nonmoving: Drop redundant write barrier on stack underflow

Previously we would push stack-carried return values to the new stack on
a stack overflow. While the precise reasoning for this barrier is
unfortunately lost to history, in hindsight I suspect it was prompted by
a missing barrier elsewhere (that has been since fixed).

Moreover, there the redundant barrier is actively harmful: the stack may
contain non-pointer values; blindly pushing these to the mark queue will
result in a crash. This is precisely what happened in the `stack003`
test. However, because of a (now fixed) deficiency in the test this
crash did not trigger on amd64.

1 files changed, 0 insertions, 10 deletions

diff --git a/rts/Threads.c b/rts/Threads.c
index 8334c5a..8b05dd7 100644
--- a/rts/Threads.c
+++ b/rts/Threads.c
@@ -723,16 +723,6 @@ threadStackUnderflow (Capability *cap, StgTSO *tso)
             barf("threadStackUnderflow: not enough space for return values");
         }
 
-        IF_NONMOVING_WRITE_BARRIER_ENABLED {
-            // ensure that values that we copy into the new stack are marked
-            // for the nonmoving collector. Note that these values won't
-            // necessarily form a full closure so we need to handle them
-            // specially.
-            for (unsigned int i = 0; i < retvals; i++) {
-                updateRemembSetPushClosure(cap, (StgClosure *) old_stack->sp[i]);
-            }
-        }
-
         memcpy(/* dest */ new_stack->sp - retvals,
                /* src  */ old_stack->sp,
                /* size */ retvals * sizeof(W_));