[REGRESSION][BISECTED] 3.12-rc "n_tty: Don't wait for buffer work in read() loop" patch breaks gcc's testsuite

[Mikael Pettersson <mikpelinux@gmail.com>]

With 3.12-rc[12] I see unexpected failures in gcc's Ada acats testsuite, e.g.

                === acats tests ===
FAIL:   a83009b
FAIL:   c37209a
FAIL:   c45531e
FAIL:   c45614a
FAIL:   c67005d
FAIL:   c730a01
FAIL:   c74302b
FAIL:   cc3004a
FAIL:   cd2a24j
FAIL:   cd2a53a
FAIL:   cxa3001
FAIL:   cxf3a07
FAIL:   cxf3a08

                === acats Summary ===
# of expected passes            2307
# of unexpected failures        13
Native configuration is x86_64-unknown-linux-gnu

The exact failures vary from run to run, but some failures always occur on my
x86_64 machines, and all three open gcc branches (trunk, 4.8, 4.7) are affected.
With a 3.11 kernel the acats testsuite is always clean.

A bisection identified:

>From f95499c3030fe1bfad57745f2db1959c5b43dca8 Mon Sep 17 00:00:00 2001
From: Peter Hurley <peter@hurleysoftware.com>
Date: Sat, 15 Jun 2013 13:14:29 +0000
Subject: n_tty: Don't wait for buffer work in read() loop

User-space read() can run concurrently with receiving from device;
waiting for receive_buf() to complete is not required.

Signed-off-by: Peter Hurley <peter@hurleysoftware.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
---
diff --git a/drivers/tty/n_tty.c b/drivers/tty/n_tty.c
index fe1c399..a6eea30 100644
--- a/drivers/tty/n_tty.c
+++ b/drivers/tty/n_tty.c
@@ -1724,7 +1724,6 @@ static inline int input_available_p(struct tty_struct *tty, int amt)
 {
        struct n_tty_data *ldata = tty->disc_data;
 
-       tty_flush_to_ldisc(tty);
        if (ldata->icanon && !L_EXTPROC(tty)) {
                if (ldata->canon_head != ldata->read_tail)
                        return 1;

as the culprit.  Reverting that from 3.12-rc2 eliminates the acats failures
and brings the gcc testsuite results to what one gets with 3.11.

I can't pretend to understand exactly what goes wrong, suffice it to say that
the gcc testsuite harness uses a combination of shell, expect, and tcl.  I
suspect ptys are also involved.

To repeat, bootstrap a recent gcc 4.8 snapshot w/ ada in --enable-languages,
then run the test suite with "make -j6 -k check; make mail-report.log".
(Adjust -jN as appropriate, but -j6 is what I'm using on my quad-core i7s.)

Please consider reverting or fixing this patch.

/Mikael