diff --git a/internal.h b/internal.h
index e9f8cb0..e4f15bf 100644
--- a/internal.h
+++ b/internal.h
@@ -442,6 +442,7 @@ VALUE rb_id_quote_unprintable(ID);
 #define QUOTE(str) rb_str_quote_unprintable(str)
 #define QUOTE_ID(id) rb_id_quote_unprintable(id)
 void rb_str_fill_terminator(VALUE str, const int termlen);
+VALUE rb_str_locktmp_ensure(VALUE str, VALUE (*func)(VALUE), VALUE arg);
 
 /* struct.c */
 VALUE rb_struct_init_copy(VALUE copy, VALUE s);
diff --git a/io.c b/io.c
index 6038609..7c9da71 100644
--- a/io.c
+++ b/io.c
@@ -2366,12 +2366,27 @@ rb_io_set_nonblock(rb_io_t *fptr)
 void
 rb_readwrite_sys_fail(int writable, const char *mesg);
 
+struct read_internal_arg {
+    int fd;
+    char *str_ptr;
+    long len;
+};
+
+static VALUE
+read_internal_call(VALUE arg)
+{
+    struct read_internal_arg *p = (struct read_internal_arg *)arg;
+    p->len = rb_read_internal(p->fd, p->str_ptr, p->len);
+    return Qundef;
+}
+
 static VALUE
 io_getpartial(int argc, VALUE *argv, VALUE io, int nonblock)
 {
     rb_io_t *fptr;
     VALUE length, str;
     long n, len;
+    struct read_internal_arg arg;
 
     rb_scan_args(argc, argv, "11", &length, &str);
 
@@ -2397,9 +2412,11 @@ io_getpartial(int argc, VALUE *argv, VALUE io, int nonblock)
             rb_io_set_nonblock(fptr);
         }
 	io_setstrbuf(&str, len);
-	rb_str_locktmp(str);
-	n = rb_read_internal(fptr->fd, RSTRING_PTR(str), len);
-	rb_str_unlocktmp(str);
+	arg.fd = fptr->fd;
+	arg.str_ptr = RSTRING_PTR(str);
+	arg.len = len;
+	rb_str_locktmp_ensure(str, read_internal_call, (VALUE)&arg);
+	n = arg.len;
         if (n < 0) {
             if (!nonblock && rb_io_wait_readable(fptr->fd))
                 goto again;
@@ -4511,6 +4528,7 @@ rb_io_sysread(int argc, VALUE *argv, VALUE io)
     VALUE len, str;
     rb_io_t *fptr;
     long n, ilen;
+    struct read_internal_arg arg;
 
     rb_scan_args(argc, argv, "11", &len, &str);
     ilen = NUM2LONG(len);
@@ -4540,8 +4558,11 @@ rb_io_sysread(int argc, VALUE *argv, VALUE io)
 
     io_setstrbuf(&str, ilen);
     rb_str_locktmp(str);
-    n = rb_read_internal(fptr->fd, RSTRING_PTR(str), ilen);
-    rb_str_unlocktmp(str);
+    arg.fd = fptr->fd;
+    arg.str_ptr = RSTRING_PTR(str);
+    arg.len = ilen;
+    rb_ensure(read_internal_call, (VALUE)&arg, rb_str_unlocktmp, str);
+    n = arg.len;
 
     if (n == -1) {
 	rb_sys_fail_path(fptr->pathv);
diff --git a/string.c b/string.c
index f101a94..e7f9b44 100644
--- a/string.c
+++ b/string.c
@@ -1871,6 +1871,13 @@ rb_str_unlocktmp(VALUE str)
     return str;
 }
 
+VALUE
+rb_str_locktmp_ensure(VALUE str, VALUE (*func)(VALUE), VALUE arg)
+{
+    rb_str_locktmp(str);
+    return rb_ensure(func, arg, rb_str_unlocktmp, str);
+}
+
 void
 rb_str_set_len(VALUE str, long len)
 {