qsbr: only mark reader thread as being waited for in contended case

[userspace-rcu.git] / urcu-static.h

diff --git a/urcu-static.h b/urcu-static.h
index f819e6fc9aa07e9f4e58329661bf5a494936bb25..efb8225161b2c3c99c450d68a83289f1acb0379d 100644 (file)
--- a/urcu-static.h
+++ b/urcu-static.h
@@ -31,6 +31,7 @@
 
 #include <stdlib.h>
 #include <pthread.h>
+#include <sched.h>
 
 #include <compiler.h>
 #include <arch.h>
@@ -117,6 +118,11 @@
  */
 #define KICK_READER_LOOPS 10000
 
+/*
+ * Active attempts to check for reader Q.S. before calling sched_yield().
+ */
+#define RCU_QS_ACTIVE_ATTEMPTS 100
+
 #ifdef DEBUG_RCU
 #define rcu_assert(args...)    assert(args)
 #else
@@ -199,6 +205,7 @@ static inline void reader_barrier()
 /* Use the amount of bits equal to half of the architecture long size */
 #define RCU_GP_CTR_BIT         (1UL << (sizeof(long) << 2))
 #define RCU_GP_CTR_NEST_MASK   (RCU_GP_CTR_BIT - 1)
+#define RCU_GP_ONGOING         (RCU_GP_CTR_BIT << 1)
 
 /*
  * Global quiescent period counter with low-order bits unused.
@@ -207,7 +214,12 @@ static inline void reader_barrier()
  */
 extern long urcu_gp_ctr;
 
-extern long __thread urcu_active_readers;
+struct urcu_reader_status {
+       long active_readers;
+       long gp_waiting;
+};
+
+extern struct urcu_reader_status __thread urcu_reader_status;
 
 static inline int rcu_old_gp_ongoing(long *value)
 {
@@ -226,25 +238,35 @@ static inline int rcu_old_gp_ongoing(long *value)
 
 static inline void _rcu_read_lock(void)
 {
-       long tmp;
+       long tmp, gp_ctr;
 
-       tmp = urcu_active_readers;
+       tmp = urcu_reader_status.active_readers;
        /* urcu_gp_ctr = RCU_GP_COUNT | (~RCU_GP_CTR_BIT or RCU_GP_CTR_BIT) */
        if (likely(!(tmp & RCU_GP_CTR_NEST_MASK))) {
-               _STORE_SHARED(urcu_active_readers, _LOAD_SHARED(urcu_gp_ctr));
+               /*
+                * volatile accesses can be reordered and optimized when within
+                * the same statement.
+                */
+               if (unlikely((gp_ctr = _LOAD_SHARED(urcu_gp_ctr))
+                               & RCU_GP_ONGOING) &&
+                   unlikely(LOAD_SHARED(urcu_reader_status.gp_waiting))) {
+                       sched_yield();
+                       gp_ctr = _LOAD_SHARED(urcu_gp_ctr);
+               }
+               _STORE_SHARED(urcu_reader_status.active_readers, gp_ctr);
                /*
                 * Set active readers count for outermost nesting level before
                 * accessing the pointer. See force_mb_all_threads().
                 */
                reader_barrier();
        } else {
-               _STORE_SHARED(urcu_active_readers, tmp + RCU_GP_COUNT);
+               _STORE_SHARED(urcu_reader_status.active_readers,
+                             tmp + RCU_GP_COUNT);
        }
 }
 
 static inline void _rcu_read_unlock(void)
 {
-       reader_barrier();
        /*
         * Finish using rcu before decrementing the pointer.
         * See force_mb_all_threads().
@@ -252,7 +274,9 @@ static inline void _rcu_read_unlock(void)
         * 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);
+       reader_barrier();
+       _STORE_SHARED(urcu_reader_status.active_readers,
+                     urcu_reader_status.active_readers - RCU_GP_COUNT);
 }
 
 /**