Merge tag 'timers-core-2022-12-10' of git://git.kernel.org/pub/scm/linux/kernel/git/tip/tip

Pull timer updates from Thomas Gleixner:
 "Updates for timers, timekeeping and drivers:

  Core:

   - The timer_shutdown[_sync]() infrastructure:

     Tearing down timers can be tedious when there are circular
     dependencies to other things which need to be torn down. A prime
     example is timer and workqueue where the timer schedules work and
     the work arms the timer.

     What needs to prevented is that pending work which is drained via
     destroy_workqueue() does not rearm the previously shutdown timer.
     Nothing in that shutdown sequence relies on the timer being
     functional.

     The conclusion was that the semantics of timer_shutdown_sync()
     should be:
	- timer is not enqueued
    	- timer callback is not running
    	- timer cannot be rearmed

     Preventing the rearming of shutdown timers is done by discarding
     rearm attempts silently.

     A warning for the case that a rearm attempt of a shutdown timer is
     detected would not be really helpful because it's entirely unclear
     how it should be acted upon. The only way to address such a case is
     to add 'if (in_shutdown)' conditionals all over the place. This is
     error prone and in most cases of teardown not required all.

   - The real fix for the bluetooth HCI teardown based on
     timer_shutdown_sync().

     A larger scale conversion to timer_shutdown_sync() is work in
     progress.

   - Consolidation of VDSO time namespace helper functions

   - Small fixes for timer and timerqueue

  Drivers:

   - Prevent integer overflow on the XGene-1 TVAL register which causes
     an never ending interrupt storm.

   - The usual set of new device tree bindings

   - Small fixes and improvements all over the place"

* tag 'timers-core-2022-12-10' of git://git.kernel.org/pub/scm/linux/kernel/git/tip/tip: (34 commits)
  dt-bindings: timer: renesas,cmt: Add r8a779g0 CMT support
  dt-bindings: timer: renesas,tmu: Add r8a779g0 support
  clocksource/drivers/arm_arch_timer: Use kstrtobool() instead of strtobool()
  clocksource/drivers/timer-ti-dm: Fix missing clk_disable_unprepare in dmtimer_systimer_init_clock()
  clocksource/drivers/timer-ti-dm: Clear settings on probe and free
  clocksource/drivers/timer-ti-dm: Make timer_get_irq static
  clocksource/drivers/timer-ti-dm: Fix warning for omap_timer_match
  clocksource/drivers/arm_arch_timer: Fix XGene-1 TVAL register math error
  clocksource/drivers/timer-npcm7xx: Enable timer 1 clock before use
  dt-bindings: timer: nuvoton,npcm7xx-timer: Allow specifying all clocks
  dt-bindings: timer: rockchip: Add rockchip,rk3128-timer
  clockevents: Repair kernel-doc for clockevent_delta2ns()
  clocksource/drivers/ingenic-ost: Define pm functions properly in platform_driver struct
  clocksource/drivers/sh_cmt: Access registers according to spec
  vdso/timens: Refactor copy-pasted find_timens_vvar_page() helper into one copy
  Bluetooth: hci_qca: Fix the teardown problem for real
  timers: Update the documentation to reflect on the new timer_shutdown() API
  timers: Provide timer_shutdown[_sync]()
  timers: Add shutdown mechanism to the internal functions
  timers: Split [try_to_]del_timer[_sync]() to prepare for shutdown mode
  ...

3 files changed, 348 insertions, 99 deletions

diff --git a/kernel/time/clockevents.c b/kernel/time/clockevents.c
index 5d85014d59b5..960143b183cd 100644
--- a/kernel/time/clockevents.c
+++ b/kernel/time/clockevents.c
@@ -76,7 +76,7 @@ static u64 cev_delta2ns(unsigned long latch, struct clock_event_device *evt,
 }
 
 /**
- * clockevents_delta2ns - Convert a latch value (device ticks) to nanoseconds
+ * clockevent_delta2ns - Convert a latch value (device ticks) to nanoseconds
  * @latch:	value to convert
  * @evt:	pointer to clock event device descriptor
  *
diff --git a/kernel/time/namespace.c b/kernel/time/namespace.c
index aec832801c26..0775b9ec952a 100644
--- a/kernel/time/namespace.c
+++ b/kernel/time/namespace.c
@@ -192,6 +192,24 @@ static void timens_setup_vdso_data(struct vdso_data *vdata,
 	offset[CLOCK_BOOTTIME_ALARM]	= boottime;
 }
 
