Performance: remove rcu read lock from ring buffer get/put cpu

The tracepoints are already protected by a RCU-bp read-side lock, so
trying to take this nested lock is useless. We gain 132 ns/event on the
ARM32 Cubietruck by removing this nested rcu read-side lock.

Signed-off-by: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>

diff --git a/libringbuffer/frontend_api.h b/libringbuffer/frontend_api.h
index 56dbef2aa4326c8a4067ecc96ada66b887358afd..77a269184fcf1f3dbc26380f7e2e201cd25bd497 100644 (file)
--- a/libringbuffer/frontend_api.h
+++ b/libringbuffer/frontend_api.h
@@ -37,10 +37,10 @@
 /**
  * lib_ring_buffer_get_cpu - Precedes ring buffer reserve/commit.
  *
- * Grabs RCU read-side lock and keeps a ring buffer nesting count as
- * supplementary safety net to ensure tracer client code will never
- * trigger an endless recursion. Returns the processor ID on success,
- * -EPERM on failure (nesting count too high).
+ * Keeps a ring buffer nesting count as supplementary safety net to
+ * ensure tracer client code will never trigger an endless recursion.
+ * Returns the processor ID on success, -EPERM on failure (nesting count
+ * too high).
  *
  * asm volatile and "memory" clobber prevent the compiler from moving
  * instructions out of the ring buffer nesting count. This is required to ensure
@@ -53,7 +53,6 @@ int lib_ring_buffer_get_cpu(const struct lttng_ust_lib_ring_buffer_config *confi
 {
        int cpu, nesting;
 
-       rcu_read_lock();
        cpu = lttng_ust_get_cpu();
        nesting = ++URCU_TLS(lib_ring_buffer_nesting);
        cmm_barrier();
@@ -75,7 +74,6 @@ void lib_ring_buffer_put_cpu(const struct lttng_ust_lib_ring_buffer_config *conf
 {
        cmm_barrier();
        URCU_TLS(lib_ring_buffer_nesting)--;            /* TLS */
-       rcu_read_unlock();
 }
 
 /*