linux-kernel - [PATCH rcu] Move TINY_RCU quiescent state out of extended quiescent state

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Message-ID: <1348193336.2371.76.camel@ThinkPad-T420>
Date:	Fri, 21 Sep 2012 10:08:56 +0800
From:	Li Zhong <zhong@...ux.vnet.ibm.com>
To:	LKML <linux-kernel@...r.kernel.org>
Cc:	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>,
	dipankar@...ibm.com
Subject: [PATCH rcu] Move TINY_RCU quiescent state out of extended quiescent
 state

TINY_RCU's rcu_idle_enter_common() invokes rcu_sched_qs() in order
to inform the RCU core of the quiescent state implied by idle entry.
Of course, idle is also an extended quiescent state, so that the call
to rcu_sched_qs() speeds up RCU's invoking of any callbacks that might
be queued.  This speed-up is important when entering into dyntick-idle
mode -- if there are no further scheduling-clock interrupts, the callbacks
might never be invoked, which could result in a system hang.

However, processing callbacks does event tracing, which in turn
implies RCU read-side critical sections, which are illegal in extended
quiescent states.  This patch therefore moves the call to rcu_sched_qs()
so that it precedes the point at which we set the new value of 
rcu_dynticks_nesting, which may indicate RCU is in an extended quiescent
state. 

Signed-off-by: Li Zhong <zhong@...ux.vnet.ibm.com>
Signed-off-by: Paul E. McKenney <paulmck@...ux.vnet.ibm.com>
---
 kernel/rcutiny.c |    2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/kernel/rcutiny.c b/kernel/rcutiny.c
index 2e073a2..e4c6a59 100644
--- a/kernel/rcutiny.c
+++ b/kernel/rcutiny.c
@@ -75,9 +75,9 @@ static void rcu_idle_enter_common(long long newval)
 			  current->pid, current->comm,
 			  idle->pid, idle->comm); /* must be idle task! */
 	}
+	rcu_sched_qs(0); /* implies rcu_bh_qsctr_inc(0) */
 	barrier();
 	rcu_dynticks_nesting = newval;
-	rcu_sched_qs(0); /* implies rcu_bh_qsctr_inc(0) */
 }

 /*
-- 
1.7.9.5

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