+struct page *find_timens_vvar_page(struct vm_area_struct *vma)
+{
+	if (likely(vma->vm_mm == current->mm))
+		return current->nsproxy->time_ns->vvar_page;
+
+	/*
+	 * VM_PFNMAP | VM_IO protect .fault() handler from being called
+	 * through interfaces like /proc/$pid/mem or
+	 * process_vm_{readv,writev}() as long as there's no .access()
+	 * in special_mapping_vmops().
+	 * For more details check_vma_flags() and __access_remote_vm()
+	 */
+
+	WARN(1, "vvar_page accessed remotely");
+
+	return NULL;
+}
+
 /*
  * Protects possibly multiple offsets writers racing each other
  * and tasks entering the namespace.
diff --git a/kernel/time/timer.c b/kernel/time/timer.c
index 717fcb9fb14a..63a8ce7177dd 100644
--- a/kernel/time/timer.c
+++ b/kernel/time/timer.c
@@ -1017,7 +1017,7 @@ __mod_timer(struct timer_list *timer, unsigned long expires, unsigned int option
 	unsigned int idx = UINT_MAX;
 	int ret = 0;
 
-	BUG_ON(!timer->function);
+	debug_assert_init(timer);
 
 	/*
 	 * This is a common optimization triggered by the networking code - if
@@ -1044,6 +1044,14 @@ __mod_timer(struct timer_list *timer, unsigned long expires, unsigned int option
 		 * dequeue/enqueue dance.
 		 */
 		base = lock_timer_base(timer, &flags);
+		/*
+		 * Has @timer been shutdown? This needs to be evaluated
+		 * while holding base lock to prevent a race against the
+		 * shutdown code.
+		 */
+		if (!timer->function)
+			goto out_unlock;
+
 		forward_timer_base(base);
 
 		if (timer_pending(timer) && (options & MOD_TIMER_REDUCE) &&
@@ -1070,6 +1078,14 @@ __mod_timer(struct timer_list *timer, unsigned long expires, unsigned int option
 		}
 	} else {
 		base = lock_timer_base(timer, &flags);
+		/*
+		 * Has @timer been shutdown? This needs to be evaluated
+		 * while holding base lock to prevent a race against the
+		 * shutdown code.
+		 */
+		if (!timer->function)
+			goto out_unlock;
+
 		forward_timer_base(base);
 	}
 
@@ -1083,7 +1099,7 @@ __mod_timer(struct timer_list *timer, unsigned long expires, unsigned int option
 		/*
 		 * We are trying to schedule the timer on the new base.
 		 * However we can't change timer's base while it is running,
-		 * otherwise del_timer_sync() can't detect that the timer's
+		 * otherwise timer_delete_sync() can't detect that the timer's
 		 * handler yet has not finished. This also guarantees that the
 		 * timer is serialized wrt itself.
 		 */
@@ -1121,14 +1137,20 @@ out_unlock:
 }
 
 /**
- * mod_timer_pending - modify a pending timer's timeout
- * @timer: the pending timer to be modified
- * @expires: new timeout in jiffies
+ * mod_timer_pending - Modify a pending timer's timeout
+ * @timer:	The pending timer to be modified
+ * @expires:	New absolute timeout in jiffies
+ *
+ * mod_timer_pending() is the same for pending timers as mod_timer(), but
+ * will not activate inactive timers.
  *
- * mod_timer_pending() is the same for pending timers as mod_timer(),
- * but will not re-activate and modify already deleted timers.
+ * If @timer->function == NULL then the start operation is silently
+ * discarded.
  *
- * It is useful for unserialized use of timers.
+ * Return:
+ * * %0 - The timer was inactive and not modified or was in
+ *	  shutdown state and the operation was discarded
+ * * %1 - The timer was active and requeued to expire at @expires
  */
 int mod_timer_pending(struct timer_list *timer, unsigned long expires)
 {
@@ -1137,24 +1159,31 @@ int mod_timer_pending(struct timer_list *timer, unsigned long expires)
 EXPORT_SYMBOL(mod_timer_pending);
 
