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Subject[tip:timers/core] nohz: Fix !HIGH_RES_TIMERS hang
Commit-ID:  6b442bc81337913eb775965a67ffdb8a36935422
Gitweb: http://git.kernel.org/tip/6b442bc81337913eb775965a67ffdb8a36935422
Author: Thomas Gleixner <tglx@linutronix.de>
AuthorDate: Thu, 7 May 2015 14:35:59 +0200
Committer: Ingo Molnar <mingo@kernel.org>
CommitDate: Thu, 7 May 2015 16:15:50 +0200

nohz: Fix !HIGH_RES_TIMERS hang

Simon Horman reported this crash on a system with
high-res timers disabled but nohz enabled:

> ------------[ cut here ]------------
> kernel BUG at kernel/irq_work.c:135!

BUG_ON(!irqs_disabled());

So something enabled interrupts in the periodic tick handling machinery,
and that code path indeed has a local_irq_disable()/enable pair in
tick_nohz_switch_to_nohz() which causes havoc. Fix it.

This patch also fixes a +nohz -hrtimers hang reported by Ingo Molnar.

Reported-by: Simon Horman <horms@verge.net.au>
Reported-by: Ingo Molnar <mingo@kernel.org>
Tested-by: Simon Horman <horms@verge.net.au>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Cc: Borislav Petkov <bp@alien8.de>
Cc: H. Peter Anvin <hpa@zytor.com>
Cc: LAK <linux-arm-kernel@lists.infradead.org>
Cc: Magnus Damm <magnus.damm@gmail.com>
Cc: Peter Zijlstra <peterz@infradead.org>
Link: http://lkml.kernel.org/r/alpine.DEB.2.11.1505071425520.4225@nanos
Signed-off-by: Ingo Molnar <mingo@kernel.org>
---
kernel/time/tick-sched.c | 9 +++------
1 file changed, 3 insertions(+), 6 deletions(-)

diff --git a/kernel/time/tick-sched.c b/kernel/time/tick-sched.c
index 753c211..812f7a3 100644
--- a/kernel/time/tick-sched.c
+++ b/kernel/time/tick-sched.c
@@ -967,11 +967,9 @@ static void tick_nohz_switch_to_nohz(void)
if (!tick_nohz_enabled)
return;

- local_irq_disable();
- if (tick_switch_to_oneshot(tick_nohz_handler)) {
- local_irq_enable();
+ if (tick_switch_to_oneshot(tick_nohz_handler))
return;
- }
+
tick_nohz_active = 1;
ts->nohz_mode = NOHZ_MODE_LOWRES;

@@ -986,7 +984,6 @@ static void tick_nohz_switch_to_nohz(void)
hrtimer_forward_now(&ts->sched_timer, tick_period);
hrtimer_set_expires(&ts->sched_timer, next);
tick_program_event(next, 1);
- local_irq_enable();
}

/*
@@ -1171,7 +1168,7 @@ void tick_oneshot_notify(void)
* Called cyclic from the hrtimer softirq (driven by the timer
* softirq) allow_nohz signals, that we can switch into low-res nohz
* mode, because high resolution timers are disabled (either compile
- * or runtime).
+ * or runtime). Called with interrupts disabled.
*/
int tick_check_oneshot_change(int allow_nohz)
{

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