linux-kernel - Re: timers: HARDIRQ-safe -> HARDIRQ-unsafe lock order detected

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Date:	Tue, 12 Jan 2016 21:18:56 +0100
From:	Peter Zijlstra <peterz@...radead.org>
To:	Sasha Levin <sasha.levin@...cle.com>
Cc:	Thomas Gleixner <tglx@...utronix.de>,
	LKML <linux-kernel@...r.kernel.org>,
	Paul McKenney <paulmck@...ux.vnet.ibm.com>
Subject: Re: timers: HARDIRQ-safe -> HARDIRQ-unsafe lock order detected

On Tue, Jan 12, 2016 at 03:03:27PM -0500, Sasha Levin wrote:
> [ 3408.703754] Call Trace:

> [ 3408.733192] rcu_read_unlock_special (kernel/rcu/tree_plugin.h:503)
> [ 3408.735155] __rcu_read_unlock (kernel/rcu/update.c:223)
> [ 3408.736090] __lock_timer (include/linux/rcupdate.h:495 include/linux/rcupdate.h:930 kernel/time/posix-timers.c:709)

I'm thinking this is one of those magic preemptible RCU bits..

---
 kernel/time/posix-timers.c | 12 ++++++++++--
 1 file changed, 10 insertions(+), 2 deletions(-)

diff --git a/kernel/time/posix-timers.c b/kernel/time/posix-timers.c
index 31d11ac9fa47..09e28733e725 100644
--- a/kernel/time/posix-timers.c
+++ b/kernel/time/posix-timers.c
@@ -701,17 +701,25 @@ static struct k_itimer *__lock_timer(timer_t timer_id, unsigned long *flags)
 	if ((unsigned long long)timer_id > INT_MAX)
 		return NULL;
 
+	/*
+	 * One of the few rules of preemptible RCU is that one cannot do
+	 * rcu_read_unlock() while holding a scheduler (or nested) lock when
+	 * part of the read side critical section was irqs-enabled -- see
+	 * rcu_read_unlock_special().
+	 */
+	local_irq_safe(*flags);
 	rcu_read_lock();
 	timr = posix_timer_by_id(timer_id);
 	if (timr) {
-		spin_lock_irqsave(&timr->it_lock, *flags);
+		spin_lock(&timr->it_lock);
 		if (timr->it_signal == current->signal) {
 			rcu_read_unlock();
 			return timr;
 		}
-		spin_unlock_irqrestore(&timr->it_lock, *flags);
+		spin_unlock(&timr->it_lock);
 	}
 	rcu_read_unlock();
+	local_irq_restore(*flags);
 
 	return NULL;
 }