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-rw-r--r--Documentation/power/freezing-of-tasks.txt31
1 files changed, 19 insertions, 12 deletions
diff --git a/Documentation/power/freezing-of-tasks.txt b/Documentation/power/freezing-of-tasks.txt
index d5c65e8d6a37..38b57248fd61 100644
--- a/Documentation/power/freezing-of-tasks.txt
+++ b/Documentation/power/freezing-of-tasks.txt
@@ -19,12 +19,13 @@ we only consider hibernation, but the description also applies to suspend).
19Namely, as the first step of the hibernation procedure the function 19Namely, as the first step of the hibernation procedure the function
20freeze_processes() (defined in kernel/power/process.c) is called. It executes 20freeze_processes() (defined in kernel/power/process.c) is called. It executes
21try_to_freeze_tasks() that sets TIF_FREEZE for all of the freezable tasks and 21try_to_freeze_tasks() that sets TIF_FREEZE for all of the freezable tasks and
22sends a fake signal to each of them. A task that receives such a signal and has 22either wakes them up, if they are kernel threads, or sends fake signals to them,
23TIF_FREEZE set, should react to it by calling the refrigerator() function 23if they are user space processes. A task that has TIF_FREEZE set, should react
24(defined in kernel/power/process.c), which sets the task's PF_FROZEN flag, 24to it by calling the function called refrigerator() (defined in
25changes its state to TASK_UNINTERRUPTIBLE and makes it loop until PF_FROZEN is 25kernel/power/process.c), which sets the task's PF_FROZEN flag, changes its state
26cleared for it. Then, we say that the task is 'frozen' and therefore the set of 26to TASK_UNINTERRUPTIBLE and makes it loop until PF_FROZEN is cleared for it.
27functions handling this mechanism is called 'the freezer' (these functions are 27Then, we say that the task is 'frozen' and therefore the set of functions
28handling this mechanism is referred to as 'the freezer' (these functions are
28defined in kernel/power/process.c and include/linux/freezer.h). User space 29defined in kernel/power/process.c and include/linux/freezer.h). User space
29processes are generally frozen before kernel threads. 30processes are generally frozen before kernel threads.
30 31
@@ -35,21 +36,27 @@ task enter refrigerator() if the flag is set.
35 36
36For user space processes try_to_freeze() is called automatically from the 37For user space processes try_to_freeze() is called automatically from the
37signal-handling code, but the freezable kernel threads need to call it 38signal-handling code, but the freezable kernel threads need to call it
38explicitly in suitable places. The code to do this may look like the following: 39explicitly in suitable places or use the wait_event_freezable() or
40wait_event_freezable_timeout() macros (defined in include/linux/freezer.h)
41that combine interruptible sleep with checking if TIF_FREEZE is set and calling
42try_to_freeze(). The main loop of a freezable kernel thread may look like the
43following one:
39 44
45 set_freezable();
40 do { 46 do {
41 hub_events(); 47 hub_events();
42 wait_event_interruptible(khubd_wait, 48 wait_event_freezable(khubd_wait,
43 !list_empty(&hub_event_list)); 49 !list_empty(&hub_event_list) ||
44 try_to_freeze(); 50 kthread_should_stop());
45 } while (!signal_pending(current)); 51 } while (!kthread_should_stop() || !list_empty(&hub_event_list));
46 52
47(from drivers/usb/core/hub.c::hub_thread()). 53(from drivers/usb/core/hub.c::hub_thread()).
48 54
49If a freezable kernel thread fails to call try_to_freeze() after the freezer has 55If a freezable kernel thread fails to call try_to_freeze() after the freezer has
50set TIF_FREEZE for it, the freezing of tasks will fail and the entire 56set TIF_FREEZE for it, the freezing of tasks will fail and the entire
51hibernation operation will be cancelled. For this reason, freezable kernel 57hibernation operation will be cancelled. For this reason, freezable kernel
52threads must call try_to_freeze() somewhere. 58threads must call try_to_freeze() somewhere or use one of the
59wait_event_freezable() and wait_event_freezable_timeout() macros.
53 60
54After the system memory state has been restored from a hibernation image and 61After the system memory state has been restored from a hibernation image and
55devices have been reinitialized, the function thaw_processes() is called in 62devices have been reinitialized, the function thaw_processes() is called in