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@gmail.com>
Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
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 @@
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 void rcu_all_qs(void)
{
+ barrier(); /* Avoid RCU read-side critical sections leaking across. */
}
#endif /* __LINUX_RCUTINY_H */