Improve the support of atomics

This change:

* Starts using GCC's and clang's `__atomic_*` intrinsics instead of the
  `__sync_*` ones, since the former supercede the latter (and can be
  safely replaced by their equivalent `__atomic_*` version with the
  sequentially consistent model).
* Makes `git_atomic64`'s value `volatile`. Otherwise, this will make
  ThreadSanitizer complain.
* Adds ways to load the values from atomics. As it turns out,
  unsynchronized read are okay only in some architectures, but if we
  want to be correct (and make ThreadSanitizer happy), those loads
  should also be performed with the atomic builtins.
* Fixes two ThreadSanitizer warnings, as a proof-of-concept that this
  works:
  - Avoid directly accessing `git_refcount`'s `owner` directly, and
    instead makes all callers go through the `GIT_REFCOUNT_*()` macros,
    which also use the atomic utilities.
  - Makes `pool_system_page_size()` race-free.

Part of: #5592

src/cache.c

@@ -168,7 +168,7 @@ static void *cache_store(git_cache *cache, git_cached_obj *entry)
 		return entry;
 
 	/* soften the load on the cache */
-	if (git_cache__current_storage.val > git_cache__max_storage)
+	if (git_atomic_ssize_get(&git_cache__current_storage) > git_cache__max_storage)
 		cache_evict_entries(cache);
 
 	/* not found */

src/odb.c

@@ -44,8 +44,8 @@ typedef struct
 
 static git_cache *odb_cache(git_odb *odb)
 {
-	if (odb->rc.owner != NULL) {
-		git_repository *owner = odb->rc.owner;
+	git_repository *owner = GIT_REFCOUNT_OWNER(odb);
+	if (owner != NULL) {
 		return &owner->objects;
 	}
 
@@ -664,7 +664,7 @@ int git_odb_open(git_odb **out, const char *objects_dir)
 int git_odb__set_caps(git_odb *odb, int caps)
 {
 	if (caps == GIT_ODB_CAP_FROM_OWNER) {
-		git_repository *repo = odb->rc.owner;
+		git_repository *repo = GIT_REFCOUNT_OWNER(odb);
 		int val;
 
 		if (!repo) {

src/pool.c

@@ -23,17 +23,18 @@ static void *pool_alloc_page(git_pool *pool, size_t size);
 
 static size_t pool_system_page_size(void)
 {
-	static size_t size = 0;
+	static git_atomic_ssize cached_size = {0};
+	size_t page_size = 0;
 
-	if (!size) {
-		size_t page_size;
+	if ((page_size = (size_t)git_atomic_ssize_get(&cached_size)) == 0) {
 		if (git__page_size(&page_size) < 0)
 			page_size = 4096;
 		/* allow space for malloc overhead */
-		size = (page_size - (2 * sizeof(void *)) - sizeof(git_pool_page));
+		page_size -= (2 * sizeof(void *)) + sizeof(git_pool_page);
+		git_atomic_ssize_set(&cached_size, (int64_t)page_size);
 	}
 
-	return size;
+	return page_size;
 }
 
 #ifndef GIT_DEBUG_POOL

src/thread-utils.h

@@ -7,12 +7,31 @@
 #ifndef INCLUDE_thread_utils_h__
 #define INCLUDE_thread_utils_h__
 
-#if defined(__GNUC__) && defined(GIT_THREADS)
+#if defined(GIT_THREADS)
+
+#if defined(__clang__)
+
+# if (__clang_major__ < 3 || (__clang_major__ == 3 && __clang_minor__ < 1))
+#  error Atomic primitives do not exist on this version of clang; configure libgit2 with -DTHREADSAFE=OFF
+# else
+#  define GIT_BUILTIN_ATOMIC
+# endif
+
+#elif defined(__GNUC__)
+
 # if (__GNUC__ < 4 || (__GNUC__ == 4 && __GNUC_MINOR__ < 1))
 #  error Atomic primitives do not exist on this version of gcc; configure libgit2 with -DTHREADSAFE=OFF
 # endif
+# if (__GNUC__ > 4 || (__GNUC__ == 4 && __GNUC_MINOR__ >= 7))
+#  define GIT_BUILTIN_ATOMIC
+# else
+#  define GIT_BUILTIN_SYNC
+# endif
+
 #endif
 
