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.

src/global.h

@@ -10,14 +10,6 @@
 #include "mwindow.h"
 #include "hash.h"
 
-#if defined(GIT_THREADS) && defined(_MSC_VER)
-# define GIT_MEMORY_BARRIER MemoryBarrier()
-#elif defined(GIT_THREADS)
-# define GIT_MEMORY_BARRIER __sync_synchronize()
-#else
-# define GIT_MEMORY_BARRIER /* noop */
-#endif
-
 typedef struct {
 	git_error *last_error;
 	git_error error_t;

src/repository.c

@@ -32,43 +32,57 @@
 
 #define GIT_TEMPLATE_DIR "/usr/share/git-core/templates"
 
-#define repo_swap_ptr(repo,item_ptr,new_value) \
-	git__swap(&repo->lock, (void **)item_ptr, new_value)
-
-static void drop_odb(git_repository *repo)
+static void set_odb(git_repository *repo, git_odb *odb)
 {
-	git_odb *dropme = repo_swap_ptr(repo, &repo->_odb, NULL);
-	if (dropme != NULL) {
-		GIT_REFCOUNT_OWN(dropme, NULL);
-		git_odb_free(dropme);
+	if (odb) {
+		GIT_REFCOUNT_OWN(odb, repo);
+		GIT_REFCOUNT_INC(odb);
+	}
+
+	if ((odb = git__swap(repo->_odb, odb)) != NULL) {
+		GIT_REFCOUNT_OWN(odb, NULL);
+		git_odb_free(odb);
 	}
 }
 
-static void drop_refdb(git_repository *repo)
+static void set_refdb(git_repository *repo, git_refdb *refdb)
 {
-	git_refdb *dropme = repo_swap_ptr(repo, &repo->_refdb, NULL);
-	if (dropme != NULL) {
-		GIT_REFCOUNT_OWN(dropme, NULL);
-		git_refdb_free(dropme);
+	if (refdb) {
+		GIT_REFCOUNT_OWN(refdb, repo);
+		GIT_REFCOUNT_INC(refdb);
+	}
+
+	if ((refdb = git__swap(repo->_refdb, refdb)) != NULL) {
+		GIT_REFCOUNT_OWN(refdb, NULL);
+		git_refdb_free(refdb);
 	}
 }
 
-static void drop_config(git_repository *repo)
+static void set_config(git_repository *repo, git_config *config)
 {
-	git_config *dropme = repo_swap_ptr(repo, &repo->_config, NULL);
-	if (dropme != NULL) {
-		GIT_REFCOUNT_OWN(dropme, NULL);
-		git_config_free(dropme);
-		git_repository__cvar_cache_clear(repo);
+	if (config) {
+		GIT_REFCOUNT_OWN(config, repo);
+		GIT_REFCOUNT_INC(config);
+	}
+
+	if ((config = git__swap(repo->_config, config)) != NULL) {
+		GIT_REFCOUNT_OWN(config, NULL);
+		git_config_free(config);
 	}
+
+	git_repository__cvar_cache_clear(repo);
 }
 
-static void drop_index(git_repository *repo)
+static void set_index(git_repository *repo, git_index *index)
 {
-	git_index *dropme = repo_swap_ptr(repo, &repo->_index, NULL);
-	if (dropme != NULL) {
-		GIT_REFCOUNT_OWN(dropme, NULL);
-		git_index_free(dropme);
+	if (index) {
+		GIT_REFCOUNT_OWN(index, repo);
+		GIT_REFCOUNT_INC(index);
+	}
+
+	if ((index = git__swap(repo->_index, index)) != NULL) {
+		GIT_REFCOUNT_OWN(index, NULL);
+		git_index_free(index);
 	}
 }
 
@@ -81,15 +95,14 @@ void git_repository_free(git_repository *repo)
 	git_attr_cache_flush(repo);
 	git_submodule_config_free(repo);
 
-	drop_config(repo);
-	drop_index(repo);
-	drop_odb(repo);
-	drop_refdb(repo);
+	set_config(repo, NULL);
+	set_index(repo, NULL);
+	set_odb(repo, NULL);
+	set_refdb(repo, NULL);
 
 	git__free(repo->path_repository);
 	git__free(repo->workdir);
 
-	git_mutex_free(&repo->lock);
 	git__free(repo);
 }
 
@@ -122,8 +135,6 @@ static git_repository *repository_alloc(void)
 
 	memset(repo, 0x0, sizeof(git_repository));
 
-	git_mutex_init(&repo->lock);
-
 	if (git_cache_init(&repo->objects) < 0) {
 		git__free(repo);
 		return NULL;
@@ -581,7 +592,7 @@ int git_repository_config__weakptr(git_config **out, git_repository *repo)
 		if (!error) {
 			GIT_REFCOUNT_OWN(config, repo);
 
-			config = repo_swap_ptr(repo, &repo->_config, config);
+			config = git__compare_and_swap(&repo->_config, NULL, config);
 			if (config != NULL) {
 				GIT_REFCOUNT_OWN(config, NULL);
 				git_config_free(config);
@@ -609,17 +620,7 @@ int git_repository_config(git_config **out, git_repository *repo)
 void git_repository_set_config(git_repository *repo, git_config *config)
 {
 	assert(repo && config);
-
-	GIT_REFCOUNT_OWN(config, repo);
-	GIT_REFCOUNT_INC(config);
-
-	config = repo_swap_ptr(repo, &repo->_config, config);
-	if (config != NULL) {
-		GIT_REFCOUNT_OWN(config, NULL);
-		git_config_free(config);
-	}
-
-	git_repository__cvar_cache_clear(repo);
+	set_config(repo, config);
 }
 
 int git_repository_odb__weakptr(git_odb **out, git_repository *repo)
@@ -638,7 +639,7 @@ int git_repository_odb__weakptr(git_odb **out, git_repository *repo)
 		if (!error) {
 			GIT_REFCOUNT_OWN(odb, repo);
 