 /**
- * mod_timer - modify a timer's timeout
- * @timer: the timer to be modified
- * @expires: new timeout in jiffies
- *
- * mod_timer() is a more efficient way to update the expire field of an
- * active timer (if the timer is inactive it will be activated)
+ * mod_timer - Modify a timer's timeout
+ * @timer:	The timer to be modified
+ * @expires:	New absolute timeout in jiffies
  *
  * mod_timer(timer, expires) is equivalent to:
  *
  *     del_timer(timer); timer->expires = expires; add_timer(timer);
  *
+ * mod_timer() is more efficient than the above open coded sequence. In
+ * case that the timer is inactive, the del_timer() part is a NOP. The
+ * timer is in any case activated with the new expiry time @expires.
+ *
  * Note that if there are multiple unserialized concurrent users of the
  * same timer, then mod_timer() is the only safe way to modify the timeout,
  * since add_timer() cannot modify an already running timer.
  *
- * The function returns whether it has modified a pending timer or not.
- * (ie. mod_timer() of an inactive timer returns 0, mod_timer() of an
- * active timer returns 1.)
+ * If @timer->function == NULL then the start operation is silently
+ * discarded. In this case the return value is 0 and meaningless.
+ *
+ * Return:
+ * * %0 - The timer was inactive and started or was in shutdown
+ *	  state and the operation was discarded
+ * * %1 - The timer was active and requeued to expire at @expires or
+ *	  the timer was active and not modified because @expires did
+ *	  not change the effective expiry time
  */
 int mod_timer(struct timer_list *timer, unsigned long expires)
 {
@@ -1165,11 +1194,22 @@ EXPORT_SYMBOL(mod_timer);
 /**
  * timer_reduce - Modify a timer's timeout if it would reduce the timeout
  * @timer:	The timer to be modified
- * @expires:	New timeout in jiffies
+ * @expires:	New absolute timeout in jiffies
  *
  * timer_reduce() is very similar to mod_timer(), except that it will only
- * modify a running timer if that would reduce the expiration time (it will
- * start a timer that isn't running).
+ * modify an enqueued timer if that would reduce the expiration time. If
+ * @timer is not enqueued it starts the timer.
+ *
+ * If @timer->function == NULL then the start operation is silently
+ * discarded.
+ *
+ * Return:
+ * * %0 - The timer was inactive and started or was in shutdown
+ *	  state and the operation was discarded
+ * * %1 - The timer was active and requeued to expire at @expires or
+ *	  the timer was active and not modified because @expires
+ *	  did not change the effective expiry time such that the
+ *	  timer would expire earlier than already scheduled
  */
 int timer_reduce(struct timer_list *timer, unsigned long expires)
 {
@@ -1178,39 +1218,51 @@ int timer_reduce(struct timer_list *timer, unsigned long expires)
 EXPORT_SYMBOL(timer_reduce);
 
 /**
- * add_timer - start a timer
- * @timer: the timer to be added
+ * add_timer - Start a timer
+ * @timer:	The timer to be started
  *
- * The kernel will do a ->function(@timer) callback from the
- * timer interrupt at the ->expires point in the future. The
- * current time is 'jiffies'.
+ * Start @timer to expire at @timer->expires in the future. @timer->expires
+ * is the absolute expiry time measured in 'jiffies'. When the timer expires
+ * timer->function(timer) will be invoked from soft interrupt context.
  *
- * The timer's ->expires, ->function fields must be set prior calling this
- * function.
+ * The @timer->expires and @timer->function fields must be set prior
+ * to calling this function.
  *
- * Timers with an ->expires field in the past will be executed in the next
- * timer tick.
+ * If @timer->function == NULL then the start operation is silently
+ * discarded.
+ *
+ * If @timer->expires is already in the past @timer will be queued to
+ * expire at the next timer tick.
+ *
+ * This can only operate on an inactive timer. Attempts to invoke this on
+ * an active timer are rejected with a warning.
  */
 void add_timer(struct timer_list *timer)
 {
-	BUG_ON(timer_pending(timer));
+	if (WARN_ON_ONCE(timer_pending(timer)))
+		return;
 	__mod_timer(timer, timer->expires, MOD_TIMER_NOTPENDING);
 }
 EXPORT_SYMBOL(add_timer);
 
