[PATCH rcu] Move TINY_RCU quiescent state out of extended quiescent state

Li Zhong
2012-09-21T02:09:35Z

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@linux.vnet.ibm.com>
Signed-off-by: Paul E. McKenney <paulmck@linux.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