PM / Sleep: Recommend [un]lock_system_sleep() over using pm_mutex directly

Update the documentation to explain the perils of directly using
mutex_[un]lock(&pm_mutex) and recommend the usage of the safe
APIs [un]lock_system_sleep() instead.
Signed-off-by: Srivatsa S. Bhat <srivatsa.bhat@linux.vnet.ibm.com>
Signed-off-by: Rafael J. Wysocki <rjw@sisk.pl>

Documentation/power/freezing-of-tasks.txt

@@ -176,3 +176,28 @@ tasks, since it generally exists anyway.
 A driver must have all firmwares it may need in RAM before suspend() is called.
 If keeping them is not practical, for example due to their size, they must be
 requested early enough using the suspend notifier API described in notifiers.txt.
+
+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
+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
+that could lead to freezing failures, because if the suspend/hibernate code
+successfully acquired the 'pm_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
+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.