Add futex support to accelerate synchronize_rcu() on UP

[urcu.git] / urcu-static.h

diff --git a/urcu-static.h b/urcu-static.h
index f819e6fc9aa07e9f4e58329661bf5a494936bb25..3caa0f93d55058528a07c5fb20ba3a0e80191e79 100644 (file)
--- a/urcu-static.h
+++ b/urcu-static.h
@@ -31,6 +31,8 @@
 
 #include <stdlib.h>
 #include <pthread.h>
+#include <syscall.h>
+#include <unistd.h>
 
 #include <compiler.h>
 #include <arch.h>
@@ -94,6 +96,10 @@
                                (_________p1);                          \
                                })
 
+#define futex(...)             syscall(__NR_futex, __VA_ARGS__)
+#define FUTEX_WAIT             0
+#define FUTEX_WAKE             1
+
 /*
  * This code section can only be included in LGPL 2.1 compatible source code.
  * See below for the function call wrappers which can be used in code meant to
@@ -117,6 +123,11 @@
  */
 #define KICK_READER_LOOPS 10000
 
+/*
+ * Active attempts to check for reader Q.S. before calling futex().
+ */
+#define RCU_QS_ACTIVE_ATTEMPTS 100
+
 #ifdef DEBUG_RCU
 #define rcu_assert(args...)    assert(args)
 #else
@@ -209,6 +220,20 @@ extern long urcu_gp_ctr;
 
 extern long __thread urcu_active_readers;
 
+extern int gp_futex;
+
+/*
+ * Wake-up waiting synchronize_rcu(). Called from many concurrent threads.
+ */
+static inline void wake_up_gp(void)
+{
+       if (unlikely(atomic_read(&gp_futex) == -1)) {
+               atomic_set(&gp_futex, 0);
+               futex(&gp_futex, FUTEX_WAKE, 1,
+                     NULL, NULL, 0);
+       }
+}
+
 static inline int rcu_old_gp_ongoing(long *value)
 {
        long v;
@@ -244,15 +269,24 @@ static inline void _rcu_read_lock(void)
 
 static inline void _rcu_read_unlock(void)
 {
-       reader_barrier();
+       long tmp;
+
+       tmp = urcu_active_readers;
        /*
         * Finish using rcu before decrementing the pointer.
         * See force_mb_all_threads().
-        * Formally only needed for outermost nesting level, but leave barrier
-        * in place for nested unlocks to remove a branch from the common case
-        * (no nesting).
         */
-       _STORE_SHARED(urcu_active_readers, urcu_active_readers - RCU_GP_COUNT);
+       if (likely((tmp & RCU_GP_CTR_NEST_MASK) == RCU_GP_COUNT)) {
+               reader_barrier();
+               _STORE_SHARED(urcu_active_readers,
+                             urcu_active_readers - RCU_GP_COUNT);
+               /* write urcu_active_readers before read futex */
+               reader_barrier();
+               wake_up_gp();
+       } else {
+               _STORE_SHARED(urcu_active_readers,
+                             urcu_active_readers - RCU_GP_COUNT);
+       }
 }
 
 /**