Fix for Fix (c): broken atomic lock in own bmesh code.

That was a nasty one, Debug build would never have any issue (even tried
with 64 threads!), but Release build would deadlock nearly immediately,
even with only 2 threads!

What happened here (I think) is that gcc optimizer would generate a
specific path endlessly looping when initial value of virtual_lock was
FLT_MAX, by-passing re-assignment from v_no[0] and the atomic cas
completely. Which would have been correct, should v_no[0] not have been
shared (and modified) by multiple threads. ;)

Idea of that (broken) for loop was to avoid completely calling the
atomic cas as long as v_no[0] was locked by some other thread, but...
Guess the avoided/missing memory barrier was the root of the issue here.

Lesson of the evening: Remember kids, do not trust your compiler to
understand all possible threading-related side effects, and be explicit
rather than elegant when using atomic ops!

Side-effect lesson: do check both release and debug builds when messing
with said atomic ops...

1 files changed, 7 insertions, 5 deletions

diff --git a/source/blender/bmesh/intern/bmesh_mesh.c b/source/blender/bmesh/intern/bmesh_mesh.c
index c0cd2e31753..0f97321d9b9 100644
--- a/source/blender/bmesh/intern/bmesh_mesh.c
+++ b/source/blender/bmesh/intern/bmesh_mesh.c
@@ -412,14 +412,16 @@ static void mesh_verts_calc_normals_accum_cb(void *userdata, MempoolIterData *mp
 		 * It also assumes that collisions between threads are highly unlikely,
 		 * else performances would be quite bad here. */
 		float virtual_lock = v_no[0];
-		for (virtual_lock = v_no[0];
-		     (virtual_lock == FLT_MAX) || (atomic_cas_float(&v_no[0], virtual_lock, FLT_MAX) != virtual_lock);
-		     virtual_lock = v_no[0])
-		{
+		while (true) {
 			/* This loops until following conditions are met:
 			 *   - v_no[0] has same value as virtual_lock (i.e. it did not change since last try).
-			 *   - v_no_[0] was not FLT_MAX, i.e. it was not locked by another thread.
+			 *   - v_no[0] was not FLT_MAX, i.e. it was not locked by another thread.
 			 */
+			const float vl = atomic_cas_float(&v_no[0], virtual_lock, FLT_MAX);
+			if (vl == virtual_lock && vl != FLT_MAX) {
+				break;
+			}
+			virtual_lock = vl;
 		}
 		BLI_assert(v_no[0] == FLT_MAX);
 		/* Now we own that normal value, and can change it.