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linux-next/Documentation/power/kernel_threads.txt
Christoph Lameter 3e1d1d28d9 [PATCH] Cleanup patch for process freezing
1. Establish a simple API for process freezing defined in linux/include/sched.h:

   frozen(process)		Check for frozen process
   freezing(process)		Check if a process is being frozen
   freeze(process)		Tell a process to freeze (go to refrigerator)
   thaw_process(process)	Restart process
   frozen_process(process)	Process is frozen now

2. Remove all references to PF_FREEZE and PF_FROZEN from all
   kernel sources except sched.h

3. Fix numerous locations where try_to_freeze is manually done by a driver

4. Remove the argument that is no longer necessary from two function calls.

5. Some whitespace cleanup

6. Clear potential race in refrigerator (provides an open window of PF_FREEZE
   cleared before setting PF_FROZEN, recalc_sigpending does not check
   PF_FROZEN).

This patch does not address the problem of freeze_processes() violating the rule
that a task may only modify its own flags by setting PF_FREEZE. This is not clean
in an SMP environment. freeze(process) is therefore not SMP safe!

Signed-off-by: Christoph Lameter <christoph@lameter.com>
Signed-off-by: Linus Torvalds <torvalds@osdl.org>
2005-06-25 17:10:13 -07:00

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KERNEL THREADS
Freezer
Upon entering a suspended state the system will freeze all
tasks. This is done by delivering pseudosignals. This affects
kernel threads, too. To successfully freeze a kernel thread
the thread has to check for the pseudosignal and enter the
refrigerator. Code to do this looks like this:
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()
The Unfreezable
Some kernel threads however, must not be frozen. The kernel must
be able to finish pending IO operations and later on be able to
write the memory image to disk. Kernel threads needed to do IO
must stay awake. Such threads must mark themselves unfreezable
like this:
/*
* This thread doesn't need any user-level access,
* so get rid of all our resources.
*/
daemonize("usb-storage");
current->flags |= PF_NOFREEZE;
from drivers/usb/storage/usb.c::usb_stor_control_thread()
Such drivers are themselves responsible for staying quiet during
the actual snapshotting.