Add GC write barrier in jl_reserve_excstack (#51096)

`jl_push_excstack` and `jl_reserve_excstack` take an address to the
field `excstack` in `jl_task_t`, and may update the field. In such a
case, a GC write barrier is needed. This PR adds the missing write
barrier.

Note: I am not sure if this affects the correctness of Julia GC or not.
This could be special cased in the GC code, as the task is always
rooted. However, generally if the task is in the old generation, it does
not have to be scanned even if it is a root. A write barrier is needed
to make sure the GC is aware of the update.

src/julia_internal.h

@@ -1219,7 +1219,7 @@ STATIC_INLINE size_t jl_excstack_next(jl_excstack_t *stack, size_t itr) JL_NOTSA
     return itr-2 - jl_excstack_bt_size(stack, itr);
 }
 // Exception stack manipulation
-void jl_push_excstack(jl_excstack_t **stack JL_REQUIRE_ROOTED_SLOT JL_ROOTING_ARGUMENT,
+void jl_push_excstack(jl_task_t* task, jl_excstack_t **stack JL_REQUIRE_ROOTED_SLOT JL_ROOTING_ARGUMENT,
                       jl_value_t *exception JL_ROOTED_ARGUMENT,
                       jl_bt_element_t *bt_data, size_t bt_size);
 

src/rtutils.c

@@ -325,7 +325,7 @@ static void jl_copy_excstack(jl_excstack_t *dest, jl_excstack_t *src) JL_NOTSAFE
     dest->top = src->top;
 }
 
-static void jl_reserve_excstack(jl_excstack_t **stack JL_REQUIRE_ROOTED_SLOT,
+static void jl_reserve_excstack(jl_task_t* task, jl_excstack_t **stack JL_REQUIRE_ROOTED_SLOT,
                                 size_t reserved_size)
 {
     jl_excstack_t *s = *stack;
@@ -339,13 +339,14 @@ static void jl_reserve_excstack(jl_excstack_t **stack JL_REQUIRE_ROOTED_SLOT,
     if (s)
         jl_copy_excstack(new_s, s);
     *stack = new_s;
+    jl_gc_wb(task, new_s);
 }
 
-void jl_push_excstack(jl_excstack_t **stack JL_REQUIRE_ROOTED_SLOT JL_ROOTING_ARGUMENT,
+void jl_push_excstack(jl_task_t* task, jl_excstack_t **stack JL_REQUIRE_ROOTED_SLOT JL_ROOTING_ARGUMENT,
                       jl_value_t *exception JL_ROOTED_ARGUMENT,
                       jl_bt_element_t *bt_data, size_t bt_size)
 {
-    jl_reserve_excstack(stack, (*stack ? (*stack)->top : 0) + bt_size + 2);
+    jl_reserve_excstack(task, stack, (*stack ? (*stack)->top : 0) + bt_size + 2);
     jl_excstack_t *s = *stack;
     jl_bt_element_t *rawstack = jl_excstack_raw(s);
     memcpy(rawstack + s->top, bt_data, sizeof(jl_bt_element_t)*bt_size);

src/task.c

@@ -721,7 +721,7 @@ JL_DLLEXPORT JL_NORETURN void jl_no_exc_handler(jl_value_t *e, jl_task_t *ct)
         /* The temporary ptls->bt_data is rooted by special purpose code in the\
            GC. This exists only for the purpose of preserving bt_data until we \
            set ptls->bt_size=0 below. */                                       \
-        jl_push_excstack(&ct->excstack, exception,                             \
+        jl_push_excstack(ct, &ct->excstack, exception,                             \
                           ptls->bt_data, ptls->bt_size);                       \
         ptls->bt_size = 0;                                                     \
     }                                                                          \
@@ -1224,7 +1224,7 @@ CFI_NORETURN
     jl_timing_block_task_enter(ct, ptls, NULL);
     if (jl_atomic_load_relaxed(&ct->_isexception)) {
         record_backtrace(ptls, 0);
-        jl_push_excstack(&ct->excstack, ct->result,
+        jl_push_excstack(ct, &ct->excstack, ct->result,
                          ptls->bt_data, ptls->bt_size);
         res = ct->result;
     }