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Subject[PATCH 08/17] freezer: use dedicated lock instead of task_lock() + memory barrier
Date
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: Tejun Heo <tj@kernel.org>
---
kernel/freezer.c | 84 +++++++++++++++++++++---------------------------
kernel/power/process.c | 2 -
2 files changed, 37 insertions(+), 49 deletions(-)
diff --git a/kernel/freezer.c b/kernel/freezer.c
index 8faa0e0..10dd2e0 100644
--- a/kernel/freezer.c
+++ b/kernel/freezer.c
@@ -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();
- if (frozen(p))
- return false;
-
- if (!sig_only || should_send_signal(p))
- set_freeze_flag(p);
- else
- return false;
- }
+ 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))
+ goto out_unlock;
+
+ set_freeze_flag(p);

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);
}
diff --git a/kernel/power/process.c b/kernel/power/process.c
index 9db048f..bd420ca 100644
--- a/kernel/power/process.c
+++ b/kernel/power/process.c
@@ -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 {
--
1.7.3.1


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