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Date: Tue, 6 Oct 2015 22:05:38 +0200 From: Peter Zijlstra <peterz@...radead.org> To: "Paul E. McKenney" <paulmck@...ux.vnet.ibm.com> Cc: Josh Triplett <josh@...htriplett.org>, linux-kernel@...r.kernel.org, mingo@...nel.org, jiangshanlai@...il.com, dipankar@...ibm.com, akpm@...ux-foundation.org, mathieu.desnoyers@...icios.com, tglx@...utronix.de, rostedt@...dmis.org, dhowells@...hat.com, edumazet@...gle.com, dvhart@...ux.intel.com, fweisbec@...il.com, oleg@...hat.com, bobby.prani@...il.com, Boqun Feng <boqun.feng@...il.com> Subject: Re: [PATCH tip/core/rcu 04/13] rcu: Don't disable preemption for Tiny and Tree RCU readers 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. -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@...r.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
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