freezer,sched: Rewrite core freezer logic

Rewrite the core freezer to behave better wrt thawing and be simpler
in general.

By replacing PF_FROZEN with TASK_FROZEN, a special block state, it is
ensured frozen tasks stay frozen until thawed and don't randomly wake
up early, as is currently possible.

As such, it does away with PF_FROZEN and PF_FREEZER_SKIP, freeing up
two PF_flags (yay!).

Specifically; the current scheme works a little like:

	freezer_do_not_count();
	schedule();
	freezer_count();

And either the task is blocked, or it lands in try_to_freezer()
through freezer_count(). Now, when it is blocked, the freezer
considers it frozen and continues.

However, on thawing, once pm_freezing is cleared, freezer_count()
stops working, and any random/spurious wakeup will let a task run
before its time.

That is, thawing tries to thaw things in explicit order; kernel
threads and workqueues before doing bringing SMP back before userspace
etc.. However due to the above mentioned races it is entirely possible
for userspace tasks to thaw (by accident) before SMP is back.

This can be a fatal problem in asymmetric ISA architectures (eg ARMv9)
where the userspace task requires a special CPU to run.

As said; replace this with a special task state TASK_FROZEN and add
the following state transitions:

	TASK_FREEZABLE	-> TASK_FROZEN
	__TASK_STOPPED	-> TASK_FROZEN
	__TASK_TRACED	-> TASK_FROZEN

The new TASK_FREEZABLE can be set on any state part of TASK_NORMAL
(IOW. TASK_INTERRUPTIBLE and TASK_UNINTERRUPTIBLE) -- any such state
is already required to deal with spurious wakeups and the freezer
causes one such when thawing the task (since the original state is
lost).

The special __TASK_{STOPPED,TRACED} states *can* be restored since
their canonical state is in ->jobctl.

With this, frozen tasks need an explicit TASK_FROZEN wakeup and are
free of undue (early / spurious) wakeups.

Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Reviewed-by: Ingo Molnar <mingo@kernel.org>
Acked-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>
Link: https://lore.kernel.org/r/20220822114649.055452969@infradead.org

1 files changed, 10 insertions, 235 deletions

diff --git a/include/linux/freezer.h b/include/linux/freezer.h
index 0621c5f86c39..b303472255be 100644
--- a/include/linux/freezer.h
+++ b/include/linux/freezer.h
@@ -8,9 +8,11 @@
 #include <linux/sched.h>
 #include <linux/wait.h>
 #include <linux/atomic.h>
+#include <linux/jump_label.h>
 
 #ifdef CONFIG_FREEZER
-extern atomic_t system_freezing_cnt;	/* nr of freezing conds in effect */
+DECLARE_STATIC_KEY_FALSE(freezer_active);
+
 extern bool pm_freezing;		/* PM freezing in effect */
 extern bool pm_nosig_freezing;		/* PM nosig freezing in effect */
 
@@ -22,10 +24,7 @@ extern unsigned int freeze_timeout_msecs;
 /*
  * Check if a process has been frozen
  */
-static inline bool frozen(struct task_struct *p)
-{
-	return p->flags & PF_FROZEN;
-}
+extern bool frozen(struct task_struct *p);
 
 extern bool freezing_slow_path(struct task_struct *p);
 
@@ -34,9 +33,10 @@ extern bool freezing_slow_path(struct task_struct *p);
  */
 static inline bool freezing(struct task_struct *p)
 {
-	if (likely(!atomic_read(&system_freezing_cnt)))
-		return false;
-	return freezing_slow_path(p);
+	if (static_branch_unlikely(&freezer_active))
+		return freezing_slow_path(p);
+
+	return false;
 }
 
 /* Takes and releases task alloc lock using task_lock() */
@@ -48,23 +48,14 @@ extern int freeze_kernel_threads(void);
 extern void thaw_processes(void);
 extern void thaw_kernel_threads(void);
 