 /**
- * add_timer_on - start a timer on a particular CPU
- * @timer: the timer to be added
- * @cpu: the CPU to start it on
+ * add_timer_on - Start a timer on a particular CPU
+ * @timer:	The timer to be started
+ * @cpu:	The CPU to start it on
+ *
+ * Same as add_timer() except that it starts the timer on the given CPU.
  *
- * This is not very scalable on SMP. Double adds are not possible.
+ * See add_timer() for further details.
  */
 void add_timer_on(struct timer_list *timer, int cpu)
 {
 	struct timer_base *new_base, *base;
 	unsigned long flags;
 
-	BUG_ON(timer_pending(timer) || !timer->function);
+	debug_assert_init(timer);
+
+	if (WARN_ON_ONCE(timer_pending(timer)))
+		return;
 
 	new_base = get_timer_cpu_base(timer->flags, cpu);
 
@@ -1220,6 +1272,13 @@ void add_timer_on(struct timer_list *timer, int cpu)
 	 * wrong base locked.  See lock_timer_base().
 	 */
 	base = lock_timer_base(timer, &flags);
+	/*
+	 * Has @timer been shutdown? This needs to be evaluated while
+	 * holding base lock to prevent a race against the shutdown code.
+	 */
+	if (!timer->function)
+		goto out_unlock;
+
 	if (base != new_base) {
 		timer->flags |= TIMER_MIGRATING;
 
@@ -1233,22 +1292,27 @@ void add_timer_on(struct timer_list *timer, int cpu)
 
 	debug_timer_activate(timer);
 	internal_add_timer(base, timer);
+out_unlock:
 	raw_spin_unlock_irqrestore(&base->lock, flags);
 }
 EXPORT_SYMBOL_GPL(add_timer_on);
 
 /**
- * del_timer - deactivate a timer.
- * @timer: the timer to be deactivated
- *
- * del_timer() deactivates a timer - this works on both active and inactive
- * timers.
- *
- * The function returns whether it has deactivated a pending timer or not.
- * (ie. del_timer() of an inactive timer returns 0, del_timer() of an
- * active timer returns 1.)
+ * __timer_delete - Internal function: Deactivate a timer
+ * @timer:	The timer to be deactivated
+ * @shutdown:	If true, this indicates that the timer is about to be
+ *		shutdown permanently.
+ *
+ * If @shutdown is true then @timer->function is set to NULL under the
+ * timer base lock which prevents further rearming of the time. In that
+ * case any attempt to rearm @timer after this function returns will be
+ * silently ignored.
+ *
+ * Return:
+ * * %0 - The timer was not pending
+ * * %1 - The timer was pending and deactivated
  */
-int del_timer(struct timer_list *timer)
+static int __timer_delete(struct timer_list *timer, bool shutdown)
 {
 	struct timer_base *base;
 	unsigned long flags;
@@ -1256,24 +1320,90 @@ int del_timer(struct timer_list *timer)
 
 	debug_assert_init(timer);
 
-	if (timer_pending(timer)) {
+	/*
+	 * If @shutdown is set then the lock has to be taken whether the
+	 * timer is pending or not to protect against a concurrent rearm
+	 * which might hit between the lockless pending check and the lock
+	 * aquisition. By taking the lock it is ensured that such a newly
+	 * enqueued timer is dequeued and cannot end up with
+	 * timer->function == NULL in the expiry code.
+	 *
+	 * If timer->function is currently executed, then this makes sure
+	 * that the callback cannot requeue the timer.
+	 */
+	if (timer_pending(timer) || shutdown) {
 		base = lock_timer_base(timer, &flags);
 		ret = detach_if_pending(timer, base, true);
+		if (shutdown)
+			timer->function = NULL;
 		raw_spin_unlock_irqrestore(&base->lock, flags);
 	}
 
 	return ret;
 }
-EXPORT_SYMBOL(del_timer);
 
