v
                              Send PM_SUSPEND_PREPARE
@@ -96,10 +96,10 @@ execution during resume):
 
 * thaw tasks
 * send PM_POST_SUSPEND notifications
-* Release pm_mutex lock.
+* Release system_transition_mutex lock.
 
 
-It is to be noted here that the pm_mutex lock is acquired at the very
+It is to be noted here that the system_transition_mutex lock is acquired at the very
 beginning, when we are just starting out to suspend, and then released only
 after the entire cycle is complete (i.e., suspend + resume).