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

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

[Li Zhong]

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

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

[Paul E. McKenney]

On Fri, Sep 21, 2012 at 10:08:56AM +0800, Li Zhong wrote:
> 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. 

Thank you, Zhong.  Queued, as those checking the SOBs below might
guess.  ;-)

							Thanx, paul

> 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
> 
> 
>