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Commit 0c9af092 authored by Tejun Heo's avatar Tejun Heo
Browse files

freezer: use dedicated lock instead of task_lock() + memory barrier 



Freezer synchronization is needlessly complicated - it's by no means a
hot path and the priority is staying unintrusive and safe.  This patch
makes it simply use a dedicated lock instead of piggy-backing on
task_lock() and playing with memory barriers.

On the failure path of try_to_freeze_tasks(), locking is moved from it
to cancel_freezing().  This makes the frozen() test racy but the race
here is a non-issue as the warning is printed for tasks which failed
to enter frozen for 20 seconds and race on PF_FROZEN at the last
moment doesn't change anything.

This simplifies freezer implementation and eases further changes
including some race fixes.

Signed-off-by: default avatarTejun Heo <tj@kernel.org>
parent 6cd8dedc

kernel/freezer.c

+37 −47
Original line number Diff line number Diff line
@@ -11,17 +11,8 @@ 
#include <linux/freezer.h>

#include <linux/kthread.h>



/*

 * freezing is complete, mark current process as frozen

 */

static inline void frozen_process(void)

{

	if (!unlikely(current->flags & PF_NOFREEZE)) {

		current->flags |= PF_FROZEN;

		smp_wmb();

	}

	clear_freeze_flag(current);

}

/* protects freezing and frozen transitions */

static DEFINE_SPINLOCK(freezer_lock);



/* Refrigerator is place where frozen processes are stored :-). */

bool __refrigerator(bool check_kthr_stop)
@@ -31,14 +22,16 @@ bool __refrigerator(bool check_kthr_stop) 
	bool was_frozen = false;

	long save;



	task_lock(current);

	if (freezing(current)) {

		frozen_process();

		task_unlock(current);

	} else {

		task_unlock(current);

	spin_lock_irq(&freezer_lock);

	if (!freezing(current)) {

		spin_unlock_irq(&freezer_lock);

		return was_frozen;

	}

	if (!(current->flags & PF_NOFREEZE))

		current->flags |= PF_FROZEN;

	clear_freeze_flag(current);

	spin_unlock_irq(&freezer_lock);



	save = current->state;

	pr_debug("%s entered refrigerator\n", current->comm);
@@ -99,21 +92,18 @@ static void fake_signal_wake_up(struct task_struct *p) 
 */

bool freeze_task(struct task_struct *p, bool sig_only)

{

	/*

	 * We first check if the task is freezing and next if it has already

	 * been frozen to avoid the race with frozen_process() which first marks

	 * the task as frozen and next clears its TIF_FREEZE.

	 */

	if (!freezing(p)) {

		smp_rmb();

	unsigned long flags;

	bool ret = false;



	spin_lock_irqsave(&freezer_lock, flags);



	if (sig_only && !should_send_signal(p))

		goto out_unlock;



	if (frozen(p))

			return false;

		goto out_unlock;



		if (!sig_only || should_send_signal(p))

	set_freeze_flag(p);

		else

			return false;

	}



	if (should_send_signal(p)) {

		fake_signal_wake_up(p);
@@ -123,26 +113,28 @@ bool freeze_task(struct task_struct *p, bool sig_only) 
		 * TASK_RUNNING transition can't race with task state

		 * testing in try_to_freeze_tasks().

		 */

	} else if (sig_only) {

		return false;

	} else {

		wake_up_state(p, TASK_INTERRUPTIBLE);

	}



	return true;

	ret = true;

out_unlock:

	spin_unlock_irqrestore(&freezer_lock, flags);

	return ret;

}



void cancel_freezing(struct task_struct *p)

{

	unsigned long flags;



	spin_lock_irqsave(&freezer_lock, flags);

	if (freezing(p)) {

		pr_debug("  clean up: %s\n", p->comm);

		clear_freeze_flag(p);

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

		spin_lock(&p->sighand->siglock);

		recalc_sigpending_and_wake(p);

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

		spin_unlock(&p->sighand->siglock);

	}

	spin_unlock_irqrestore(&freezer_lock, flags);

}



/*
@@ -156,16 +148,14 @@ void cancel_freezing(struct task_struct *p) 
 */

void __thaw_task(struct task_struct *p)

{

	bool was_frozen;

	unsigned long flags;



	task_lock(p);

	was_frozen = frozen(p);

	if (was_frozen)

	spin_lock_irqsave(&freezer_lock, flags);

	if (frozen(p)) {

		p->flags &= ~PF_FROZEN;

	else

		clear_freeze_flag(p);

	task_unlock(p);



	if (was_frozen)

		wake_up_process(p);

	} else {

		clear_freeze_flag(p);

	}

	spin_unlock_irqrestore(&freezer_lock, flags);

}

kernel/power/process.c

+0 −2
Original line number Diff line number Diff line
@@ -118,11 +118,9 @@ static int try_to_freeze_tasks(bool sig_only) 


		read_lock(&tasklist_lock);

		do_each_thread(g, p) {

			task_lock(p);

			if (!wakeup && freezing(p) && !freezer_should_skip(p))

				sched_show_task(p);

			cancel_freezing(p);

			task_unlock(p);

		} while_each_thread(g, p);

		read_unlock(&tasklist_lock);

	} else {