Add support for x86 older than P4, with CONFIG_HAS_FENCE option

[urcu.git] / urcu.h

diff --git a/urcu.h b/urcu.h
index 1b663c7ff1492e3beeab7832a592e6c2b7d90925..3e1ad78324cda478711e1c3f521bd62359bd2212 100644 (file)
--- a/urcu.h
+++ b/urcu.h
@@ -26,10 +26,23 @@
 #define likely(x)       __builtin_expect(!!(x), 1)
 #define unlikely(x)     __builtin_expect(!!(x), 0)
 
+/* Assume P4 or newer */
+#define CONFIG_HAS_FENCE 1
+
 /* x86 32/64 specific */
+#ifdef CONFIG_HAS_FENCE
 #define mb()    asm volatile("mfence":::"memory")
 #define rmb()   asm volatile("lfence":::"memory")
-#define wmb()   asm volatile("sfence" ::: "memory")
+#define wmb()   asm volatile("sfence"::: "memory")
+#else
+/*
+ * Some non-Intel clones support out of order store. wmb() ceases to be a
+ * nop for these.
+ */
+#define mb()    asm volatile("lock; addl $0,0(%%esp)":::"memory")
+#define rmb()   asm volatile("lock; addl $0,0(%%esp)":::"memory")
+#define wmb()   asm volatile("lock; addl $0,0(%%esp)"::: "memory")
+#endif
 
 /* Assume SMP machine, given we don't have this information */
 #define CONFIG_SMP 1
@@ -130,6 +143,13 @@ static inline unsigned long __xchg(unsigned long x, volatile void *ptr,
 
 #define SIGURCU SIGUSR1
 
+/*
+ * If a reader is really non-cooperative and refuses to commit its
+ * urcu_active_readers count to memory (there is no barrier in the reader
+ * per-se), kick it after a few loops waiting for it.
+ */
+#define KICK_READER_LOOPS 10000
+
 #ifdef DEBUG_YIELD
 #include <sched.h>
 #include <time.h>
@@ -234,8 +254,10 @@ static inline void rcu_read_lock(void)
 
        tmp = urcu_active_readers;
        /* urcu_gp_ctr = RCU_GP_COUNT | (~RCU_GP_CTR_BIT or RCU_GP_CTR_BIT) */
+       /* The data dependency "read urcu_gp_ctr, write urcu_active_readers",
+        * serializes those two memory operations. */
        if (likely(!(tmp & RCU_GP_CTR_NEST_MASK)))
-               urcu_active_readers = urcu_gp_ctr;
+               urcu_active_readers = ACCESS_ONCE(urcu_gp_ctr);
        else
                urcu_active_readers = tmp + RCU_GP_COUNT;
        /*