linux-kernel - [RFC v3 2/3] kernel/time/clockevents: make setting of ->mult and ->mult

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Date:	Wed, 13 Jul 2016 15:00:16 +0200
From:	Nicolai Stange <nicstange@...il.com>
To:	Thomas Gleixner <tglx@...utronix.de>
Cc:	John Stultz <john.stultz@...aro.org>, linux-kernel@...r.kernel.org,
	Nicolai Stange <nicstange@...il.com>
Subject: [RFC v3 2/3] kernel/time/clockevents: make setting of ->mult and ->mult_mono atomic

In order to avoid races between setting a struct clock_event_device's
->mult_mono in clockevents_update_freq() and yet to be implemented updates
triggered from the timekeeping core, the setting of ->mult and ->mult_mono
should be made atomic.

Protect the update in clockevents_update_freq() by locking the
clockevents_lock spinlock. Frequency updates are expected to be done
seldomly and thus, taking this subsystem lock should not have any impact
on performance.

Use a raw_spin_lock_irq_save()/raw_spin_unlock_irq_restore() pair for
locking/unlocking the clockevents_lock spinlock.
Purge the now redundant local_irq_save()/local_irq_restore() pair from
clockevents_update_freq(). Since the call to tick_broadcast_update_freq()
isn't done with interrupts disabled anymore,  its
raw_spin_lock()/raw_spin_unlock() pair must be converted to
raw_spin_lock_irq_save()/raw_spin_unlock_irq_restore().

Signed-off-by: Nicolai Stange <nicstange@...il.com>
---
 kernel/time/clockevents.c    | 7 ++++---
 kernel/time/tick-broadcast.c | 5 +++--
 2 files changed, 7 insertions(+), 5 deletions(-)

diff --git a/kernel/time/clockevents.c b/kernel/time/clockevents.c
index ba7fea4..ec01375 100644
--- a/kernel/time/clockevents.c
+++ b/kernel/time/clockevents.c
@@ -589,11 +589,12 @@ int clockevents_update_freq(struct clock_event_device *dev, u32 freq)
 	unsigned long flags;
 	int ret;
 
-	local_irq_save(flags);
 	ret = tick_broadcast_update_freq(dev, freq);
-	if (ret == -ENODEV)
+	if (ret == -ENODEV) {
+		raw_spin_lock_irqsave(&clockevents_lock, flags);
 		ret = __clockevents_update_freq(dev, freq);
-	local_irq_restore(flags);
+		raw_spin_unlock_irqrestore(&clockevents_lock, flags);
+	}
 	return ret;
 }
 
diff --git a/kernel/time/tick-broadcast.c b/kernel/time/tick-broadcast.c
index f6aae79..9c94c41 100644
--- a/kernel/time/tick-broadcast.c
+++ b/kernel/time/tick-broadcast.c
@@ -125,11 +125,12 @@ int tick_is_broadcast_device(struct clock_event_device *dev)
 int tick_broadcast_update_freq(struct clock_event_device *dev, u32 freq)
 {
 	int ret = -ENODEV;
+	unsigned long flags;
 
 	if (tick_is_broadcast_device(dev)) {
-		raw_spin_lock(&tick_broadcast_lock);
+		raw_spin_lock_irqsave(&tick_broadcast_lock, flags);
 		ret = __clockevents_update_freq(dev, freq);
-		raw_spin_unlock(&tick_broadcast_lock);
+		raw_spin_unlock_irqrestore(&tick_broadcast_lock, flags);
 	}
 	return ret;
 }
-- 
2.9.0