+#endif /* GIT_THREADS */
+
 /* Common operations even if threading has been disabled */
 typedef struct {
 #if defined(GIT_WIN32)
@@ -26,21 +45,25 @@ typedef struct {
 
 typedef struct {
 #if defined(GIT_WIN32)
-	__int64 val;
+	volatile __int64 val;
 #else
-	int64_t val;
+	volatile int64_t val;
 #endif
 } git_atomic64;
 
 typedef git_atomic64 git_atomic_ssize;
 
+#define git_atomic_ssize_set git_atomic64_set
 #define git_atomic_ssize_add git_atomic64_add
+#define git_atomic_ssize_get git_atomic64_get
 
 #else
 
 typedef git_atomic git_atomic_ssize;
 
+#define git_atomic_ssize_set git_atomic_set
 #define git_atomic_ssize_add git_atomic_add
+#define git_atomic_ssize_get git_atomic_get
 
 #endif
 
@@ -56,7 +79,9 @@ GIT_INLINE(void) git_atomic_set(git_atomic *a, int val)
 {
 #if defined(GIT_WIN32)
 	InterlockedExchange(&a->val, (LONG)val);
-#elif defined(__GNUC__)
+#elif defined(GIT_BUILTIN_ATOMIC)
+	__atomic_store_n(&a->val, val, __ATOMIC_SEQ_CST);
+#elif defined(GIT_BUILTIN_SYNC)
 	__sync_lock_test_and_set(&a->val, val);
 #else
 #	error "Unsupported architecture for atomic operations"
@@ -67,7 +92,9 @@ GIT_INLINE(int) git_atomic_inc(git_atomic *a)
 {
 #if defined(GIT_WIN32)
 	return InterlockedIncrement(&a->val);
-#elif defined(__GNUC__)
+#elif defined(GIT_BUILTIN_ATOMIC)
+	return __atomic_add_fetch(&a->val, 1, __ATOMIC_SEQ_CST);
+#elif defined(GIT_BUILTIN_SYNC)
 	return __sync_add_and_fetch(&a->val, 1);
 #else
 #	error "Unsupported architecture for atomic operations"
@@ -78,7 +105,9 @@ GIT_INLINE(int) git_atomic_add(git_atomic *a, int32_t addend)
 {
 #if defined(GIT_WIN32)
 	return InterlockedExchangeAdd(&a->val, addend);
-#elif defined(__GNUC__)
+#elif defined(GIT_BUILTIN_ATOMIC)
+	return __atomic_add_fetch(&a->val, addend, __ATOMIC_SEQ_CST);
+#elif defined(GIT_BUILTIN_SYNC)
 	return __sync_add_and_fetch(&a->val, addend);
 #else
 #	error "Unsupported architecture for atomic operations"
@@ -89,25 +118,45 @@ GIT_INLINE(int) git_atomic_dec(git_atomic *a)
 {
 #if defined(GIT_WIN32)
 	return InterlockedDecrement(&a->val);
-#elif defined(__GNUC__)
+#elif defined(GIT_BUILTIN_ATOMIC)
+	return __atomic_sub_fetch(&a->val, 1, __ATOMIC_SEQ_CST);
+#elif defined(GIT_BUILTIN_SYNC)
 	return __sync_sub_and_fetch(&a->val, 1);
 #else
 #	error "Unsupported architecture for atomic operations"
 #endif
 }
 
