On Tue, Oct 06, 2015 at 10:42:04AM -0700, Paul E. McKenney wrote:
>
> Ah. The reason is that Tiny RCU and Tree RCU (the !PREEMPT ones) act
> by implicitly extending (and, if need be, merging) the RCU read-side
> critical sections to include all the code between successive quiescent
> states, for example, all the code between a pair of calls to schedule().
>
> Therefore, there need to be barrier() calls in the quiescent-state
> functions. Some could be argued to be implicitly present due to
> translation-unit boundaries, but paranoia and all that.
>
> Would adding that sort of explanation help?
> +++ b/include/linux/rcutiny.h
> @@ -216,6 +216,7 @@ static inline bool rcu_is_watching(void)
>
> static inline void rcu_all_qs(void)
> {
> + barrier(); /* Avoid RCU read-side critical sections leaking across. */
> }
>
> #endif /* __LINUX_RCUTINY_H */
This is more than sheer paranoia I think, inlined functions are not a
compiler barrier.
> diff --git a/kernel/rcu/tree.c b/kernel/rcu/tree.c
> index b9d9e0249e2f..93c0f23c3e45 100644
> --- a/kernel/rcu/tree.c
> +++ b/kernel/rcu/tree.c
> @@ -337,12 +337,14 @@ static void rcu_momentary_dyntick_idle(void)
> */
> void rcu_note_context_switch(void)
> {
> + barrier(); /* Avoid RCU read-side critical sections leaking down. */
> trace_rcu_utilization(TPS("Start context switch"));
> rcu_sched_qs();
> rcu_preempt_note_context_switch();
> if (unlikely(raw_cpu_read(rcu_sched_qs_mask)))
> rcu_momentary_dyntick_idle();
> trace_rcu_utilization(TPS("End context switch"));
> + barrier(); /* Avoid RCU read-side critical sections leaking up. */
> }
> EXPORT_SYMBOL_GPL(rcu_note_context_switch);
These OTOH could be fixed with a noinline, such that the compiler may
never inline it, even with whole-program-optimizations, thereby
guaranteeing a function call boundary or compiler barrier.
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