 /**
- * try_to_del_timer_sync - Try to deactivate a timer
- * @timer: timer to delete
+ * timer_delete - Deactivate a timer
+ * @timer:	The timer to be deactivated
+ *
+ * The function only deactivates a pending timer, but contrary to
+ * timer_delete_sync() it does not take into account whether the timer's
+ * callback function is concurrently executed on a different CPU or not.
+ * It neither prevents rearming of the timer.  If @timer can be rearmed
+ * concurrently then the return value of this function is meaningless.
+ *
+ * Return:
+ * * %0 - The timer was not pending
+ * * %1 - The timer was pending and deactivated
+ */
+int timer_delete(struct timer_list *timer)
+{
+	return __timer_delete(timer, false);
+}
+EXPORT_SYMBOL(timer_delete);
+
+/**
+ * timer_shutdown - Deactivate a timer and prevent rearming
+ * @timer:	The timer to be deactivated
  *
- * This function tries to deactivate a timer. Upon successful (ret >= 0)
- * exit the timer is not queued and the handler is not running on any CPU.
+ * The function does not wait for an eventually running timer callback on a
+ * different CPU but it prevents rearming of the timer. Any attempt to arm
+ * @timer after this function returns will be silently ignored.
+ *
+ * This function is useful for teardown code and should only be used when
+ * timer_shutdown_sync() cannot be invoked due to locking or context constraints.
+ *
+ * Return:
+ * * %0 - The timer was not pending
+ * * %1 - The timer was pending
  */
-int try_to_del_timer_sync(struct timer_list *timer)
+int timer_shutdown(struct timer_list *timer)
+{
+	return __timer_delete(timer, true);
+}
+EXPORT_SYMBOL_GPL(timer_shutdown);
+
+/**
+ * __try_to_del_timer_sync - Internal function: Try to deactivate a timer
+ * @timer:	Timer to deactivate
+ * @shutdown:	If true, this indicates that the timer is about to be
+ *		shutdown permanently.
+ *
+ * If @shutdown is true then @timer->function is set to NULL under the
+ * timer base lock which prevents further rearming of the timer. Any
+ * attempt to rearm @timer after this function returns will be silently
+ * ignored.
+ *
+ * This function cannot guarantee that the timer cannot be rearmed
+ * right after dropping the base lock if @shutdown is false. That
+ * needs to be prevented by the calling code if necessary.
+ *
+ * Return:
+ * * %0  - The timer was not pending
+ * * %1  - The timer was pending and deactivated
+ * * %-1 - The timer callback function is running on a different CPU
+ */
+static int __try_to_del_timer_sync(struct timer_list *timer, bool shutdown)
 {
 	struct timer_base *base;
 	unsigned long flags;
@@ -1285,11 +1415,34 @@ int try_to_del_timer_sync(struct timer_list *timer)
 
 	if (base->running_timer != timer)
 		ret = detach_if_pending(timer, base, true);
+	if (shutdown)
+		timer->function = NULL;
 
 	raw_spin_unlock_irqrestore(&base->lock, flags);
 
 	return ret;
 }
+
+/**
+ * try_to_del_timer_sync - Try to deactivate a timer
+ * @timer:	Timer to deactivate
+ *
+ * This function tries to deactivate a timer. On success the timer is not
+ * queued and the timer callback function is not running on any CPU.
+ *
+ * This function does not guarantee that the timer cannot be rearmed right
+ * after dropping the base lock. That needs to be prevented by the calling
+ * code if necessary.
+ *
+ * Return:
+ * * %0  - The timer was not pending
+ * * %1  - The timer was pending and deactivated
+ * * %-1 - The timer callback function is running on a different CPU
+ */
+int try_to_del_timer_sync(struct timer_list *timer)
+{
+	return __try_to_del_timer_sync(timer, false);
+}
 EXPORT_SYMBOL(try_to_del_timer_sync);
 
 #ifdef CONFIG_PREEMPT_RT
@@ -1365,44 +1518,29 @@ static inline void timer_sync_wait_running(struct timer_base *base) { }
 static inline void del_timer_wait_running(struct timer_list *timer) { }
 #endif
 
