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    SubjectRe: [PATCH tip/core/rcu 04/13] rcu: Don't disable preemption for Tiny and Tree RCU readers
    On Tue, Oct 06, 2015 at 10:16:30AM -0700, Josh Triplett wrote:
    > On Tue, Oct 06, 2015 at 10:01:01AM -0700, Paul E. McKenney wrote:
    > > 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.
    >
    > That much seemed clear from the comments, but that doesn't explain *why*
    > those functions need barriers of their own even though rcu_read_lock()
    > and rcu_read_unlock() don't.

    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?

    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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