erts: Remove erts_staging_bp_index

It should always be erts_active_bp_index ^ 1.
Avoid tiny race when we had to change two atomics.
We only call erts_staging_bp_ix() when we change trace settings.

Author: sverker

erts/emulator/beam/beam_bp.c

@@ -175,7 +175,6 @@ static void bp_hash_dealloc(bp_trace_hash_t *hash);
 void 
 erts_bp_init(void) {
     erts_atomic32_init_nob(&erts_active_bp_index, 0);
-    erts_atomic32_init_nob(&erts_staging_bp_index, 1);
     erts_mtx_init(&erts_dirty_bp_ix_mtx, "dirty_break_point_index", NIL,
         ERTS_LOCK_FLAGS_PROPERTY_STATIC | ERTS_LOCK_FLAGS_CATEGORY_DEBUG);
 }
@@ -510,11 +509,10 @@ consolidate_bp_data_session(GenericBp* g)
 void
 erts_commit_staged_bp(void)
 {
-    ErtsBpIndex staging = erts_staging_bp_ix();
-    ErtsBpIndex active = erts_active_bp_ix();
+    const ErtsBpIndex new_active = erts_active_bp_ix() ^ 1;
 
-    erts_atomic32_set_nob(&erts_active_bp_index, staging);
-    erts_atomic32_set_nob(&erts_staging_bp_index, active);
+    ASSERT(new_active < 2);
+    erts_atomic32_set_nob(&erts_active_bp_index, new_active);
 }
 
 void

erts/emulator/beam/beam_bp.h

@@ -217,7 +217,7 @@ ERTS_GLB_INLINE ErtsBpIndex erts_active_bp_ix(void)
 
 ERTS_GLB_INLINE ErtsBpIndex erts_staging_bp_ix(void)
 {
-    return erts_atomic32_read_nob(&erts_staging_bp_index);
+    return erts_atomic32_read_nob(&erts_active_bp_index) ^ 1;
 }
 
 ERTS_GLB_INLINE

erts/emulator/beam/erl_trace.c

@@ -64,7 +64,6 @@
 #endif
 
 erts_atomic32_t erts_active_bp_index;
-erts_atomic32_t erts_staging_bp_index;
 
 /* Pseudo export entries. Never filled in with data, only used to
    yield unique pointers of the correct type. */

erts/emulator/beam/erl_trace.h

@@ -67,7 +67,6 @@ typedef struct
 #define ERTS_NUM_BP_IX 2
 typedef Uint32 ErtsBpIndex;
 extern erts_atomic32_t erts_active_bp_index;
-extern erts_atomic32_t erts_staging_bp_index;
 
 struct trace_pattern_flags {
     unsigned int breakpoint : 1; /* Set if any other is set */