-#if defined(CONFIG_SMP) || defined(CONFIG_PREEMPT_RT)
 /**
- * del_timer_sync - deactivate a timer and wait for the handler to finish.
- * @timer: the timer to be deactivated
- *
- * This function only differs from del_timer() on SMP: besides deactivating
- * the timer it also makes sure the handler has finished executing on other
- * CPUs.
- *
- * Synchronization rules: Callers must prevent restarting of the timer,
- * otherwise this function is meaningless. It must not be called from
- * interrupt contexts unless the timer is an irqsafe one. The caller must
- * not hold locks which would prevent completion of the timer's
- * handler. The timer's handler must not call add_timer_on(). Upon exit the
- * timer is not queued and the handler is not running on any CPU.
- *
- * Note: For !irqsafe timers, you must not hold locks that are held in
- *   interrupt context while calling this function. Even if the lock has
- *   nothing to do with the timer in question.  Here's why::
- *
- *    CPU0                             CPU1
- *    ----                             ----
- *                                     <SOFTIRQ>
- *                                       call_timer_fn();
- *                                       base->running_timer = mytimer;
- *    spin_lock_irq(somelock);
- *                                     <IRQ>
- *                                        spin_lock(somelock);
- *    del_timer_sync(mytimer);
- *    while (base->running_timer == mytimer);
- *
- * Now del_timer_sync() will never return and never release somelock.
- * The interrupt on the other CPU is waiting to grab somelock but
- * it has interrupted the softirq that CPU0 is waiting to finish.
- *
- * The function returns whether it has deactivated a pending timer or not.
+ * __timer_delete_sync - Internal function: Deactivate a timer and wait
+ *			 for the handler to finish.
+ * @timer:	The timer to be deactivated
+ * @shutdown:	If true, @timer->function will be set to NULL under the
+ *		timer base lock which prevents rearming of @timer
+ *
+ * If @shutdown is not set the timer can be rearmed later. If the timer can
+ * be rearmed concurrently, i.e. after dropping the base lock then the
+ * return value is meaningless.
+ *
+ * If @shutdown is set then @timer->function is set to NULL under timer
+ * base lock which prevents rearming of the timer. Any attempt to rearm
+ * a shutdown timer is silently ignored.
+ *
+ * If the timer should be reused after shutdown it has to be initialized
+ * again.
+ *
+ * Return:
+ * * %0	- The timer was not pending
+ * * %1	- The timer was pending and deactivated
  */
-int del_timer_sync(struct timer_list *timer)
+static int __timer_delete_sync(struct timer_list *timer, bool shutdown)
 {
 	int ret;
 
@@ -1422,7 +1560,7 @@ int del_timer_sync(struct timer_list *timer)
 	 * don't use it in hardirq context, because it
 	 * could lead to deadlock.
 	 */
-	WARN_ON(in_irq() && !(timer->flags & TIMER_IRQSAFE));
+	WARN_ON(in_hardirq() && !(timer->flags & TIMER_IRQSAFE));
 
 	/*
 	 * Must be able to sleep on PREEMPT_RT because of the slowpath in
@@ -1432,7 +1570,7 @@ int del_timer_sync(struct timer_list *timer)
 		lockdep_assert_preemption_enabled();
 
 	do {
-		ret = try_to_del_timer_sync(timer);
+		ret = __try_to_del_timer_sync(timer, shutdown);
 
 		if (unlikely(ret < 0)) {
 			del_timer_wait_running(timer);
@@ -1442,8 +1580,96 @@ int del_timer_sync(struct timer_list *timer)
 
