libatomic: perform write before context switch

Atomic actions block a thread at the "switch_to_master" function call, so under
the current structure (which isn't quite fit for relaxed modeling yet...) we
should perform the memory write before calling "switch_to_master".

If not, we can observe sequences like the following, where x is an atomic
variable. All actions are seq_cst:

 Initially, x = 0

 Thread    Action
 ------    ------
 1         r1 = x;       // r1 = 0
 1          x = r1 + 1;  //  x = 1, not stored yet?
 2         r2 = x;       // r2 = 0
 2          x = r2       //  x = 1, not stored yet?

Then, depending on scheduling, Thread 1 or Thread 2 might complete first, with
its write being performed *after* it receives control again.

diff --git a/libatomic.cc b/libatomic.cc
index 88d7fbf79900859575b7b6d3bf1b78609caddd3b..507f6853ca503e75664a86a5711f8f03c5cbc10a 100644 (file)
--- a/libatomic.cc
+++ b/libatomic.cc
@@ -5,8 +5,8 @@
 void atomic_store_explicit(struct atomic_object *obj, int value, memory_order order)
 {
        DBG();
-       model->switch_to_master(new ModelAction(ATOMIC_WRITE, order, obj, value));
        obj->value = value;
+       model->switch_to_master(new ModelAction(ATOMIC_WRITE, order, obj, value));
 }
 
 int atomic_load_explicit(struct atomic_object *obj, memory_order order)