-			odb = repo_swap_ptr(repo, &repo->_odb, odb);
+			odb = git__compare_and_swap(&repo->_odb, NULL, odb);
 			if (odb != NULL) {
 				GIT_REFCOUNT_OWN(odb, NULL);
 				git_odb_free(odb);
@@ -664,15 +665,7 @@ int git_repository_odb(git_odb **out, git_repository *repo)
 void git_repository_set_odb(git_repository *repo, git_odb *odb)
 {
 	assert(repo && odb);
-
-	GIT_REFCOUNT_OWN(odb, repo);
-	GIT_REFCOUNT_INC(odb);
-
-	odb = repo_swap_ptr(repo, &repo->_odb, odb);
-	if (odb != NULL) {
-		GIT_REFCOUNT_OWN(odb, NULL);
-		git_odb_free(odb);
-	}
+	set_odb(repo, odb);
 }
 
 int git_repository_refdb__weakptr(git_refdb **out, git_repository *repo)
@@ -688,7 +681,7 @@ int git_repository_refdb__weakptr(git_refdb **out, git_repository *repo)
 		if (!error) {
 			GIT_REFCOUNT_OWN(refdb, repo);
 
-			refdb = repo_swap_ptr(repo, &repo->_refdb, refdb);
+			refdb = git__compare_and_swap(&repo->_refdb, NULL, refdb);
 			if (refdb != NULL) {
 				GIT_REFCOUNT_OWN(refdb, NULL);
 				git_refdb_free(refdb);
@@ -712,15 +705,7 @@ int git_repository_refdb(git_refdb **out, git_repository *repo)
 void git_repository_set_refdb(git_repository *repo, git_refdb *refdb)
 {
 	assert(repo && refdb);
-
-	GIT_REFCOUNT_OWN(refdb, repo);
-	GIT_REFCOUNT_INC(refdb);
-
-	refdb = repo_swap_ptr(repo, &repo->_refdb, refdb);
-	if (refdb != NULL) {
-		GIT_REFCOUNT_OWN(refdb, NULL);
-		git_refdb_free(refdb);
-	}
+	set_refdb(repo, refdb);
 }
 
 int git_repository_index__weakptr(git_index **out, git_repository *repo)
@@ -739,7 +724,7 @@ int git_repository_index__weakptr(git_index **out, git_repository *repo)
 		if (!error) {
 			GIT_REFCOUNT_OWN(index, repo);
 
-			index = repo_swap_ptr(repo, &repo->_index, index);
+			index = git__compare_and_swap(&repo->_index, NULL, index);
 			if (index != NULL) {
 				GIT_REFCOUNT_OWN(index, NULL);
 				git_index_free(index);
@@ -767,15 +752,7 @@ int git_repository_index(git_index **out, git_repository *repo)
 void git_repository_set_index(git_repository *repo, git_index *index)
 {
 	assert(repo && index);
-
-	GIT_REFCOUNT_OWN(index, repo);
-	GIT_REFCOUNT_INC(index);
-
-	index = repo_swap_ptr(repo, &repo->_index, index);
-	if (index != NULL) {
-		GIT_REFCOUNT_OWN(index, NULL);
-		git_index_free(index);
-	}
+	set_index(repo, index);
 }
 
 static int check_repositoryformatversion(git_config *config)
@@ -1465,14 +1442,13 @@ static int at_least_one_cb(const char *refname, void *payload)
 
 static int repo_contains_no_reference(git_repository *repo)
 {
-	int error;
-	
-	error = git_reference_foreach(repo, GIT_REF_LISTALL, at_least_one_cb, NULL);
+	int error = git_reference_foreach(repo, GIT_REF_LISTALL, at_least_one_cb, NULL);
 
 	if (error == GIT_EUSER)
 		return 0;
-
-	return error == 0 ? 1 : error;
+	if (!error)
+		return 1;
+	return error;
 }
 
 int git_repository_is_empty(git_repository *repo)

src/repository.h

@@ -83,7 +83,6 @@ struct git_repository {
 	git_refdb *_refdb;
 	git_config *_config;
 	git_index *_index;
-	git_mutex lock;
 
 	git_cache objects;
 	git_attr_cache attrcache;

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__ */

src/util.h

@@ -313,23 +313,4 @@ int git__date_parse(git_time_t *out, const char *date);
  */
 extern size_t git__unescape(char *str);
 
-/*
- * Swap a pointer with thread safety, returning old value.
- */
-GIT_INLINE(void *) git__swap(git_mutex *lock, void **ptr_ptr, void *new_ptr)
-{
-	void *old_ptr;
-
-	if (*ptr_ptr == new_ptr)
-		return NULL;
-	if (git_mutex_lock(lock) < 0)
-		return new_ptr;
-
-	old_ptr = *ptr_ptr;
-	*ptr_ptr = new_ptr;
-
-	git_mutex_unlock(lock);
-	return old_ptr;
-}
-
 #endif /* INCLUDE_util_h__ */