+GIT_INLINE(int) git_atomic_get(git_atomic *a)
+{
+#if defined(GIT_WIN32)
+	return (int)InterlockedCompareExchange(&a->val, 0, 0);
+#elif defined(GIT_BUILTIN_ATOMIC)
+	return __atomic_load_n(&a->val, __ATOMIC_SEQ_CST);
+#elif defined(GIT_BUILTIN_SYNC)
+	return __sync_val_compare_and_swap(&a->val, 0, 0);
+#else
+#	error "Unsupported architecture for atomic operations"
+#endif
+}
+
 GIT_INLINE(void *) git___compare_and_swap(
 	void * volatile *ptr, void *oldval, void *newval)
 {
-	volatile void *foundval;
 #if defined(GIT_WIN32)
+	volatile void *foundval;
 	foundval = InterlockedCompareExchangePointer((volatile PVOID *)ptr, newval, oldval);
-#elif defined(__GNUC__)
+	return (foundval == oldval) ? oldval : newval;
+#elif defined(GIT_BUILTIN_ATOMIC)
+	bool success = __atomic_compare_exchange(ptr, &oldval, &newval, false, __ATOMIC_SEQ_CST, __ATOMIC_SEQ_CST);
+	return success ? oldval : newval;
+#elif defined(GIT_BUILTIN_SYNC)
+	volatile void *foundval;
 	foundval = __sync_val_compare_and_swap(ptr, oldval, newval);
+	return (foundval == oldval) ? oldval : newval;
 #else
 #	error "Unsupported architecture for atomic operations"
 #endif
-	return (foundval == oldval) ? oldval : newval;
 }
 
 GIT_INLINE(volatile void *) git___swap(
@@ -115,8 +164,30 @@ GIT_INLINE(volatile void *) git___swap(
 {
 #if defined(GIT_WIN32)
 	return InterlockedExchangePointer(ptr, newval);
-#else
+#elif defined(GIT_BUILTIN_ATOMIC)
+	void * volatile foundval;
+	__atomic_exchange(ptr, &newval, &foundval, __ATOMIC_SEQ_CST);
+	return foundval;
+#elif defined(GIT_BUILTIN_SYNC)
 	return __sync_lock_test_and_set(ptr, newval);
+#else
+#	error "Unsupported architecture for atomic operations"
+#endif
+}
+
+GIT_INLINE(volatile void *) git___load(void * volatile *ptr)
+{
+#if defined(GIT_WIN32)
+	void *newval = NULL, *oldval = NULL;
+	volatile void *foundval = NULL;
+	foundval = InterlockedCompareExchangePointer((volatile PVOID *)ptr, newval, oldval);
+	return foundval;
+#elif defined(GIT_BUILTIN_ATOMIC)
+	return (volatile void *)__atomic_load_n(ptr, __ATOMIC_SEQ_CST);
+#elif defined(GIT_BUILTIN_SYNC)
+	return (volatile void *)__sync_val_compare_and_swap(ptr, 0, 0);
+#else
+#	error "Unsupported architecture for atomic operations"
 #endif
 }
 
@@ -126,13 +197,41 @@ GIT_INLINE(int64_t) git_atomic64_add(git_atomic64 *a, int64_t addend)
 {
 #if defined(GIT_WIN32)
 	return InterlockedExchangeAdd64(&a->val, addend);
-#elif defined(__GNUC__)
+#elif defined(GIT_BUILTIN_ATOMIC)
+	return __atomic_add_fetch(&a->val, addend, __ATOMIC_SEQ_CST);
+#elif defined(GIT_BUILTIN_SYNC)
 	return __sync_add_and_fetch(&a->val, addend);
 #else
 #	error "Unsupported architecture for atomic operations"
 #endif
 }
 