-/*
- * DO NOT ADD ANY NEW CALLERS OF THIS FUNCTION
- * If try_to_freeze causes a lockdep warning it means the caller may deadlock
- */
-static inline bool try_to_freeze_unsafe(void)
+static inline bool try_to_freeze(void)
 {
 	might_sleep();
 	if (likely(!freezing(current)))
 		return false;
-	return __refrigerator(false);
-}
-
-static inline bool try_to_freeze(void)
-{
 	if (!(current->flags & PF_NOFREEZE))
 		debug_check_no_locks_held();
-	return try_to_freeze_unsafe();
+	return __refrigerator(false);
 }
 
 extern bool freeze_task(struct task_struct *p);
@@ -79,195 +70,6 @@ static inline bool cgroup_freezing(struct task_struct *task)
 }
 #endif /* !CONFIG_CGROUP_FREEZER */
 
-/*
- * The PF_FREEZER_SKIP flag should be set by a vfork parent right before it
- * calls wait_for_completion(&vfork) and reset right after it returns from this
- * function.  Next, the parent should call try_to_freeze() to freeze itself
- * appropriately in case the child has exited before the freezing of tasks is
- * complete.  However, we don't want kernel threads to be frozen in unexpected
- * places, so we allow them to block freeze_processes() instead or to set
- * PF_NOFREEZE if needed. Fortunately, in the ____call_usermodehelper() case the
- * parent won't really block freeze_processes(), since ____call_usermodehelper()
- * (the child) does a little before exec/exit and it can't be frozen before
- * waking up the parent.
- */
-
-
-/**
- * freezer_do_not_count - tell freezer to ignore %current
- *
- * Tell freezers to ignore the current task when determining whether the
- * target frozen state is reached.  IOW, the current task will be
- * considered frozen enough by freezers.
- *
- * The caller shouldn't do anything which isn't allowed for a frozen task
- * until freezer_cont() is called.  Usually, freezer[_do_not]_count() pair
- * wrap a scheduling operation and nothing much else.
- */
-static inline void freezer_do_not_count(void)
-{
-	current->flags |= PF_FREEZER_SKIP;
-}
-
-/**
- * freezer_count - tell freezer to stop ignoring %current
- *
- * Undo freezer_do_not_count().  It tells freezers that %current should be
- * considered again and tries to freeze if freezing condition is already in
- * effect.
- */
-static inline void freezer_count(void)
-{
-	current->flags &= ~PF_FREEZER_SKIP;
-	/*
-	 * If freezing is in progress, the following paired with smp_mb()
-	 * in freezer_should_skip() ensures that either we see %true
-	 * freezing() or freezer_should_skip() sees !PF_FREEZER_SKIP.
-	 */
-	smp_mb();
-	try_to_freeze();
-}
-
-/* DO NOT ADD ANY NEW CALLERS OF THIS FUNCTION */
-static inline void freezer_count_unsafe(void)
-{
-	current->flags &= ~PF_FREEZER_SKIP;
-	smp_mb();
-	try_to_freeze_unsafe();
-}
-
-/**
- * freezer_should_skip - whether to skip a task when determining frozen
- *			 state is reached
- * @p: task in quesion
- *
- * This function is used by freezers after establishing %true freezing() to
- * test whether a task should be skipped when determining the target frozen
- * state is reached.  IOW, if this function returns %true, @p is considered
- * frozen enough.
- */
-static inline bool freezer_should_skip(struct task_struct *p)
-{
-	/*
-	 * The following smp_mb() paired with the one in freezer_count()
-	 * ensures that either freezer_count() sees %true freezing() or we
-	 * see cleared %PF_FREEZER_SKIP and return %false.  This makes it
-	 * impossible for a task to slip frozen state testing after
-	 * clearing %PF_FREEZER_SKIP.
-	 */
-	smp_mb();
-	return p->flags & PF_FREEZER_SKIP;
-}
-
-/*
- * These functions are intended to be used whenever you want allow a sleeping
- * task to be frozen. Note that neither return any clear indication of
- * whether a freeze event happened while in this function.
- */
-
-/* Like schedule(), but should not block the freezer. */
-static inline void freezable_schedule(void)
-{
-	freezer_do_not_count();
-	schedule();
-	freezer_count();
-}
-
-/* DO NOT ADD ANY NEW CALLERS OF THIS FUNCTION */
-static inline void freezable_schedule_unsafe(void)
-{
-	freezer_do_not_count();
-	schedule();
-	freezer_count_unsafe();
-}
-
-/*
- * Like schedule_timeout(), but should not block the freezer.  Do not
- * call this with locks held.
- */
-static inline long freezable_schedule_timeout(long timeout)
-{
-	long __retval;
-	freezer_do_not_count();
-	__retval = schedule_timeout(timeout);
-	freezer_count();
-	return __retval;
-}
-
-/*
- * Like schedule_timeout_interruptible(), but should not block the freezer.  Do not
- * call this with locks held.
- */
-static inline long freezable_schedule_timeout_interruptible(long timeout)
-{
-	long __retval;
-	freezer_do_not_count();
-	__retval = schedule_timeout_interruptible(timeout);
-	freezer_count();
-	return __retval;
-}
-
-/* DO NOT ADD ANY NEW CALLERS OF THIS FUNCTION */
-static inline long freezable_schedule_timeout_interruptible_unsafe(long timeout)
-{
-	long __retval;
-
-	freezer_do_not_count();
-	__retval = schedule_timeout_interruptible(timeout);
-	freezer_count_unsafe();
-	return __retval;
-}
-
-/* Like schedule_timeout_killable(), but should not block the freezer. */
-static inline long freezable_schedule_timeout_killable(long timeout)
-{
-	long __retval;
-	freezer_do_not_count();
-	__retval = schedule_timeout_killable(timeout);
-	freezer_count();
-	return __retval;
-}
-
-/* DO NOT ADD ANY NEW CALLERS OF THIS FUNCTION */
-static inline long freezable_schedule_timeout_killable_unsafe(long timeout)
-{
-	long __retval;
-	freezer_do_not_count();
-	__retval = schedule_timeout_killable(timeout);
-	freezer_count_unsafe();
-	return __retval;
-}
-
-/*
- * Like schedule_hrtimeout_range(), but should not block the freezer.  Do not
- * call this with locks held.
- */
-static inline int freezable_schedule_hrtimeout_range(ktime_t *expires,
-		u64 delta, const enum hrtimer_mode mode)
-{
-	int __retval;
-	freezer_do_not_count();
-	__retval = schedule_hrtimeout_range(expires, delta, mode);
-	freezer_count();
-	return __retval;
-}
-
-/*
- * Freezer-friendly wrappers around wait_event_interruptible(),
- * wait_event_killable() and wait_event_interruptible_timeout(), originally
- * defined in <linux/wait.h>
- */
-
-/* DO NOT ADD ANY NEW CALLERS OF THIS FUNCTION */
-#define wait_event_freezekillable_unsafe(wq, condition)			\
-({									\
-	int __retval;							\
-	freezer_do_not_count();						\
-	__retval = wait_event_killable(wq, (condition));		\
-	freezer_count_unsafe();						\
-	__retval;							\
-})
-
 #else /* !CONFIG_FREEZER */
 static inline bool frozen(struct task_struct *p) { return false; }
 static inline bool freezing(struct task_struct *p) { return false; }
@@ -281,35 +83,8 @@ static inline void thaw_kernel_threads(void) {}
 
