freezer: make freezing indicate freeze condition in effect
Currently freezing (TIF_FREEZE) and frozen (PF_FROZEN) states are
interlocked - freezing is set to request freeze and when the task
actually freezes, it clears freezing and sets frozen.
This interlocking makes things more complex than necessary - freezing
doesn't mean there's freezing condition in effect and frozen doesn't
match the task actually entering and leaving frozen state (it's
cleared by the thawing task).
This patch makes freezing indicate that freeze condition is in effect.
A task enters and stays frozen if freezing. This makes PF_FROZEN
manipulation done only by the task itself and prevents wakeup from
__thaw_task() leaking outside of refrigerator.
The only place which needs to tell freezing && !frozen is
try_to_freeze_task() to whine about tasks which don't enter frozen.
It's updated to test the condition explicitly.
With the change, frozen() state my linger after __thaw_task() until
the task wakes up and exits fridge. This can trigger BUG_ON() in
update_if_frozen(). Work it around by testing freezing() && frozen()
instead of frozen().
-v2: Oleg pointed out missing re-check of freezing() when trying to
clear FROZEN and possible spurious BUG_ON() trigger in
update_if_frozen(). Both fixed.
Signed-off-by: Tejun Heo <tj@kernel.org>
Cc: Oleg Nesterov <oleg@redhat.com>
Cc: Paul Menage <paul@paulmenage.org>
diff --git a/kernel/cgroup_freezer.c b/kernel/cgroup_freezer.c
index a6d405a..cd27b08 100644
--- a/kernel/cgroup_freezer.c
+++ b/kernel/cgroup_freezer.c
@@ -231,7 +231,7 @@
cgroup_iter_start(cgroup, &it);
while ((task = cgroup_iter_next(cgroup, &it))) {
ntotal++;
- if (frozen(task))
+ if (freezing(task) && frozen(task))
nfrozen++;
}
diff --git a/kernel/freezer.c b/kernel/freezer.c
index 4130e48..a8822be 100644
--- a/kernel/freezer.c
+++ b/kernel/freezer.c
@@ -22,14 +22,19 @@
bool was_frozen = false;
long save;
+ /*
+ * Enter FROZEN. If NOFREEZE, schedule immediate thawing by
+ * clearing freezing.
+ */
spin_lock_irq(&freezer_lock);
+repeat:
if (!freezing(current)) {
spin_unlock_irq(&freezer_lock);
return was_frozen;
}
- if (!(current->flags & PF_NOFREEZE))
- current->flags |= PF_FROZEN;
- clear_freeze_flag(current);
+ if (current->flags & PF_NOFREEZE)
+ clear_freeze_flag(current);
+ current->flags |= PF_FROZEN;
spin_unlock_irq(&freezer_lock);
save = current->state;
@@ -44,7 +49,7 @@
for (;;) {
set_current_state(TASK_UNINTERRUPTIBLE);
- if (!frozen(current) ||
+ if (!freezing(current) ||
(check_kthr_stop && kthread_should_stop()))
break;
was_frozen = true;
@@ -54,6 +59,13 @@
/* Remove the accounting blocker */
current->flags &= ~PF_FREEZING;
+ /* leave FROZEN */
+ spin_lock_irq(&freezer_lock);
+ if (freezing(current))
+ goto repeat;
+ current->flags &= ~PF_FROZEN;
+ spin_unlock_irq(&freezer_lock);
+
pr_debug("%s left refrigerator\n", current->comm);
/*
@@ -137,25 +149,19 @@
spin_unlock_irqrestore(&freezer_lock, flags);
}
-/*
- * Wake up a frozen task
- *
- * task_lock() is needed to prevent the race with refrigerator() which may
- * occur if the freezing of tasks fails. Namely, without the lock, if the
- * freezing of tasks failed, thaw_tasks() might have run before a task in
- * refrigerator() could call frozen_process(), in which case the task would be
- * frozen and no one would thaw it.
- */
void __thaw_task(struct task_struct *p)
{
unsigned long flags;
+ /*
+ * Clear freezing and kick @p if FROZEN. Clearing is guaranteed to
+ * be visible to @p as waking up implies wmb. Waking up inside
+ * freezer_lock also prevents wakeups from leaking outside
+ * refrigerator.
+ */
spin_lock_irqsave(&freezer_lock, flags);
- if (frozen(p)) {
- p->flags &= ~PF_FROZEN;
+ clear_freeze_flag(p);
+ if (frozen(p))
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 bd420ca..e6e2739 100644
--- a/kernel/power/process.c
+++ b/kernel/power/process.c
@@ -118,7 +118,8 @@
read_lock(&tasklist_lock);
do_each_thread(g, p) {
- if (!wakeup && freezing(p) && !freezer_should_skip(p))
+ if (!wakeup && !freezer_should_skip(p) &&
+ freezing(p) && !frozen(p))
sched_show_task(p);
cancel_freezing(p);
} while_each_thread(g, p);