A samll set of timer related fixes:

- Plug a race between rearm and process tick in the posix CPU timers code
 
  - Make the optimization to avoid recalculation of the next timer interrupt
    work correctly when there are no timers pending.
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Merge tag 'timers-urgent-2021-07-25' of git://git.kernel.org/pub/scm/linux/kernel/git/tip/tip

Pull timer fixes from Thomas Gleixner:
 "A small set of timer related fixes:

   - Plug a race between rearm and process tick in the posix CPU timers
     code

   - Make the optimization to avoid recalculation of the next timer
     interrupt work correctly when there are no timers pending"

* tag 'timers-urgent-2021-07-25' of git://git.kernel.org/pub/scm/linux/kernel/git/tip/tip:
  timers: Fix get_next_timer_interrupt() with no timers pending
  posix-cpu-timers: Fix rearm racing against process tick

kernel/time/posix-cpu-timers.c

@@ -991,6 +991,11 @@ static void posix_cpu_timer_rearm(struct k_itimer *timer)
 	if (!p)
 		goto out;
 
+	/* Protect timer list r/w in arm_timer() */
+	sighand = lock_task_sighand(p, &flags);
+	if (unlikely(sighand == NULL))
+		goto out;
+
 	/*
 	 * Fetch the current sample and update the timer's expiry time.
 	 */
@@ -1001,11 +1006,6 @@ static void posix_cpu_timer_rearm(struct k_itimer *timer)
 
 	bump_cpu_timer(timer, now);
 
-	/* Protect timer list r/w in arm_timer() */
-	sighand = lock_task_sighand(p, &flags);
-	if (unlikely(sighand == NULL))
-		goto out;
-
 	/*
 	 * Now re-arm for the new expiry time.
 	 */

kernel/time/timer.c

@@ -207,6 +207,7 @@ struct timer_base {
 	unsigned int		cpu;
 	bool			next_expiry_recalc;
 	bool			is_idle;
+	bool			timers_pending;
 	DECLARE_BITMAP(pending_map, WHEEL_SIZE);
 	struct hlist_head	vectors[WHEEL_SIZE];
 } ____cacheline_aligned;
@@ -595,6 +596,7 @@ static void enqueue_timer(struct timer_base *base, struct timer_list *timer,
 		 * can reevaluate the wheel:
 		 */
 		base->next_expiry = bucket_expiry;
+		base->timers_pending = true;
 		base->next_expiry_recalc = false;
 		trigger_dyntick_cpu(base, timer);
 	}
@@ -1582,6 +1584,7 @@ static unsigned long __next_timer_interrupt(struct timer_base *base)
 	}
 
 	base->next_expiry_recalc = false;
+	base->timers_pending = !(next == base->clk + NEXT_TIMER_MAX_DELTA);
 
 	return next;
 }
@@ -1633,7 +1636,6 @@ u64 get_next_timer_interrupt(unsigned long basej, u64 basem)
 	struct timer_base *base = this_cpu_ptr(&timer_bases[BASE_STD]);
 	u64 expires = KTIME_MAX;
 	unsigned long nextevt;
-	bool is_max_delta;
 
 	/*
 	 * Pretend that there is no timer pending if the cpu is offline.
@@ -1646,7 +1648,6 @@ u64 get_next_timer_interrupt(unsigned long basej, u64 basem)
 	if (base->next_expiry_recalc)
 		base->next_expiry = __next_timer_interrupt(base);
 	nextevt = base->next_expiry;
-	is_max_delta = (nextevt == base->clk + NEXT_TIMER_MAX_DELTA);
 
 	/*
 	 * We have a fresh next event. Check whether we can forward the
@@ -1664,7 +1665,7 @@ u64 get_next_timer_interrupt(unsigned long basej, u64 basem)
 		expires = basem;
 		base->is_idle = false;
 	} else {
-		if (!is_max_delta)
+		if (base->timers_pending)
 			expires = basem + (u64)(nextevt - basej) * TICK_NSEC;
 		/*
 		 * If we expect to sleep more than a tick, mark the base idle.
@@ -1947,6 +1948,7 @@ int timers_prepare_cpu(unsigned int cpu)
 		base = per_cpu_ptr(&timer_bases[b], cpu);
 		base->clk = jiffies;
 		base->next_expiry = base->clk + NEXT_TIMER_MAX_DELTA;
+		base->timers_pending = false;
 		base->is_idle = false;
 	}
 	return 0;