Fix implementation of atomic update max and move to a central location

While unlikely to have had any serious effects because of limited use, the
previous implementation was not actually atomic due to a data race and
incorrectly coded CAS loop. We also had duplicates of this code in a few
places, it's now been moved to a single location with all other atomic
operations.

2 files changed, 12 insertions, 0 deletions

diff --git a/intern/atomic/atomic_ops.h b/intern/atomic/atomic_ops.h
index 1e9528f9ed9..72813c39ac2 100644
--- a/intern/atomic/atomic_ops.h
+++ b/intern/atomic/atomic_ops.h
@@ -100,6 +100,7 @@ ATOMIC_INLINE size_t atomic_sub_and_fetch_z(size_t *p, size_t x);
 ATOMIC_INLINE size_t atomic_fetch_and_add_z(size_t *p, size_t x);
 ATOMIC_INLINE size_t atomic_fetch_and_sub_z(size_t *p, size_t x);
 ATOMIC_INLINE size_t atomic_cas_z(size_t *v, size_t old, size_t _new);
+ATOMIC_INLINE size_t atomic_fetch_and_update_max_z(size_t *p, size_t x); /* Uses CAS loop, see warning below. */
 
 ATOMIC_INLINE unsigned int atomic_add_and_fetch_u(unsigned int *p, unsigned int x);
 ATOMIC_INLINE unsigned int atomic_sub_and_fetch_u(unsigned int *p, unsigned int x);
diff --git a/intern/atomic/intern/atomic_ops_ext.h b/intern/atomic/intern/atomic_ops_ext.h
index b72c94563fc..8d5f2e5dad7 100644
--- a/intern/atomic/intern/atomic_ops_ext.h
+++ b/intern/atomic/intern/atomic_ops_ext.h
@@ -111,6 +111,17 @@ ATOMIC_INLINE size_t atomic_cas_z(size_t *v, size_t old, size_t _new)
 #endif
 }
 
+ATOMIC_INLINE size_t atomic_fetch_and_update_max_z(size_t *p, size_t x)
+{
+	size_t prev_value;
+	while((prev_value = *p) < x) {
+		if(atomic_cas_z(p, prev_value, x) == prev_value) {
+			break;
+		}
+	}
+	return prev_value;
+}
+
 /******************************************************************************/
 /* unsigned operations. */
 ATOMIC_INLINE unsigned int atomic_add_and_fetch_u(unsigned int *p, unsigned int x)