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From: Julien Grall <julien.grall@arm.com>
To: linux-rt-users@vger.kernel.org
Cc: tglx@linutronix.de, linux-kernel@vger.kernel.org, maz@kernel.org,
	bigeasy@linutronix.de, rostedt@goodmis.org,
	Julien Grall <julien.grall@arm.com>
Subject: [RT PATCH 1/3] hrtimer: Use READ_ONCE to access timer->base in hrimer_grab_expiry_lock()
Date: Wed, 21 Aug 2019 10:24:07 +0100
Message-ID: <20190821092409.13225-2-julien.grall@arm.com> (raw)
In-Reply-To: <20190821092409.13225-1-julien.grall@arm.com>

The update to timer->base is protected by the base->cpu_base->lock().
However, hrtimer_grab_expirty_lock() does not access it with the lock.

So it would theorically be possible to have timer->base changed under
our feet. We need to prevent the compiler to refetch timer->base so the
check and the access is performed on the same base.

Other access of timer->base are either done with a lock or protected
with READ_ONCE(). So use READ_ONCE() in hrtimer_grab_expirty_lock().

Signed-off-by: Julien Grall <julien.grall@arm.com>

---

This is rather theoritical so far as I don't have a reproducer for this.
---
 kernel/time/hrtimer.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/kernel/time/hrtimer.c b/kernel/time/hrtimer.c
index 7d7db8802131..b869e816e96a 100644
--- a/kernel/time/hrtimer.c
+++ b/kernel/time/hrtimer.c
@@ -932,7 +932,7 @@ EXPORT_SYMBOL_GPL(hrtimer_forward);
 
 void hrtimer_grab_expiry_lock(const struct hrtimer *timer)
 {
-	struct hrtimer_clock_base *base = timer->base;
+	struct hrtimer_clock_base *base = READ_ONCE(timer->base);
 
 	if (base && base->cpu_base) {
 		spin_lock(&base->cpu_base->softirq_expiry_lock);
-- 
2.11.0


  reply index

Thread overview: 11+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2019-08-21  9:24 [RT PATCH 0/3] hrtimer: RT fixes for hrtimer_grab_expiry_lock() Julien Grall
2019-08-21  9:24 ` Julien Grall [this message]
2019-08-21 13:44   ` [RT PATCH 1/3] hrtimer: Use READ_ONCE to access timer->base in hrimer_grab_expiry_lock() Sebastian Andrzej Siewior
2019-08-21 13:50     ` Thomas Gleixner
2019-08-21 13:59       ` Sebastian Andrzej Siewior
2019-09-26 21:47       ` Eric Dumazet
2019-08-21  9:24 ` [RT PATCH 2/3] hrtimer: Don't grab the expiry lock for non-soft hrtimer Julien Grall
2019-08-21 13:49   ` Sebastian Andrzej Siewior
2019-08-21  9:24 ` [RT PATCH 3/3] hrtimer: Prevent using uninitialized spin_lock in hrtimer_grab_expiry_lock() Julien Grall
2019-08-21 14:02   ` Thomas Gleixner
2019-08-22 10:59     ` Julien Grall

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