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 a6d405a86ee04d78127c59d19465924f1f38b7fa..cd27b0825560fe82695f0f4f3188bfe421810f3a 100644 (file)
--- a/kernel/cgroup_freezer.c
+++ b/kernel/cgroup_freezer.c
@@ -231,7 +231,7 @@ static void update_if_frozen(struct cgroup *cgroup,
        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 4130e48649bb166d7b4a192bb8932492a17b8a1f..a8822be43da01330e41b7857d014c8c472695dd3 100644 (file)
--- a/kernel/freezer.c
+++ b/kernel/freezer.c
@@ -22,14 +22,19 @@ bool __refrigerator(bool check_kthr_stop)
        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 @@ bool __refrigerator(bool check_kthr_stop)
 
        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 @@ bool __refrigerator(bool check_kthr_stop)
        /* 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 @@ void cancel_freezing(struct task_struct *p)
        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 bd420ca48261ba76dd04c5a4200b8763ea578255..e6e2739190b5f483715d54a1a0b29d805c07b439 100644 (file)
--- a/kernel/power/process.c
+++ b/kernel/power/process.c
@@ -118,7 +118,8 @@ static int try_to_freeze_tasks(bool sig_only)
 
                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);