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+=================
+Freezing of tasks
+=================
+
+(C) 2007 Rafael J. Wysocki <rjw@sisk.pl>, GPL
+
+I. What is the freezing of tasks?
+=================================
+
+The freezing of tasks is a mechanism by which user space processes and some
+kernel threads are controlled during hibernation or system-wide suspend (on some
+architectures).
+
+II. How does it work?
+=====================
+
+There are three per-task flags used for that, PF_NOFREEZE, PF_FROZEN
+and PF_FREEZER_SKIP (the last one is auxiliary).  The tasks that have
+PF_NOFREEZE unset (all user space processes and some kernel threads) are
+regarded as 'freezable' and treated in a special way before the system enters a
+suspend state as well as before a hibernation image is created (in what follows
+we only consider hibernation, but the description also applies to suspend).
+
+Namely, as the first step of the hibernation procedure the function
+freeze_processes() (defined in kernel/power/process.c) is called.  A system-wide
+variable system_freezing_cnt (as opposed to a per-task flag) is used to indicate
+whether the system is to undergo a freezing operation. And freeze_processes()
+sets this variable.  After this, it executes try_to_freeze_tasks() that sends a
+fake signal to all user space processes, and wakes up all the kernel threads.
+All freezable tasks must react to that by calling try_to_freeze(), which
+results in a call to __refrigerator() (defined in kernel/freezer.c), which sets
+the task's PF_FROZEN flag, changes its state to TASK_UNINTERRUPTIBLE and makes
+it loop until PF_FROZEN is cleared for it. Then, we say that the task is
+'frozen' and therefore the set of functions handling this mechanism is referred
+to as 'the freezer' (these functions are defined in kernel/power/process.c,
+kernel/freezer.c & include/linux/freezer.h). User space processes are generally
+frozen before kernel threads.
+
+__refrigerator() must not be called directly.  Instead, use the
+try_to_freeze() function (defined in include/linux/freezer.h), that checks
+if the task is to be frozen and makes the task enter __refrigerator().
+
+For user space processes try_to_freeze() is called automatically from the
+signal-handling code, but the freezable kernel threads need to call it
+explicitly in suitable places or use the wait_event_freezable() or
+wait_event_freezable_timeout() macros (defined in include/linux/freezer.h)
+that combine interruptible sleep with checking if the task is to be frozen and
+calling try_to_freeze().  The main loop of a freezable kernel thread may look
+like the following one::
+
+	set_freezable();
+	do {
+		hub_events();
+		wait_event_freezable(khubd_wait,
+				!list_empty(&hub_event_list) ||
+				kthread_should_stop());
+	} while (!kthread_should_stop() || !list_empty(&hub_event_list));
+
+(from drivers/usb/core/hub.c::hub_thread()).
+
+If a freezable kernel thread fails to call try_to_freeze() after the freezer has
+initiated a freezing operation, the freezing of tasks will fail and the entire
+hibernation operation will be cancelled.  For this reason, freezable kernel
+threads must call try_to_freeze() somewhere or use one of the
+wait_event_freezable() and wait_event_freezable_timeout() macros.
+
+After the system memory state has been restored from a hibernation image and
+devices have been reinitialized, the function thaw_processes() is called in
+order to clear the PF_FROZEN flag for each frozen task.  Then, the tasks that
+have been frozen leave __refrigerator() and continue running.
+
+
+Rationale behind the functions dealing with freezing and thawing of tasks
+-------------------------------------------------------------------------
+
+freeze_processes():
+  - freezes only userspace tasks
+
+freeze_kernel_threads():
+  - freezes all tasks (including kernel threads) because we can't freeze
+    kernel threads without freezing userspace tasks
+
+thaw_kernel_threads():
+  - thaws only kernel threads; this is particularly useful if we need to do
+    anything special in between thawing of kernel threads and thawing of
+    userspace tasks, or if we want to postpone the thawing of userspace tasks
+
+thaw_processes():
+  - thaws all tasks (including kernel threads) because we can't thaw userspace
+    tasks without thawing kernel threads
+
+
+III. Which kernel threads are freezable?
+========================================
+
+Kernel threads are not freezable by default.  However, a kernel thread may clear
+PF_NOFREEZE for itself by calling set_freezable() (the resetting of PF_NOFREEZE
+directly is not allowed).  From this point it is regarded as freezable
+and must call try_to_freeze() in a suitable place.
+
+IV. Why do we do that?
+======================
+
+Generally speaking, there is a couple of reasons to use the freezing of tasks:
+
+1. The principal reason is to prevent filesystems from being damaged after
+   hibernation.  At the moment we have no simple means of checkpointing
+   filesystems, so if there are any modifications made to filesystem data and/or
+   metadata on disks, we cannot bring them back to the state from before the
+   modifications.  At the same time each hibernation image contains some
+   filesystem-related information that must be consistent with the state of the
+   on-disk data and metadata after the system memory state has been restored
+   from the image (otherwise the filesystems will be damaged in a nasty way,
+   usually making them almost impossible to repair).  We therefore freeze
+   tasks that might cause the on-disk filesystems' data and metadata to be
+   modified after the hibernation image has been created and before the
+   system is finally powered off. The majority of these are user space
+   processes, but if any of the kernel threads may cause something like this
+   to happen, they have to be freezable.
