* [PATCH RFC] rcu: Don't disable preemption for Tiny and Tree RCU readers
@ 2015-07-31 4:37 Paul E. McKenney
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From: Paul E. McKenney @ 2015-07-31 4:37 UTC (permalink / raw)
To: josh; +Cc: linux-kernel
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 (though probably not significantly).
This commit therefore removes the preempt_disable() and preempt_enable()
from the non-preemptible implementations of __rcu_read_lock() and
__rcu_read_unlock(), respectively.
Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
diff --git a/include/linux/rcupdate.h b/include/linux/rcupdate.h
index 78af46c98e55..927c5e60c1dd 100644
--- a/include/linux/rcupdate.h
+++ b/include/linux/rcupdate.h
@@ -298,12 +298,14 @@ void synchronize_rcu(void);
static inline void __rcu_read_lock(void)
{
- preempt_disable();
+ if (!IS_ENABLED(CONFIG_TINY_RCU))
+ preempt_disable();
}
static inline void __rcu_read_unlock(void)
{
- preempt_enable();
+ if (!IS_ENABLED(CONFIG_TINY_RCU))
+ preempt_enable();
}
static inline void synchronize_rcu(void)
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