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 d63bb77dab35a4ecf56fd33e9d9ad21d93ab2876..6c3ceceb614898790d6b59219c12e81cafb287ed 100644 (file)
--- 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 c8a0722f77ea07ea74823dca6ab725273a27918e..4c1aaf9cce7b3ac767c69d7849715d69089a9b48 100644 (file)
--- 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 b9d9e0249e2f3a7d39dacd851a629b80369a0c3d..93c0f23c3e45d42fccb9fa946767450ea836ac5a 100644 (file)
--- 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);