+
+2. Next, to create the hibernation image we need to free a sufficient amount of
+   memory (approximately 50% of available RAM) and we need to do that before
+   devices are deactivated, because we generally need them for swapping out.
+   Then, after the memory for the image has been freed, we don't want tasks
+   to allocate additional memory and we prevent them from doing that by
+   freezing them earlier. [Of course, this also means that device drivers
+   should not allocate substantial amounts of memory from their .suspend()
+   callbacks before hibernation, but this is a separate issue.]
+
+3. The third reason is to prevent user space processes and some kernel threads
+   from interfering with the suspending and resuming of devices.  A user space
+   process running on a second CPU while we are suspending devices may, for
+   example, be troublesome and without the freezing of tasks we would need some
+   safeguards against race conditions that might occur in such a case.
+
+Although Linus Torvalds doesn't like the freezing of tasks, he said this in one
+of the discussions on LKML (http://lkml.org/lkml/2007/4/27/608):
+
+"RJW:> Why we freeze tasks at all or why we freeze kernel threads?
+
+Linus: In many ways, 'at all'.
+
+I **do** realize the IO request queue issues, and that we cannot actually do
+s2ram with some devices in the middle of a DMA.  So we want to be able to
+avoid *that*, there's no question about that.  And I suspect that stopping
+user threads and then waiting for a sync is practically one of the easier
+ways to do so.
+
+So in practice, the 'at all' may become a 'why freeze kernel threads?' and
+freezing user threads I don't find really objectionable."
+
+Still, there are kernel threads that may want to be freezable.  For example, if
+a kernel thread that belongs to a device driver accesses the device directly, it
+in principle needs to know when the device is suspended, so that it doesn't try
+to access it at that time.  However, if the kernel thread is freezable, it will
+be frozen before the driver's .suspend() callback is executed and it will be
+thawed after the driver's .resume() callback has run, so it won't be accessing
+the device while it's suspended.
+
+4. Another reason for freezing tasks is to prevent user space processes from
+   realizing that hibernation (or suspend) operation takes place.  Ideally, user
+   space processes should not notice that such a system-wide operation has
+   occurred and should continue running without any problems after the restore
+   (or resume from suspend).  Unfortunately, in the most general case this
+   is quite difficult to achieve without the freezing of tasks.  Consider,
+   for example, a process that depends on all CPUs being online while it's
+   running.  Since we need to disable nonboot CPUs during the hibernation,
+   if this process is not frozen, it may notice that the number of CPUs has
+   changed and may start to work incorrectly because of that.
+
+V. Are there any problems related to the freezing of tasks?
+===========================================================
+
+Yes, there are.
+
+First of all, the freezing of kernel threads may be tricky if they depend one
+on another.  For example, if kernel thread A waits for a completion (in the
+TASK_UNINTERRUPTIBLE state) that needs to be done by freezable kernel thread B
+and B is frozen in the meantime, then A will be blocked until B is thawed, which
+may be undesirable.  That's why kernel threads are not freezable by default.
+
+Second, there are the following two problems related to the freezing of user
+space processes:
+
+1. Putting processes into an uninterruptible sleep distorts the load average.
+2. Now that we have FUSE, plus the framework for doing device drivers in
+   userspace, it gets even more complicated because some userspace processes are
+   now doing the sorts of things that kernel threads do
+   (https://lists.linux-foundation.org/pipermail/linux-pm/2007-May/012309.html).
+
+The problem 1. seems to be fixable, although it hasn't been fixed so far.  The
+other one is more serious, but it seems that we can work around it by using
+hibernation (and suspend) notifiers (in that case, though, we won't be able to
+avoid the realization by the user space processes that the hibernation is taking
+place).
+
+There are also problems that the freezing of tasks tends to expose, although
+they are not directly related to it.  For example, if request_firmware() is
+called from a device driver's .resume() routine, it will timeout and eventually
+fail, because the user land process that should respond to the request is frozen
+at this point.  So, seemingly, the failure is due to the freezing of tasks.
+Suppose, however, that the firmware file is located on a filesystem accessible
+only through another device that hasn't been resumed yet.  In that case,
+request_firmware() will fail regardless of whether or not the freezing of tasks
+is used.  Consequently, the problem is not really related to the freezing of
+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
+Documentation/driver-api/pm/notifiers.rst.
+
+VI. Are there any precautions to be taken to prevent freezing failures?
+=======================================================================
+
+Yes, there are.
+
+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 '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
+'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 '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(&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
+all user space processes or all freezable kernel threads, in unit of
+millisecond.  The default value is 20000, with range of unsigned integer.