linux-kernel - [RFC PATCH] hrtimer: remove deadlock due to waiting on IPI in softirq context

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Date:	Wed, 5 Mar 2014 16:25:26 -0500
From:	Rik van Riel <riel@...hat.com>
To:	linux-kernel@...r.kernel.org
Cc:	Mateusz Guzik <mguzik@...hat.com>,
	Benjamin Herrenschmidt <benh@...nel.crashing.org>,
	Ingo Molnar <mingo@...hat.com>,
	Thomas Gleixner <tglx@...utronix.de>,
	Prarit Bhargava <prarit@...hat.com>,
	Frederic Weisbecker <fweisbec@...il.com>,
	Clark Williams <williams@...hat.com>
Subject: [RFC PATCH] hrtimer: remove deadlock due to waiting on IPI in
 softirq context

There appears to be a deadlock in the hrtimer code. Specifically,
clock_was_set() calls an IPI with wait=1, from softirq context.

Waiting for IPIs to complete in irq context can lead to a deadlock,
because the current code (that was interrupted) might be holding some
kind of lock, that another CPU is waiting for with spin_lock_irq or
similar.

In other words, the current CPU may need to release a resource, before
the IPI can be handled by one of the destination CPUs.

To my untrained eye, it does not look like this patch introduces a
new bug to the timer code, but that is hard to ascertain with the
timer code. so I am posting this as an RFC for the timer gods to hurt
their brains on :)

This bug was introduced by 54cdfdb4 in early 2007 (the original
hrtimer code patch).

Not-yet-signed-off-by: Rik van Riel <riel@...hat.com>
Reported-by: Mateusz Guzik <mguzik@...hat.com>
Cc: Benjamin Herrenschmidt <benh@...nel.crashing.org>
Cc: Ingo Molnar <mingo@...hat.com>
Cc: Thomas Gleixner <tglx@...utronix.de>
Cc: Prarit Bhargava <prarit@...hat.com>
Cc: Frederic Weisbecker <fweisbec@...il.com>
Cc: Clark Williams <williams@...hat.com>
---
 kernel/hrtimer.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/kernel/hrtimer.c b/kernel/hrtimer.c
index 0909436..19145ec 100644
--- a/kernel/hrtimer.c
+++ b/kernel/hrtimer.c
@@ -771,7 +771,7 @@ void clock_was_set(void)
 {
 #ifdef CONFIG_HIGH_RES_TIMERS
 	/* Retrigger the CPU local events everywhere */
-	on_each_cpu(retrigger_next_event, NULL, 1);
+	on_each_cpu(retrigger_next_event, NULL, 0);
 #endif
 	timerfd_clock_was_set();
 }
--
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