simplify altstack and enable reuse with thread cache

Instead of allocating and registering the altstack in different
places, do it together to reduce code and improve readability.
When thread cache is enabled, storing altstack in rb_thread_t
is wasteful and we may reuse altstack in the same pthread.

This also lets us clearly allow use of xmalloc to allow GC to
recover from ENOMEM.

[ruby-core:85621] [Feature #14487]

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@63213 b2dd03c8-39d4-4d8f-98ff-823fe69b080e

gc.c

@@ -2403,16 +2403,6 @@ Init_heap(void)
     heap_add_pages(objspace, heap_eden, gc_params.heap_init_slots / HEAP_PAGE_OBJ_LIMIT);
     init_mark_stack(&objspace->mark_stack);
 
-#ifdef USE_SIGALTSTACK
-    {
-	/* altstack of another threads are allocated in another place */
-	rb_thread_t *th = GET_THREAD();
-	void *tmp = th->altstack;
-	th->altstack = malloc(rb_sigaltstack_size());
-	free(tmp); /* free previously allocated area */
-    }
-#endif
-
     objspace->profile.invoke_time = getrusage_time();
     finalizer_table = st_init_numtable();
 }

internal.h

@@ -1704,7 +1704,6 @@ int rb_match_nth_defined(int nth, VALUE match);
 /* signal.c */
 extern int ruby_enable_coredump;
 int rb_get_next_signal(void);
-int rb_sigaltstack_size(void);
 
 /* st.c */
 extern void rb_hash_bulk_insert(long, const VALUE *, VALUE);

signal.c

@@ -554,7 +554,7 @@ typedef RETSIGTYPE ruby_sigaction_t(int);
 #endif
 
 #ifdef USE_SIGALTSTACK
-int
+static int
 rb_sigaltstack_size(void)
 {
     /* XXX: BSD_vfprintf() uses >1500KiB stack and x86-64 need >5KiB stack. */
@@ -577,19 +577,18 @@ rb_sigaltstack_size(void)
 }
 
 /* alternate stack for SIGSEGV */
-void
-rb_register_sigaltstack(rb_thread_t *th)
+void *
+rb_register_sigaltstack(void)
 {
     stack_t newSS, oldSS;
 
-    if (!th->altstack)
-	rb_bug("rb_register_sigaltstack: th->altstack not initialized\n");
-
-    newSS.ss_sp = th->altstack;
     newSS.ss_size = rb_sigaltstack_size();
+    newSS.ss_sp = xmalloc(newSS.ss_size);
     newSS.ss_flags = 0;
 
     sigaltstack(&newSS, &oldSS); /* ignore error. */
+
+    return newSS.ss_sp;
 }
 #endif /* USE_SIGALTSTACK */
 
@@ -1532,9 +1531,7 @@ Init_signal(void)
 	install_sighandler(SIGILL, (sighandler_t)sigill);
 #endif
 #ifdef SIGSEGV
-# ifdef USE_SIGALTSTACK
-	rb_register_sigaltstack(GET_THREAD());
-# endif
+	RB_ALTSTACK_INIT(GET_VM()->main_altstack);
 	install_sighandler(SIGSEGV, (sighandler_t)sigsegv);
 #endif
     }

thread.c

@@ -652,11 +652,6 @@ thread_start_func_2(rb_thread_t *th, VALUE *stack_start, VALUE *register_stack_s
     rb_thread_list_t *join_list;
     rb_thread_t *main_th;
     VALUE errinfo = Qnil;
-# ifdef USE_SIGALTSTACK
-    void rb_register_sigaltstack(rb_thread_t *th);
-
-    rb_register_sigaltstack(th);
-# endif
 
     if (th == th->vm->main_thread)
 	rb_bug("thread_start_func_2 must not be used for main thread");

thread_pthread.c

@@ -840,6 +840,7 @@ static void *
 thread_start_func_1(void *th_ptr)
 {
     rb_thread_t *th = th_ptr;
+    RB_ALTSTACK_INIT(void *altstack);
 #if USE_THREAD_CACHE
   thread_start:
 #endif
@@ -868,6 +869,7 @@ thread_start_func_1(void *th_ptr)
 	}
     }
 #endif
+    RB_ALTSTACK_FREE(altstack);
     return 0;
 }
 

vm.c

@@ -2216,6 +2216,7 @@ ruby_vm_destruct(rb_vm_t *vm)
 	    vm->frozen_strings = 0;
 	}
 	rb_vm_gvl_destroy(vm);
+	RB_ALTSTACK_FREE(vm->main_altstack);
 	if (objspace) {
 	    rb_objspace_free(objspace);
 	}
@@ -2473,9 +2474,6 @@ thread_free(void *ptr)
 	RUBY_GC_INFO("main thread\n");
     }
     else {
-#ifdef USE_SIGALTSTACK
-	free(th->altstack);
-#endif
 	ruby_xfree(ptr);
     }
 
@@ -2535,11 +2533,6 @@ th_init(rb_thread_t *th, VALUE self)
     th->self = self;
     rb_threadptr_root_fiber_setup(th);
 
-    /* allocate thread stack */
-#ifdef USE_SIGALTSTACK
-    /* altstack of main thread is reallocated in another place */
-    th->altstack = malloc(rb_sigaltstack_size());
-#endif
     {
 	/* vm_stack_size is word number.
 	 * th->vm->default_params.thread_vm_stack_size is byte size. */

vm_core.h

@@ -101,7 +101,13 @@
 #endif
 
 #if defined(SIGSEGV) && defined(HAVE_SIGALTSTACK) && defined(SA_SIGINFO) && !defined(__NetBSD__)
-#define USE_SIGALTSTACK
+#  define USE_SIGALTSTACK
+void *rb_register_sigaltstack(void);
+#  define RB_ALTSTACK_INIT(var) var = rb_register_sigaltstack()
+#  define RB_ALTSTACK_FREE(var) xfree(var)
+#else /* noop */
+#  define RB_ALTSTACK_INIT(var)
+#  define RB_ALTSTACK_FREE(var)
 #endif
 
 /*****************/
@@ -538,6 +544,9 @@ typedef struct rb_vm_struct {
 
     struct rb_thread_struct *main_thread;
     struct rb_thread_struct *running_thread;
+#ifdef USE_SIGALTSTACK
+    void *main_altstack;
+#endif
 
     rb_serial_t fork_gen;
     struct list_head waiting_fds; /* <=> struct waiting_fd */
@@ -883,9 +892,6 @@ typedef struct rb_thread_struct {
     /* misc */
     unsigned int abort_on_exception: 1;
     unsigned int report_on_exception: 1;
-#ifdef USE_SIGALTSTACK
-    void *altstack;
-#endif
     uint32_t running_time_us; /* 12500..800000 */
     VALUE name;
 } rb_thread_t;