Commit-ID: bb73c52bad3666997ed2ec83c0c80c3f8ef55008
Gitweb: http://git.kernel.org/tip/bb73c52bad3666997ed2ec83c0c80c3f8ef55008
Author: Boqun Feng <boqun.feng@xxxxxxxxx>
AuthorDate: Thu, 30 Jul 2015 16:55:38 -0700
Committer: Paul E. McKenney <paulmck@xxxxxxxxxxxxxxxxxx>
CommitDate: Tue, 6 Oct 2015 11:08:23 -0700
rcu: Don't disable preemption for Tiny and Tree RCU readers
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.
However, Tiny and Tree RCU operates by coalescing all RCU read-side
critical sections on a given CPU that lie between successive quiescent
states. It is therefore necessary to compensate for removing barriers
from __rcu_read_lock() and __rcu_read_unlock() by adding them to a
couple of the RCU functions invoked during quiescent states, namely to
rcu_all_qs() and rcu_note_context_switch(). However, note that the latter
is more paranoia than necessity, at least until link-time optimizations
become more aggressive.
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.
Signed-off-by: Boqun Feng <boqun.feng@xxxxxxxxx>
Signed-off-by: Paul E. McKenney <paulmck@xxxxxxxxxxxxxxxxxx>
---
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 d63bb77..6c3cece 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 c8a0722..4c1aaf9 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 b9d9e02..93c0f23 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);
--
To unsubscribe from this list: send the line "unsubscribe linux-tip-commits" in
the body of a message to majordomo@xxxxxxxxxxxxxxx
More majordomo info at http://vger.kernel.org/majordomo-info.html
|