simple dynamic race detector for jl_array_to_string (#173)

src/array.c

@@ -457,13 +457,43 @@ JL_DLLEXPORT jl_array_t *jl_pchar_to_array(const char *str, size_t len)
     return a;
 }
 
+uv_mutex_t array_to_string_print_lock;
+
+void jl_set_in_flight_bit_for_array_to_string(jl_array_t *a)
+{
+    uintptr_t msk = (1 << ARRAY_TO_STRING_IN_FLIGHT_BIT_OFFSET);
+    uintptr_t header = jl_atomic_fetch_or((_Atomic(uintptr_t) *)jl_astaggedvalue(a), msk);
+    if (header & msk) {
+        uv_mutex_lock(&array_to_string_print_lock);
+        // Race detected... Someone already set the in-flight bit.
+        jl_safe_printf("Race detected... Someone already set the in-flight bit.\n");
+        jlbacktracet(jl_current_task);
+        uv_mutex_unlock(&array_to_string_print_lock);
+    }
+}
+
+void jl_reset_in_flight_bit_for_array_to_string(jl_array_t *a)
+{
+    uintptr_t msk = (1 << ARRAY_TO_STRING_IN_FLIGHT_BIT_OFFSET);
+    uintptr_t header = jl_atomic_fetch_and((_Atomic(uintptr_t) *)jl_astaggedvalue(a), ~msk);
+    if (!(header & msk)) {
+        uv_mutex_lock(&array_to_string_print_lock);
+        // Race detected... Someone reset the in-flight bit before we could.
+        jl_safe_printf("Race detected... Someone reset the in-flight bit before we could.\n");
+        jlbacktracet(jl_current_task);
+        uv_mutex_unlock(&array_to_string_print_lock);
+    }
+}
+
 JL_DLLEXPORT jl_value_t *jl_array_to_string(jl_array_t *a)
 {
+    jl_set_in_flight_bit_for_array_to_string(a);
     size_t len = jl_array_len(a);
     if (len == 0) {
         // this may seem like purely an optimization (which it also is), but it
         // also ensures that calling `String(a)` doesn't corrupt a previous
         // string also created the same way, where `a = StringVector(_)`.
+        jl_reset_in_flight_bit_for_array_to_string(a);
         return jl_an_empty_string;
     }
     if (a->flags.how == 3 && a->offset == 0 && a->elsize == 1 &&
@@ -476,11 +506,13 @@ JL_DLLEXPORT jl_value_t *jl_array_to_string(jl_array_t *a)
             a->nrows = 0;
             a->length = 0;
             a->maxsize = 0;
+            jl_reset_in_flight_bit_for_array_to_string(a);
             return o;
         }
     }
     a->nrows = 0;
     a->length = 0;
+    jl_reset_in_flight_bit_for_array_to_string(a);
     return jl_pchar_to_string((const char*)jl_array_data(a), len);
 }
 

src/init.c

@@ -855,6 +855,8 @@ JL_DLLEXPORT void julia_init(JL_IMAGE_SEARCH rel)
 
 void jl_init_heartbeat(void);
 
+extern uv_mutex_t array_to_string_print_lock;
+
 static NOINLINE void _finish_julia_init(JL_IMAGE_SEARCH rel, jl_ptls_t ptls, jl_task_t *ct)
 {
     JL_TIMING(JULIA_INIT, JULIA_INIT);
@@ -902,6 +904,8 @@ static NOINLINE void _finish_julia_init(JL_IMAGE_SEARCH rel, jl_ptls_t ptls, jl_
     jl_start_gc_threads();
     uv_barrier_wait(&thread_init_done);
 
+    uv_mutex_init(&array_to_string_print_lock);
+
     jl_init_heartbeat();
 
     jl_gc_enable(1);

src/julia.h

@@ -95,7 +95,11 @@ typedef struct _jl_value_t jl_value_t;
 struct _jl_taggedvalue_bits {
     uintptr_t gc:2;
     uintptr_t in_image:1;
-    uintptr_t unused:1;
+    // Bit to indicate whether a call to `jl_array_to_string` is in-flight
+    // Mostly used to implement a poor-man's dynamic race detector.
+    // See usage in `jl_array_to_string`.
+#define ARRAY_TO_STRING_IN_FLIGHT_BIT_OFFSET (3)
+    uintptr_t array_to_string_in_flight:1;
 #ifdef _P64
     uintptr_t tag:60;
 #else
@@ -2228,6 +2232,7 @@ JL_DLLEXPORT size_t jl_static_show(JL_STREAM *out, jl_value_t *v) JL_NOTSAFEPOIN
 JL_DLLEXPORT size_t jl_static_show_func_sig(JL_STREAM *s, jl_value_t *type) JL_NOTSAFEPOINT;
 JL_DLLEXPORT void jl_print_backtrace(void) JL_NOTSAFEPOINT;
 JL_DLLEXPORT void jlbacktrace(void) JL_NOTSAFEPOINT; // deprecated
+JL_DLLEXPORT void jlbacktracet(jl_task_t *t) JL_NOTSAFEPOINT;
 // Mainly for debugging, use `void*` so that no type cast is needed in C++.
 JL_DLLEXPORT void jl_(void *jl_value) JL_NOTSAFEPOINT;