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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 four per-task flags used for that, PF_NOFREEZE, PF_FROZEN, TIF_FREEZE
+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.  It executes
+try_to_freeze_tasks() that sets TIF_FREEZE for all of the freezable tasks and
+sends a fake signal to each of them.  A task that receives such a signal and has
+TIF_FREEZE set, should react to it by calling the refrigerator() function
+(defined in kernel/power/process.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 called 'the freezer' (these functions are
+defined in kernel/power/process.c and include/linux/freezer.h).  User space
+processes are generally frozen before kernel threads.
+
+It is not recommended to call refrigerator() directly.  Instead, it is
+recommended to use the try_to_freeze() function (defined in
+include/linux/freezer.h), that checks the task's TIF_FREEZE flag and makes the
+task enter refrigerator() if the flag is set.
+
+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.  The code to do this may look like the following:
+
+        do {
+                hub_events();
+                wait_event_interruptible(khubd_wait,
+                                        !list_empty(&hub_event_list));
+                try_to_freeze();
+        } while (!signal_pending(current));
+
+(from drivers/usb/core/hub.c::hub_thread()).
+
+If a freezable kernel thread fails to call try_to_freeze() after the freezer has
+set TIF_FREEZE for it, 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.
+
+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.
+
+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 strongly discouraged).  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. The second 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 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.
+
+3. 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.  [The solution to this particular
+problem is to keep the firmware in memory after it's loaded for the first time
+and upload if from memory to the device whenever necessary.]