path: root/kernel/rcu/tree.c
diff options
authorBoqun Feng <>2015-07-30 16:55:38 -0700
committerPaul E. McKenney <>2015-10-06 11:08:23 -0700
commitbb73c52bad3666997ed2ec83c0c80c3f8ef55008 (patch)
tree5dd6150075147c23984f6c4902ba5fcde4612338 /kernel/rcu/tree.c
parentdb3e8db45e1cbf8cc22f1083a559d78eb91ccd63 (diff)
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 <> Signed-off-by: Paul E. McKenney <>
Diffstat (limited to 'kernel/rcu/tree.c')
1 files changed, 9 insertions, 0 deletions
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"));
if (unlikely(raw_cpu_read(rcu_sched_qs_mask)))
trace_rcu_utilization(TPS("End context switch"));
+ barrier(); /* Avoid RCU read-side critical sections leaking up. */
@@ -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)))
+ barrier(); /* Avoid RCU read-side critical sections leaking up. */