diff --git a/ChangeLog b/ChangeLog
index c25a32cdef..fa30abb4e6 100644
--- a/ChangeLog
+++ b/ChangeLog
@@ -1,3 +1,15 @@
+Sun Sep  9 22:02:50 2012  KOSAKI Motohiro  <kosaki.motohiro@gmail.com>
+
+	* ext/socket/basicsocket.c (rsock_bsock_send):
+	  avoid unnecessary select() calls before doing I/O
+	  Patch by Eric Wong. [Feature #4538] [ruby-core:35586]
+	* ext/socket/init.c (rsock_s_recvfrom): ditto.
+	* ext/socket/init.c (rsock_s_accept): ditto.
+	* ext/socket/udpsocket.c (udp_send): ditto.
+	* io.c (io_fflush): ditto.
+	* io.c (io_binwrite): ditto.
+	* io.c (rb_io_syswrite): ditto.
+
 Mon Sep 10 01:38:51 2012  KOSAKI Motohiro  <kosaki.motohiro@gmail.com>
 
 	* io.c (nogvl_close, maygvl_close, nogvl_fclose, maygvl_fclose):
diff --git a/ext/socket/basicsocket.c b/ext/socket/basicsocket.c
index c1ae88d80c..84ea2bb6bd 100644
--- a/ext/socket/basicsocket.c
+++ b/ext/socket/basicsocket.c
@@ -558,8 +558,7 @@ rsock_bsock_send(int argc, VALUE *argv, VALUE sock)
     GetOpenFile(sock, fptr);
     arg.fd = fptr->fd;
     arg.flags = NUM2INT(flags);
-    while (rb_thread_fd_writable(arg.fd),
-	   (n = (int)BLOCKING_REGION_FD(func, &arg)) < 0) {
+    while ((n = (int)BLOCKING_REGION_FD(func, &arg)) < 0) {
 	if (rb_io_wait_writable(arg.fd)) {
 	    continue;
 	}
diff --git a/ext/socket/init.c b/ext/socket/init.c
index 696b62d82a..3960a23300 100644
--- a/ext/socket/init.c
+++ b/ext/socket/init.c
@@ -130,7 +130,6 @@ rsock_s_recvfrom(VALUE sock, int argc, VALUE *argv, enum sock_recv_type from)
     RBASIC(str)->klass = 0;
 
     while (rb_io_check_closed(fptr),
-	   rb_thread_wait_fd(arg.fd),
 	   (slen = BLOCKING_REGION_FD(recvfrom_blocking, &arg)) < 0) {
         if (!rb_io_wait_readable(fptr->fd)) {
             rb_sys_fail("recvfrom(2)");
@@ -560,7 +559,6 @@ rsock_s_accept(VALUE klass, int fd, struct sockaddr *sockaddr, socklen_t *len)
     arg.sockaddr = sockaddr;
     arg.len = len;
   retry:
-    rb_thread_wait_fd(fd);
     fd2 = (int)BLOCKING_REGION_FD(accept_blocking, &arg);
     if (fd2 < 0) {
 	switch (errno) {
diff --git a/ext/socket/udpsocket.c b/ext/socket/udpsocket.c
index 0ba4371f1a..1b8756a904 100644
--- a/ext/socket/udpsocket.c
+++ b/ext/socket/udpsocket.c
@@ -176,7 +176,6 @@ udp_send(int argc, VALUE *argv, VALUE sock)
       retry:
 	arg.to = res->ai_addr;
 	arg.tolen = res->ai_addrlen;
-	rb_thread_fd_writable(arg.fd);
 	n = (int)BLOCKING_REGION_FD(rsock_sendto_blocking, &arg);
 	if (n >= 0) {
 	    freeaddrinfo(res0);
diff --git a/io.c b/io.c
index 57f5775c41..24f141c825 100644
--- a/io.c
+++ b/io.c
@@ -975,9 +975,7 @@ io_fflush(rb_io_t *fptr)
     rb_io_check_closed(fptr);
     if (fptr->wbuf.len == 0)
         return 0;
-    if (!rb_thread_fd_writable(fptr->fd)) {
-        rb_io_check_closed(fptr);
-    }
+    rb_io_check_closed(fptr);
     while (fptr->wbuf.len > 0 && io_flush_buffer(fptr) != 0) {
 	if (!rb_io_wait_writable(fptr->fd))
 	    return -1;
@@ -1132,7 +1130,12 @@ io_binwrite(VALUE str, const char *ptr, long len, rb_io_t *fptr, int nosync)
         (fptr->wbuf.ptr && fptr->wbuf.capa <= fptr->wbuf.len + len)) {
 	struct binwrite_arg arg;
 
-        /* xxx: use writev to avoid double write if available */
+	/*
+	 * xxx: use writev to avoid double write if available
+	 * writev may help avoid context switch between "a" and "\n" in
+	 * STDERR.puts "a" [ruby-dev:25080] (rebroken since native threads
+	 * introduced in 1.9)
+	 */
         if (fptr->wbuf.len && fptr->wbuf.len+len <= fptr->wbuf.capa) {
             if (fptr->wbuf.capa < fptr->wbuf.off+fptr->wbuf.len+len) {
                 MEMMOVE(fptr->wbuf.ptr, fptr->wbuf.ptr+fptr->wbuf.off, char, fptr->wbuf.len);
@@ -1146,11 +1149,8 @@ io_binwrite(VALUE str, const char *ptr, long len, rb_io_t *fptr, int nosync)
             return -1L;
         if (n == 0)
             return len;
-        /* avoid context switch between "a" and "\n" in STDERR.puts "a".
-           [ruby-dev:25080] */
-	if (fptr->stdio_file != stderr && !rb_thread_fd_writable(fptr->fd)) {
-	    rb_io_check_closed(fptr);
-	}
+
+	rb_io_check_closed(fptr);
 	arg.fptr = fptr;
 	arg.str = str;
       retry:
@@ -4323,9 +4323,6 @@ rb_io_syswrite(VALUE io, VALUE str)
     if (fptr->wbuf.len) {
 	rb_warn("syswrite for buffered IO");
     }
-    if (!rb_thread_fd_writable(fptr->fd)) {
-        rb_io_check_closed(fptr);
-    }
 
     n = rb_write_internal(fptr->fd, RSTRING_PTR(str), RSTRING_LEN(str));