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.]
