From: Oliver Neukum This is additional documentation for power management. Pavel Machek has given his acknowledgement. Signed-Off-By: Oliver Neukum Signed-off-by: Andrew Morton --- 25-akpm/Documentation/power/kernel_threads.txt | 41 +++++++++++++++++++++++++ 1 files changed, 41 insertions(+) diff -puN /dev/null Documentation/power/kernel_threads.txt --- /dev/null 2003-09-15 06:40:47.000000000 -0700 +++ 25-akpm/Documentation/power/kernel_threads.txt 2004-10-05 02:27:34.060467400 -0700 @@ -0,0 +1,41 @@ +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)); + if (current->flags & PF_FREEZE) + refrigerator(PF_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. _