Messages in this thread | | | Date | Tue, 6 Oct 2015 10:01:01 -0700 | From | "Paul E. McKenney" <> | Subject | Re: [PATCH tip/core/rcu 04/13] rcu: Don't disable preemption for Tiny and Tree RCU readers |
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On Tue, Oct 06, 2015 at 09:44:45AM -0700, Josh Triplett wrote: > On Tue, Oct 06, 2015 at 09:13:39AM -0700, Paul E. McKenney wrote: > > From: Boqun Feng <boqun.feng@gmail.com> > > > > Because preempt_disable() maps to barrier() for non-debug builds, > > it forces the compiler to spill and reload registers. Because Tree > > RCU and Tiny RCU now only appear in CONFIG_PREEMPT=n builds, these > > barrier() instances generate needless extra code for each instance of > > rcu_read_lock() and rcu_read_unlock(). This extra code slows down Tree > > RCU and bloats Tiny RCU. > > > > This commit therefore removes the preempt_disable() and preempt_enable() > > from the non-preemptible implementations of __rcu_read_lock() and > > __rcu_read_unlock(), respectively. However, for debug purposes, > > preempt_disable() and preempt_enable() are still invoked if > > CONFIG_PREEMPT_COUNT=y, because this allows detection of sleeping inside > > atomic sections in non-preemptible kernels. > > > > This is based on an earlier patch by Paul E. McKenney, fixing > > a bug encountered in kernels built with CONFIG_PREEMPT=n and > > CONFIG_PREEMPT_COUNT=y. > > This also adds explicit barrier() calls to several internal RCU > functions, but the commit message doesn't explain those at all.
To compensate for them being removed from rcu_read_lock() and rcu_read_unlock(), but yes, I will update.
And good catch!
Thanx, Paul
> > Signed-off-by: Boqun Feng <boqun.feng@gmail.com> > > Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com> > > --- > > include/linux/rcupdate.h | 6 ++++-- > > include/linux/rcutiny.h | 1 + > > kernel/rcu/tree.c | 9 +++++++++ > > 3 files changed, 14 insertions(+), 2 deletions(-) > > > > diff --git a/include/linux/rcupdate.h b/include/linux/rcupdate.h > > index d63bb77dab35..6c3ceceb6148 100644 > > --- a/include/linux/rcupdate.h > > +++ b/include/linux/rcupdate.h > > @@ -297,12 +297,14 @@ void synchronize_rcu(void); > > > > static inline void __rcu_read_lock(void) > > { > > - preempt_disable(); > > + if (IS_ENABLED(CONFIG_PREEMPT_COUNT)) > > + preempt_disable(); > > } > > > > static inline void __rcu_read_unlock(void) > > { > > - preempt_enable(); > > + if (IS_ENABLED(CONFIG_PREEMPT_COUNT)) > > + preempt_enable(); > > } > > > > static inline void synchronize_rcu(void) > > diff --git a/include/linux/rcutiny.h b/include/linux/rcutiny.h > > index c8a0722f77ea..4c1aaf9cce7b 100644 > > --- a/include/linux/rcutiny.h > > +++ 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 */ > > 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); > > > > @@ -353,12 +355,19 @@ EXPORT_SYMBOL_GPL(rcu_note_context_switch); > > * RCU flavors in desperate need of a quiescent state, which will normally > > * be none of them). Either way, do a lightweight quiescent state for > > * all RCU flavors. > > + * > > + * The barrier() calls are redundant in the common case when this is > > + * called externally, but just in case this is called from within this > > + * file. > > + * > > */ > > void rcu_all_qs(void) > > { > > + barrier(); /* Avoid RCU read-side critical sections leaking down. */ > > if (unlikely(raw_cpu_read(rcu_sched_qs_mask))) > > rcu_momentary_dyntick_idle(); > > this_cpu_inc(rcu_qs_ctr); > > + barrier(); /* Avoid RCU read-side critical sections leaking up. */ > > } > > EXPORT_SYMBOL_GPL(rcu_all_qs); > > > > -- > > 2.5.2 > > >
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