diff --git a/ChangeLog b/ChangeLog
index 7fa69fe646..5ba31484a2 100644
--- a/ChangeLog
+++ b/ChangeLog
@@ -1,3 +1,8 @@
+Wed Mar 30 06:11:36 2016  NARUSE, Yui  <naruse@ruby-lang.org>
+
+	* thread_pthread.c (reserve_stack): fix reserving position where
+	  the stack growing bottom to top. [Bug #12118]
+
 Wed Mar 30 06:04:18 2016  SHIBATA Hiroshi  <hsbt@ruby-lang.org>
 
 	* .travis.yml: removed commented-out code.
diff --git a/thread_pthread.c b/thread_pthread.c
index d3162993b8..224305819e 100644
--- a/thread_pthread.c
+++ b/thread_pthread.c
@@ -680,17 +680,31 @@ reserve_stack(volatile char *limit, size_t size)
 	const volatile char *end = buf + sizeof(buf);
 	limit += size;
 	if (limit > end) {
-	    size = limit - end;
-	    limit = alloca(size);
-	    limit[stack_check_margin+size-1] = 0;
+	    /* |<-bottom (=limit(a))                                     top->|
+	     * | .. |<-buf 256B |<-end                          | stack check |
+	     * |  256B  |              =size=                   | margin (4KB)|
+	     * |              =size=         limit(b)->|  256B  |             |
+	     * |                |       alloca(sz)     |        |             |
+	     * | .. |<-buf      |<-limit(c)    [sz-1]->0>       |             |
+	     */
+	    size_t sz = limit - end;
+	    limit = alloca(sz);
+	    limit[sz-1] = 0;
 	}
     }
     else {
 	limit -= size;
 	if (buf > limit) {
-	    limit = alloca(buf - limit);
-	    limit[0] = 0; /* ensure alloca is called */
-	    limit -= stack_check_margin;
+	    /* |<-top (=limit(a))                                     bottom->|
+	     * | .. | 256B buf->|                               | stack check |
+	     * |  256B  |              =size=                   | margin (4KB)|
+	     * |              =size=         limit(b)->|  256B  |             |
+	     * |                |       alloca(sz)     |        |             |
+	     * | .. |      buf->|           limit(c)-><0>       |             |
+	     */
+	    size_t sz = buf - limit;
+	    limit = alloca(sz);
+	    limit[0] = 0;
 	}
     }
 }
diff --git a/version.h b/version.h
index 6641beb2c6..219d09281a 100644
--- a/version.h
+++ b/version.h
@@ -1,6 +1,6 @@
 #define RUBY_VERSION "2.2.5"
 #define RUBY_RELEASE_DATE "2016-03-30"
-#define RUBY_PATCHLEVEL 288
+#define RUBY_PATCHLEVEL 289
 
 #define RUBY_RELEASE_YEAR 2016
 #define RUBY_RELEASE_MONTH 3