 	return ret;
 }
-EXPORT_SYMBOL(del_timer_sync);
-#endif
+
+/**
+ * timer_delete_sync - Deactivate a timer and wait for the handler to finish.
+ * @timer:	The timer to be deactivated
+ *
+ * Synchronization rules: Callers must prevent restarting of the timer,
+ * otherwise this function is meaningless. It must not be called from
+ * interrupt contexts unless the timer is an irqsafe one. The caller must
+ * not hold locks which would prevent completion of the timer's callback
+ * function. The timer's handler must not call add_timer_on(). Upon exit
+ * the timer is not queued and the handler is not running on any CPU.
+ *
+ * For !irqsafe timers, the caller must not hold locks that are held in
+ * interrupt context. Even if the lock has nothing to do with the timer in
+ * question.  Here's why::
+ *
+ *    CPU0                             CPU1
+ *    ----                             ----
+ *                                     <SOFTIRQ>
+ *                                       call_timer_fn();
+ *                                       base->running_timer = mytimer;
+ *    spin_lock_irq(somelock);
+ *                                     <IRQ>
+ *                                        spin_lock(somelock);
+ *    timer_delete_sync(mytimer);
+ *    while (base->running_timer == mytimer);
+ *
+ * Now timer_delete_sync() will never return and never release somelock.
+ * The interrupt on the other CPU is waiting to grab somelock but it has
+ * interrupted the softirq that CPU0 is waiting to finish.
+ *
+ * This function cannot guarantee that the timer is not rearmed again by
+ * some concurrent or preempting code, right after it dropped the base
+ * lock. If there is the possibility of a concurrent rearm then the return
+ * value of the function is meaningless.
+ *
+ * If such a guarantee is needed, e.g. for teardown situations then use
+ * timer_shutdown_sync() instead.
+ *
+ * Return:
+ * * %0	- The timer was not pending
+ * * %1	- The timer was pending and deactivated
+ */
+int timer_delete_sync(struct timer_list *timer)
+{
+	return __timer_delete_sync(timer, false);
+}
+EXPORT_SYMBOL(timer_delete_sync);
+
+/**
+ * timer_shutdown_sync - Shutdown a timer and prevent rearming
+ * @timer: The timer to be shutdown
+ *
+ * When the function returns it is guaranteed that:
+ *   - @timer is not queued
+ *   - The callback function of @timer is not running
+ *   - @timer cannot be enqueued again. Any attempt to rearm
+ *     @timer is silently ignored.
+ *
+ * See timer_delete_sync() for synchronization rules.
+ *
+ * This function is useful for final teardown of an infrastructure where
+ * the timer is subject to a circular dependency problem.
+ *
+ * A common pattern for this is a timer and a workqueue where the timer can
+ * schedule work and work can arm the timer. On shutdown the workqueue must
+ * be destroyed and the timer must be prevented from rearming. Unless the
+ * code has conditionals like 'if (mything->in_shutdown)' to prevent that
+ * there is no way to get this correct with timer_delete_sync().
+ *
+ * timer_shutdown_sync() is solving the problem. The correct ordering of
+ * calls in this case is:
+ *
+ *	timer_shutdown_sync(&mything->timer);
+ *	workqueue_destroy(&mything->workqueue);
+ *
+ * After this 'mything' can be safely freed.
+ *
+ * This obviously implies that the timer is not required to be functional
+ * for the rest of the shutdown operation.
+ *
+ * Return:
+ * * %0 - The timer was not pending
+ * * %1 - The timer was pending
+ */
+int timer_shutdown_sync(struct timer_list *timer)
+{
+	return __timer_delete_sync(timer, true);
+}
+EXPORT_SYMBOL_GPL(timer_shutdown_sync);
 
 static void call_timer_fn(struct timer_list *timer,
 			  void (*fn)(struct timer_list *),
@@ -1465,8 +1691,8 @@ static void call_timer_fn(struct timer_list *timer,
 #endif
 	/*
 	 * Couple the lock chain with the lock chain at
-	 * del_timer_sync() by acquiring the lock_map around the fn()
-	 * call here and in del_timer_sync().
+	 * timer_delete_sync() by acquiring the lock_map around the fn()
+	 * call here and in timer_delete_sync().
 	 */
 	lock_map_acquire(&lockdep_map);
 
@@ -1509,6 +1735,12 @@ static void expire_timers(struct timer_base *base, struct hlist_head *head)
 
 		fn = timer->function;
 
+		if (WARN_ON_ONCE(!fn)) {
+			/* Should never happen. Emphasis on should! */
+			base->running_timer = NULL;
+			continue;
+		}
+
 		if (timer->flags & TIMER_IRQSAFE) {
 			raw_spin_unlock(&base->lock);
 			call_timer_fn(timer, fn, baseclk);
@@ -1933,7 +2165,7 @@ signed long __sched schedule_timeout(signed long timeout)
 	timer_setup_on_stack(&timer.timer, process_timeout, 0);
 	__mod_timer(&timer.timer, expire, MOD_TIMER_NOTPENDING);
 	schedule();
-	del_singleshot_timer_sync(&timer.timer);
+	del_timer_sync(&timer.timer);
 
 	/* Remove the timer from the object tracker */
 	destroy_timer_on_stack(&timer.timer);
@@ -2017,8 +2249,6 @@ int timers_dead_cpu(unsigned int cpu)
 	struct timer_base *new_base;
 	int b, i;
 
-	BUG_ON(cpu_online(cpu));
-
 	for (b = 0; b < NR_BASES; b++) {
 		old_base = per_cpu_ptr(&timer_bases[b], cpu);
 		new_base = get_cpu_ptr(&timer_bases[b]);
@@ -2035,7 +2265,8 @@ int timers_dead_cpu(unsigned int cpu)
 		 */
 		forward_timer_base(new_base);
 
-		BUG_ON(old_base->running_timer);
+		WARN_ON_ONCE(old_base->running_timer);
+		old_base->running_timer = NULL;
 
 		for (i = 0; i < WHEEL_SIZE; i++)
 			migrate_timer_list(new_base, old_base->vectors + i);