[ 3.5.y.z extended stable ] Patch "nohz: Make tick_nohz_irq_exit() irq safe" has been added to staging queue

[Luis Henriques]

This is a note to let you know that I have just added a patch titled

    nohz: Make tick_nohz_irq_exit() irq safe

to the linux-3.5.y-queue branch of the 3.5.y.z extended stable tree 
which can be found at:

 http://kernel.ubuntu.com/git?p=ubuntu/linux.git;a=shortlog;h=refs/heads/linux-3.5.y-queue

If you, or anyone else, feels it should not be added to this tree, please 
reply to this email.

For more information about the 3.5.y.z tree, see
https://wiki.ubuntu.com/Kernel/Dev/ExtendedStable

Thanks.
-Luis

------

>From 2c5afd2674bfea5f6c564dad7d96d0a42251d6ee Mon Sep 17 00:00:00 2001
From: Frederic Weisbecker <fweisbec at gmail.com>
Date: Wed, 20 Feb 2013 16:15:36 +0100
Subject: [PATCH] nohz: Make tick_nohz_irq_exit() irq safe

commit e5ab012c3271990e8457055c25cafddc1ae8aa6b upstream.

As it stands, irq_exit() may or may not be called with
irqs disabled, depending on __ARCH_IRQ_EXIT_IRQS_DISABLED
that the arch can define.

It makes tick_nohz_irq_exit() unsafe. For example two
interrupts can race in tick_nohz_stop_sched_tick(): the inner
most one computes the expiring time on top of the timer list,
then it's interrupted right before reprogramming the
clock. The new interrupt enqueues a new timer list timer,
it reprogram the clock to take it into account and it exits.
The CPUs resumes the inner most interrupt and performs the clock
reprogramming without considering the new timer list timer.

This regression has been introduced by:
     280f06774afedf849f0b34248ed6aff57d0f6908
     ("nohz: Separate out irq exit and idle loop dyntick logic")

Let's fix it right now with the appropriate protections.

A saner long term solution will be to remove
__ARCH_IRQ_EXIT_IRQS_DISABLED and mandate that irq_exit() is called
with interrupts disabled.

Signed-off-by: Frederic Weisbecker <fweisbec at gmail.com>
Cc: Peter Zijlstra <peterz at infradead.org>
Cc: Ingo Molnar <mingo at kernel.org>
Cc: Linus Torvalds <torvalds at linuxfoundation.org>
Cc: <stable at vger.kernel.org> #v3.2+
Link: http://lkml.kernel.org/r/1361373336-11337-1-git-send-email-fweisbec@gmail.com
Signed-off-by: Thomas Gleixner <tglx at linutronix.de>
[ luis: backported to 3.5 ]
Signed-off-by: Luis Henriques <luis.henriques at canonical.com>
---
 kernel/time/tick-sched.c | 5 +++++
 1 file changed, 5 insertions(+)

diff --git a/kernel/time/tick-sched.c b/kernel/time/tick-sched.c
index 4a08472..cc1825f 100644
--- a/kernel/time/tick-sched.c
+++ b/kernel/time/tick-sched.c
@@ -500,12 +500,17 @@ void tick_nohz_idle_enter(void)
  */
 void tick_nohz_irq_exit(void)
 {
+	unsigned long flags;
 	struct tick_sched *ts = &__get_cpu_var(tick_cpu_sched);

 	if (!ts->inidle)
 		return;

+	local_irq_save(flags);
+
 	tick_nohz_stop_sched_tick(ts);
+
+	local_irq_restore(flags);
 }

 /**
--
1.8.1.2