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

1 files changed, 3 insertions, 3 deletions

diff --git a/src/odb.c b/src/odb.c
index 9d9530250..129a63255 100644
--- a/src/odb.c
+++ b/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) {