Re: [PATCH 1/3] hrtimer: Fix race between hrtimer_start() and __run_hrtimer()

[Peter Zijlstra]

On Wed, Apr 15, 2015 at 12:26:58PM +0200, Thomas Gleixner wrote:
> On Wed, 15 Apr 2015, Peter Zijlstra wrote:
> > hrtimer: Fix race between hrtimer_start() and __run_hrtimer()
> 
> I don't think that subject line is correct.
> 
> Back in the early hrtimer days we made deliberately the design
> decision that this kind of usage is forbidden. The reason for this is
> that the hrtimer infrastructure cannot provide proper
> serialization. So we thought it would be a sane restruction that
> restarting a timer from the callback should not be mixed with
> concurrent restarts from a different call site.

Ah I was not aware. Until I changed the locking it was possible simply
because everything was serialized by the base lock. So the concurrent
start would either land before the callback or after it but not in the
middle like it can now.

> So I rather prefer a subject line like this
> 
> hrtimer: Allow concurrent hrtimer_start() for self restarting timers
> 

/me copy/paste, done! :-)

> > To that effect, add a WARN when someone tries to forward an already
> > enqueued timer.
> 
> The warnon itself is nice, but what about sites which use
> hrtimer_set_expires() and hrtimer_start_expires()?

They are all inlines, furthermore forward is the most common way to
change the expiry of periodic / self restarting timers so would gain us
most.

How about this then?

---
Subject: hrtimer: Allow concurrent hrtimer_start() for self restarting timers
From: Peter Zijlstra <peterz@xxxxxxxxxxxxx>
Date: Tue May 20 15:49:48 CEST 2014

Because we drop cpu_base->lock around calling hrtimer::function, it is
possible for hrtimer_start() to come in between and enqueue the timer.

If hrtimer::function then returns HRTIMER_RESTART we'll hit the BUG_ON
because HRTIMER_STATE_ENQUEUED will be set.

Since the above is a perfectly valid scenario, remove the BUG_ON and
make the enqueue_hrtimer() call conditional on the timer not being
enqueued already.

NOTE: in that concurrent scenario its entirely common for both sites
to want to modify the hrtimer, since hrtimers don't provide
serialization themselves be sure to provide some such that the
hrtimer::function and the hrtimer_start() caller don't both try and
fudge the expiration state at the same time.

To that effect, add a WARN when someone tries to forward an already
enqueued timer, the most common way to change the expiry of self
restarting timers. Ideally we'd put the WARN in everything modifying
the expiry but most of that is inlines and we don't need the bloat.

Fixes: 2d44ae4d7135 ("hrtimer: clean up cpu->base locking tricks")
Cc: Ben Segall <bsegall@xxxxxxxxxx>
Cc: Roman Gushchin <klamm@xxxxxxxxxxxxxx>
Cc: Thomas Gleixner <tglx@xxxxxxxxxxxxx>
Cc: Paul Turner <pjt@xxxxxxxxxx>
Signed-off-by: Peter Zijlstra (Intel) <peterz@xxxxxxxxxxxxx>
---
 kernel/time/hrtimer.c |   12 +++++++++---
 1 file changed, 9 insertions(+), 3 deletions(-)

--- a/kernel/time/hrtimer.c
+++ b/kernel/time/hrtimer.c
@@ -791,6 +791,9 @@ u64 hrtimer_forward(struct hrtimer *time
 	if (delta.tv64 < 0)
 		return 0;
 
+	if (WARN_ON(timer->state & HRTIMER_STATE_ENQUEUED))
+		return 0;
+
 	if (interval.tv64 < hrtimer_resolution)
 		interval.tv64 = hrtimer_resolution;
 
@@ -1131,11 +1134,14 @@ static void __run_hrtimer(struct hrtimer
 	 * Note: We clear the CALLBACK bit after enqueue_hrtimer and
 	 * we do not reprogramm the event hardware. Happens either in
 	 * hrtimer_start_range_ns() or in hrtimer_interrupt()
+	 *
+	 * Note: Because we dropped the cpu_base->lock above,
+	 * hrtimer_start_range_ns() can have popped in and enqueued the timer
+	 * for us already.
 	 */
-	if (restart != HRTIMER_NORESTART) {
-		BUG_ON(timer->state != HRTIMER_STATE_CALLBACK);
+	if (restart != HRTIMER_NORESTART &&
+	    !(timer->state & HRTIMER_STATE_ENQUEUED))
 		enqueue_hrtimer(timer, base);
-	}
 
 	WARN_ON_ONCE(!(timer->state & HRTIMER_STATE_CALLBACK));
 
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