Commit d5d8c597 authored by Rafael J. Wysocki's avatar Rafael J. Wysocki Committed by Linus Torvalds
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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
b74d0deb 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: default avatarRafael J. Wysocki <rjw@sisk.pl>
Cc: Nigel Cunningham <nigel@nigel.suspend2.net>
Cc: Pavel Machek <pavel@ucw.cz>
Signed-off-by: default avatarAndrew Morton <akpm@linux-foundation.org>
Signed-off-by: default avatarLinus Torvalds <torvalds@linux-foundation.org>
parent e42837bc

Documentation/power/freezing-of-tasks.txt

+19 −12
Original line number Diff line number Diff line
@@ -19,12 +19,13 @@ 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

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 @@ 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:

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

kernel/power/process.c

+74 −33
Original line number Diff line number Diff line
@@ -75,21 +75,79 @@ void refrigerator(void) 
	__set_current_state(save);

}



static void freeze_task(struct task_struct *p)

static void fake_signal_wake_up(struct task_struct *p, int resume)

{

	unsigned long flags;



	if (!freezing(p)) {

		rmb();

		if (!frozen(p)) {

			set_freeze_flag(p);

			if (p->state == TASK_STOPPED)

				force_sig_specific(SIGSTOP, p);

	spin_lock_irqsave(&p->sighand->siglock, flags);

			signal_wake_up(p, p->state == TASK_STOPPED);

	signal_wake_up(p, resume);

	spin_unlock_irqrestore(&p->sighand->siglock, flags);

}



static void send_fake_signal(struct task_struct *p)

{

	if (p->state == TASK_STOPPED)

		force_sig_specific(SIGSTOP, p);

	fake_signal_wake_up(p, p->state == TASK_STOPPED);

}



static int has_mm(struct task_struct *p)

{

	return (p->mm && !(p->flags & PF_BORROWED_MM));

}



/**

 *	freeze_task - send a freeze request to given task

 *	@p: task to send the request to

 *	@with_mm_only: if set, the request will only be sent if the task has its

 *		own mm

 *	Return value: 0, if @with_mm_only is set and the task has no mm of its

 *		own or the task is frozen, 1, otherwise

 *

 *	The freeze request is sent by seting the tasks's TIF_FREEZE flag and

 *	either sending a fake signal to it or waking it up, depending on whether

 *	or not it has its own mm (ie. it is a user land task).  If @with_mm_only

 *	is set and the task has no mm of its own (ie. it is a kernel thread),

 *	its TIF_FREEZE flag should not be set.

 *

 *	The task_lock() is necessary to prevent races with exit_mm() or

 *	use_mm()/unuse_mm() from occuring.

 */

static int freeze_task(struct task_struct *p, int with_mm_only)

{

	int ret = 1;



	task_lock(p);

	if (freezing(p)) {

		if (has_mm(p)) {

			if (!signal_pending(p))

				fake_signal_wake_up(p, 0);

		} else {

			if (with_mm_only)

				ret = 0;

			else

				wake_up_state(p, TASK_INTERRUPTIBLE);

		}

	} else {

		rmb();

		if (frozen(p)) {

			ret = 0;

		} else {

			if (has_mm(p)) {

				set_freeze_flag(p);

				send_fake_signal(p);

			} else {

				if (with_mm_only) {

					ret = 0;

				} else {

					set_freeze_flag(p);

					wake_up_state(p, TASK_INTERRUPTIBLE);

				}

			}

		}

	}

	task_unlock(p);

	return ret;

}



static void cancel_freezing(struct task_struct *p)
@@ -119,31 +177,14 @@ static int try_to_freeze_tasks(int freeze_user_space) 
			if (frozen(p) || !freezeable(p))

				continue;



			if (freeze_user_space) {

				if (p->state == TASK_TRACED &&

				    frozen(p->parent)) {

			if (p->state == TASK_TRACED && frozen(p->parent)) {

				cancel_freezing(p);

				continue;

			}

				/*

				 * Kernel threads should not have TIF_FREEZE set

				 * at this point, so we must ensure that either

				 * p->mm is not NULL *and* PF_BORROWED_MM is

				 * unset, or TIF_FRREZE is left unset.

				 * The task_lock() is necessary to prevent races

				 * with exit_mm() or use_mm()/unuse_mm() from

				 * occuring.

				 */

				task_lock(p);

				if (!p->mm || (p->flags & PF_BORROWED_MM)) {

					task_unlock(p);



			if (!freeze_task(p, freeze_user_space))

				continue;

				}

				freeze_task(p);

				task_unlock(p);

			} else {

				freeze_task(p);

			}



			if (!freezer_should_skip(p))

				todo++;

		} while_each_thread(g, p);

kernel/signal.c

+0 −1
Original line number Diff line number Diff line
@@ -99,7 +99,6 @@ static inline int has_pending_signals(sigset_t *signal, sigset_t *blocked) 
static int recalc_sigpending_tsk(struct task_struct *t)

{

	if (t->signal->group_stop_count > 0 ||

	    (freezing(t)) ||

	    PENDING(&t->pending, &t->blocked) ||

	    PENDING(&t->signal->shared_pending, &t->blocked)) {

		set_tsk_thread_flag(t, TIF_SIGPENDING);

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