freezer: do not send signals to kernel threads
The freezer should not send signals to kernel threads, since that may lead to
subtle problems. In particular, commit
b74d0deb968e1f85942f17080eace015ce3c332c has changed recalc_sigpending_tsk()
so that it doesn't clear TIF_SIGPENDING. For this reason, if the freezer
continues to send fake signals to kernel threads and the freezing of kernel
threads fails, some of them may be running with TIF_SIGPENDING set forever.
Accordingly, recalc_sigpending_tsk() shouldn't set the task's TIF_SIGPENDING
flag if TIF_FREEZE is set.
Signed-off-by: Rafael J. Wysocki <rjw@sisk.pl>
Cc: Nigel Cunningham <nigel@nigel.suspend2.net>
Cc: Pavel Machek <pavel@ucw.cz>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
diff --git a/Documentation/power/freezing-of-tasks.txt b/Documentation/power/freezing-of-tasks.txt
index d5c65e8..38b5724 100644
--- a/Documentation/power/freezing-of-tasks.txt
+++ b/Documentation/power/freezing-of-tasks.txt
@@ -19,12 +19,13 @@
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
+either wakes them up, if they are kernel threads, or sends fake signals to them,
+if they are user space processes. A task that has TIF_FREEZE set, should react
+to it by calling the function called refrigerator() (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 referred to as '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.
@@ -35,21 +36,27 @@
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:
+explicitly in suitable places or use the wait_event_freezable() or
+wait_event_freezable_timeout() macros (defined in include/linux/freezer.h)
+that combine interruptible sleep with checking if TIF_FREEZE is set and calling
+try_to_freeze(). The main loop of a freezable kernel thread may look like the
+following one:
+ set_freezable();
do {
hub_events();
- wait_event_interruptible(khubd_wait,
- !list_empty(&hub_event_list));
- try_to_freeze();
- } while (!signal_pending(current));
+ wait_event_freezable(khubd_wait,
+ !list_empty(&hub_event_list) ||
+ kthread_should_stop());
+ } while (!kthread_should_stop() || !list_empty(&hub_event_list));
(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.
+threads must call try_to_freeze() somewhere or use one of the
+wait_event_freezable() and wait_event_freezable_timeout() macros.
After the system memory state has been restored from a hibernation image and
devices have been reinitialized, the function thaw_processes() is called in