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

Paul E. McKenney
2012-09-21T03:05:15Z

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
>
>
>