[PATCH 3.11 61/70] hrtimer: Prevent all reprogramming if hang detected

From: Luis Henriques
Date: Wed May 07 2014 - 09:16:56 EST


3.11.10.10 -stable review patch. If anyone has any objections, please let me know.

------------------

From: Stuart Hayes <stuart.w.hayes@xxxxxxxxx>

commit 6c6c0d5a1c949d2e084706f9e5fb1fccc175b265 upstream.

If the last hrtimer interrupt detected a hang it sets hang_detected=1
and programs the clock event device with a delay to let the system
make progress.

If hang_detected == 1, we prevent reprogramming of the clock event
device in hrtimer_reprogram() but not in hrtimer_force_reprogram().

This can lead to the following situation:

hrtimer_interrupt()
hang_detected = 1;
program ce device to Xms from now (hang delay)

We have two timers pending:
T1 expires 50ms from now
T2 expires 5s from now

Now T1 gets canceled, which causes hrtimer_force_reprogram() to be
invoked, which in turn programs the clock event device to T2 (5
seconds from now).

Any hrtimer_start after that will not reprogram the hardware due to
hang_detected still being set. So we effectivly block all timers until
the T2 event fires and cleans up the hang situation.

Add a check for hang_detected to hrtimer_force_reprogram() which
prevents the reprogramming of the hang delay in the hardware
timer. The subsequent hrtimer_interrupt will resolve all outstanding
issues.

[ tglx: Rewrote subject and changelog and fixed up the comment in
hrtimer_force_reprogram() ]

Signed-off-by: Stuart Hayes <stuart.w.hayes@xxxxxxxxx>
Link: http://lkml.kernel.org/r/53602DC6.2060101@xxxxxxxxx
Signed-off-by: Thomas Gleixner <tglx@xxxxxxxxxxxxx>
Signed-off-by: Luis Henriques <luis.henriques@xxxxxxxxxxxxx>
---
kernel/hrtimer.c | 17 +++++++++++++++++
1 file changed, 17 insertions(+)

diff --git a/kernel/hrtimer.c b/kernel/hrtimer.c
index 383319b..0d6df7b 100644
--- a/kernel/hrtimer.c
+++ b/kernel/hrtimer.c
@@ -581,6 +581,23 @@ hrtimer_force_reprogram(struct hrtimer_cpu_base *cpu_base, int skip_equal)

cpu_base->expires_next.tv64 = expires_next.tv64;

+ /*
+ * If a hang was detected in the last timer interrupt then we
+ * leave the hang delay active in the hardware. We want the
+ * system to make progress. That also prevents the following
+ * scenario:
+ * T1 expires 50ms from now
+ * T2 expires 5s from now
+ *
+ * T1 is removed, so this code is called and would reprogram
+ * the hardware to 5s from now. Any hrtimer_start after that
+ * will not reprogram the hardware due to hang_detected being
+ * set. So we'd effectivly block all timers until the T2 event
+ * fires.
+ */
+ if (cpu_base->hang_detected)
+ return;
+
if (cpu_base->expires_next.tv64 != KTIME_MAX)
tick_program_event(cpu_base->expires_next, 1);
}
--
1.9.1

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