Add comment in rcu_add_lock

[urcu.git] / urcu.h

diff --git a/urcu.h b/urcu.h
index 2e8c0424abb0d413b6a64035ba05acced231d253..277b7d2f45ae565610e4db9ee89457174fafa667 100644 (file)
--- a/urcu.h
+++ b/urcu.h
@@ -162,11 +162,12 @@ static inline void debug_yield_init(void)
 #endif
 
 /*
- * Limiting the nesting level to 256 to keep instructions small in the read
- * fast-path.
+ * The trick here is that RCU_GP_CTR_BIT must be a multiple of 8 so we can use a
+ * full 8-bits, 16-bits or 32-bits bitmask for the lower order bits.
  */
-#define RCU_GP_COUNT           (1U << 0)
-#define RCU_GP_CTR_BIT         (1U << 8)
+#define RCU_GP_COUNT           (1UL << 0)
+/* 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)
 
 /*
@@ -174,13 +175,13 @@ static inline void debug_yield_init(void)
  * Using a int rather than a char to eliminate false register dependencies
  * causing stalls on some architectures.
  */
-extern int urcu_gp_ctr;
+extern long urcu_gp_ctr;
 
-extern int __thread urcu_active_readers;
+extern long __thread urcu_active_readers;
 
-static inline int rcu_old_gp_ongoing(int *value)
+static inline int rcu_old_gp_ongoing(long *value)
 {
-       int v;
+       long v;
 
        if (value == NULL)
                return 0;
@@ -193,13 +194,14 @@ static inline int rcu_old_gp_ongoing(int *value)
 
 static inline void rcu_read_lock(void)
 {
-       int tmp;
+       long tmp;
 
        debug_yield_read();
        tmp = urcu_active_readers;
        debug_yield_read();
+       /* urcu_gp_ctr = RCU_GP_COUNT | (~RCU_GP_CTR_BIT or RCU_GP_CTR_BIT) */
        if (likely(!(tmp & RCU_GP_CTR_NEST_MASK)))
-               urcu_active_readers = urcu_gp_ctr + RCU_GP_COUNT;
+               urcu_active_readers = urcu_gp_ctr;
        else
                urcu_active_readers = tmp + RCU_GP_COUNT;
        debug_yield_read();