+GIT_INLINE(void) git_atomic64_set(git_atomic64 *a, int64_t val)
+{
+#if defined(GIT_WIN32)
+	InterlockedExchange64(&a->val, val);
+#elif defined(GIT_BUILTIN_ATOMIC)
+	__atomic_store_n(&a->val, val, __ATOMIC_SEQ_CST);
+#elif defined(GIT_BUILTIN_SYNC)
+	__sync_lock_test_and_set(&a->val, val);
+#else
+#	error "Unsupported architecture for atomic operations"
+#endif
+}
+
+GIT_INLINE(int64_t) git_atomic64_get(git_atomic64 *a)
+{
+#if defined(GIT_WIN32)
+	return (int64_t)InterlockedCompareExchange64(&a->val, 0, 0);
+#elif defined(GIT_BUILTIN_ATOMIC)
+	return __atomic_load_n(&a->val, __ATOMIC_SEQ_CST);
+#elif defined(GIT_BUILTIN_SYNC)
+	return __sync_val_compare_and_swap(&a->val, 0, 0);
+#else
+#	error "Unsupported architecture for atomic operations"
+#endif
+}
+
 #endif
 
 #else
@@ -190,6 +289,11 @@ GIT_INLINE(int) git_atomic_dec(git_atomic *a)
 	return --a->val;
 }
 
+GIT_INLINE(int) git_atomic_get(git_atomic *a)
+{
+	return (int)a->val;
+}
+
 GIT_INLINE(void *) git___compare_and_swap(
 	void * volatile *ptr, void *oldval, void *newval)
 {
@@ -216,15 +320,20 @@ GIT_INLINE(int64_t) git_atomic64_add(git_atomic64 *a, int64_t addend)
 	return a->val;
 }
 
-#endif
-
-#endif
+GIT_INLINE(void) git_atomic64_set(git_atomic64 *a, int64_t val)
+{
+	a->val = val;
+}
 
-GIT_INLINE(int) git_atomic_get(git_atomic *a)
+GIT_INLINE(int64_t) git_atomic64_get(git_atomic64 *a)
 {
-	return (int)a->val;
+	return (int64_t)a->val;
 }
 
+#endif
+
+#endif
+
 /* Atomically replace oldval with newval
  * @return oldval if it was replaced or newval if it was not
  */
@@ -233,14 +342,24 @@ GIT_INLINE(int) git_atomic_get(git_atomic *a)
 
 #define git__swap(ptr, val) (void *)git___swap((void * volatile *)&ptr, val)
 
+#define git__load(ptr) (void *)git___load((void * volatile *)&ptr)
+
 extern int git_online_cpus(void);
 
-#if defined(GIT_THREADS) && defined(_MSC_VER)
-# define GIT_MEMORY_BARRIER MemoryBarrier()
-#elif defined(GIT_THREADS)
-# define GIT_MEMORY_BARRIER __sync_synchronize()
+#if defined(GIT_THREADS)
+
+# if defined(GIT_WIN32)
+#  define GIT_MEMORY_BARRIER MemoryBarrier()
+# elif defined(GIT_BUILTIN_ATOMIC)
+#  define GIT_MEMORY_BARRIER __atomic_thread_fence(__ATOMIC_SEQ_CST)
+# elif defined(GIT_BUILTIN_SYNC)
+#  define GIT_MEMORY_BARRIER __sync_synchronize()
+# endif
+
 #else
+
 # define GIT_MEMORY_BARRIER /* noop */
+
 #endif
 
 #endif

src/util.h

@@ -186,10 +186,10 @@ typedef void (*git_refcount_freeptr)(void *r);
 }
 
 #define GIT_REFCOUNT_OWN(r, o) { \
-	(r)->rc.owner = o; \
+	(void)git__swap((r)->rc.owner, o); \
 }
 
-#define GIT_REFCOUNT_OWNER(r) ((r)->rc.owner)
+#define GIT_REFCOUNT_OWNER(r) git__load((r)->rc.owner)
 
 #define GIT_REFCOUNT_VAL(r) git_atomic_get((r)->rc.refcount)