 static inline bool try_to_freeze(void) { return false; }
 
-static inline void freezer_do_not_count(void) {}
-static inline void freezer_count(void) {}
-static inline int freezer_should_skip(struct task_struct *p) { return 0; }
 static inline void set_freezable(void) {}
 
-#define freezable_schedule()  schedule()
-
-#define freezable_schedule_unsafe()  schedule()
-
-#define freezable_schedule_timeout(timeout)  schedule_timeout(timeout)
-
-#define freezable_schedule_timeout_interruptible(timeout)		\
-	schedule_timeout_interruptible(timeout)
-
-#define freezable_schedule_timeout_interruptible_unsafe(timeout)	\
-	schedule_timeout_interruptible(timeout)
-
-#define freezable_schedule_timeout_killable(timeout)			\
-	schedule_timeout_killable(timeout)
-
-#define freezable_schedule_timeout_killable_unsafe(timeout)		\
-	schedule_timeout_killable(timeout)
-
-#define freezable_schedule_hrtimeout_range(expires, delta, mode)	\
-	schedule_hrtimeout_range(expires, delta, mode)
-
-#define wait_event_freezekillable_unsafe(wq, condition)			\
-		wait_event_killable(wq, condition)
-
 #endif /* !CONFIG_FREEZER */
 
 #endif	/* FREEZER_H_INCLUDED */