Removed usage of Ruby mutexes

In rare circumstances locking/unlocking a Mutex may lead to an object
allocation which in turn crashes CRuby when this happens during a GC
run. Since the GIL already prevents multiple Ruby threads from running
in parallel we should be able to safely remove the usage of explicit
mutexes without breaking things.

ext/liballocations/liballocations.c

@@ -2,8 +2,6 @@
 st_table *object_counts;
-VALUE mutex;
-
 VALUE allocation_tracer;
 VALUE free_tracer;
@@ -25,12 +23,8 @@ void newobj_callback(VALUE tracepoint, void* data) {
         return;
     }
-    rb_mutex_lock(mutex);
-
     st_lookup(object_counts, (st_data_t) klass, &count);
     st_insert(object_counts, (st_data_t) klass, count + 1);
-
-    rb_mutex_unlock(mutex);
 }
 /**
@@ -47,8 +41,6 @@ void freeobj_callback(VALUE tracepoint, void* data) {
     VALUE obj   = rb_tracearg_object(trace_arg);
     VALUE klass = RBASIC_CLASS(obj);
-    rb_mutex_lock(mutex);
-
     if ( st_lookup(object_counts, (st_data_t) klass, &count) ) {
         if ( count > 0 && (count - 1) > 0) {
             st_insert(object_counts, (st_data_t) klass, count - 1);
@@ -58,8 +50,6 @@ void freeobj_callback(VALUE tracepoint, void* data) {
             st_delete(object_counts, (st_data_t*) &klass, NULL);
         }
     }
-
-    rb_mutex_unlock(mutex);
 }
 /**
@@ -84,18 +74,12 @@ VALUE allocations_to_hash(VALUE self) {
     st_table *local_counts;
     VALUE hash;
-    rb_mutex_lock(mutex);
-
     if ( !object_counts ) {
-        rb_mutex_unlock(mutex);
-
         return rb_hash_new();
     }
     local_counts = st_copy(object_counts);
-    rb_mutex_unlock(mutex);
-
     hash = rb_hash_new();
     st_foreach(local_counts, each_count, (st_data_t) hash);
@@ -112,11 +96,7 @@ VALUE allocations_to_hash(VALUE self) {
  *     Allocations.start -> nil
  */
 VALUE allocations_start(VALUE self) {
-    rb_mutex_lock(mutex);
-
     if ( rb_ivar_get(self, id_enabled) == Qtrue ) {
-        rb_mutex_unlock(mutex);
-
         return Qnil;
     }
@@ -124,8 +104,6 @@ VALUE allocations_start(VALUE self) {
     rb_ivar_set(self, id_enabled, Qtrue);
-    rb_mutex_unlock(mutex);
-
     rb_tracepoint_enable(allocation_tracer);
     rb_tracepoint_enable(free_tracer);
@@ -139,11 +117,7 @@ VALUE allocations_start(VALUE self) {
  *     Allocations.stop -> nil
  */
 VALUE allocations_stop(VALUE self) {
-    rb_mutex_lock(mutex);
-
     if ( rb_ivar_get(self, id_enabled) != Qtrue ) {
-        rb_mutex_unlock(mutex);
-
         return Qnil;
     }
@@ -158,8 +132,6 @@ VALUE allocations_stop(VALUE self) {
     rb_ivar_set(self, id_enabled, Qfalse);
-    rb_mutex_unlock(mutex);
-
     return Qnil;
 }
@@ -172,14 +144,10 @@ VALUE allocations_stop(VALUE self) {
 VALUE allocations_enabled_p(VALUE self) {
     VALUE enabled = Qfalse;
-    rb_mutex_lock(mutex);
-
     if ( rb_ivar_get(self, id_enabled) == Qtrue ) {
         enabled = Qtrue;
     }
-    rb_mutex_unlock(mutex);
-
     return enabled;
 }
@@ -192,8 +160,6 @@ void Init_liballocations() {
     free_tracer = rb_tracepoint_new(Qnil, RUBY_INTERNAL_EVENT_FREEOBJ,
                                     freeobj_callback, NULL);
-    mutex = rb_mutex_new();
-
     id_enabled = rb_intern("enabled");
     rb_define_singleton_method(mAllocations, "to_hash", allocations_to_hash, 0);
@@ -201,7 +167,6 @@ void Init_liballocations() {
     rb_define_singleton_method(mAllocations, "stop", allocations_stop, 0);
     rb_define_singleton_method(mAllocations, "enabled?", allocations_enabled_p, 0);
-    rb_define_const(mAllocations, "MUTEX", mutex);
     rb_define_const(mAllocations, "ALLOCATION_TRACER", allocation_tracer);
     rb_define_const(mAllocations, "FREE_TRACER", free_tracer);
 }