Add git__compare_and_swap and use it

This removes the lock from the repository object and changes the
internals to use the new atomic git__compare_and_swap to update
the _odb, _config, _index, and _refdb variables in a threadsafe
manner.

1 files changed, 41 insertions, 0 deletions

diff --git a/src/thread-utils.h b/src/thread-utils.h
index 2ca290adf..7b663182d 100644
--- a/src/thread-utils.h
+++ b/src/thread-utils.h
@@ -68,6 +68,21 @@ GIT_INLINE(int) git_atomic_dec(git_atomic *a)
 #endif
 }
 
+GIT_INLINE(void *) git___compare_and_swap(
+	volatile void **ptr, void *oldval, void *newval)
+{
+	bool swapped;
+#if defined(GIT_WIN32)
+	swapped = ((LONGLONG)oldval == InterlockedCompareExchange64(
+		(LONGLONG volatile *)ptr, (LONGLONG)newval, (LONGLONG)oldval));
+#elif defined(__GNUC__)
+	swapped = (__sync_val_compare_and_swap(ptr, oldval, newval) == oldval);
+#else
+#	error "Unsupported architecture for atomic operations"
+#endif
+	return swapped ? oldval : newval;
+}
+
 #else
 
 #define git_thread unsigned int
@@ -101,8 +116,34 @@ GIT_INLINE(int) git_atomic_dec(git_atomic *a)
 	return --a->val;
 }
 
+GIT_INLINE(void *) git___compare_and_swap(
+	volatile void **ptr, void *oldval, void *newval)
+{
+	if (*ptr == oldval)
+		*ptr = newval;
+	else
+		oldval = newval;
+	return oldval;
+}
+
 #endif
 
+/* Atomically replace oldval with newval
+ * @return oldval if it was replaced or newval if it was not
+ */
+#define git__compare_and_swap(P,O,N) \
+	git___compare_and_swap((volatile void **)P, O, N)
+
+#define git__swap(ptr, val) git__compare_and_swap(&ptr, ptr, val)
+
 extern int git_online_cpus(void);
 
+#if defined(GIT_THREADS) && defined(GIT_WIN32)
+# define GIT_MEMORY_BARRIER MemoryBarrier()
+#elif defined(GIT_THREADS)
+# define GIT_MEMORY_BARRIER __sync_synchronize()
+#else
+# define GIT_MEMORY_BARRIER /* noop */
+#endif
+
 #endif /* INCLUDE_thread_utils_h__ */