From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S933140Ab1KGS0S (ORCPT ); Mon, 7 Nov 2011 13:26:18 -0500 Received: from 95-31-19-74.broadband.corbina.ru ([95.31.19.74]:59843 "EHLO dnet.ilyx.ru" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1755817Ab1KGS0R (ORCPT ); Mon, 7 Nov 2011 13:26:17 -0500 Message-ID: <4EB822C7.6070200@ilyx.ru> Date: Mon, 07 Nov 2011 22:26:15 +0400 From: Ilya Zykov User-Agent: Mozilla/5.0 (X11; U; Linux x86_64; en-US; rv:1.9.1.16) Gecko/20110929 Iceape/2.0.11 MIME-Version: 1.0 To: Alan Cox CC: Greg Kroah-Hartman , linux-kernel@vger.kernel.org Subject: Re: PROBLEM: Race condition in tty buffer's function flush_to_ldisc(). References: <4EB7BD9A.7070600@ilyx.ru> <20111107115802.23201d45@bob.linux.org.uk> <4EB7CCFF.5090304@ilyx.ru> <20111107130643.07e84fca@bob.linux.org.uk> <4EB7E0C6.7050909@ilyx.ru> <20111107145005.68ca1050@bob.linux.org.uk> In-Reply-To: <20111107145005.68ca1050@bob.linux.org.uk> Content-Type: text/plain; charset=ISO-8859-1 Content-Transfer-Encoding: 7bit Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Alan Cox wrote: >> Of course I know about tty_buffer_flush(), it only read TTY_FLUSHING, >> it can't change TTY_FLUSHING, if flush_to_ldisc() single threaded, >> we can change TTY_FLUSHING only in one place in one time(in >> flush_to_ldisc()), therefor we can use only "set_bit(TTY_FLUSHING, >> &tty->flags)" without test. > > Yes.. if you can pin down why in your testing you see the other case > sometimes being true. > > Alan > Nested call flush_to_ldisc() happened on different CPU only. It happens because one side pty call "schedule_work(&tty->buf.work)" on one CPU from "tty_flip_buffer_push()", the other side call "schedule_work(&tty->buf.work)" from n_tty's layer from "n_tty_set_room()" on different CPU. It also can happen in interrupt if interrupt handle on different CPU. Schedule_work() schedule work on CPU which was called(IMHO). For testing I use in xterminal: cat big_file and flush_to_ldisc() with this patch: diff -uprN -X ../../../dontdiff a/drivers/tty/tty_buffer.c c/drivers/tty/tty_buffer.c --- a/drivers/tty/tty_buffer.c 2011-11-07 14:45:27.000000000 +0400 +++ c/drivers/tty/tty_buffer.c 2011-11-07 21:48:49.000000000 +0400 @@ -405,13 +405,14 @@ static void flush_to_ldisc(struct work_s container_of(work, struct tty_struct, buf.work); unsigned long flags; struct tty_ldisc *disc; + static int mthread; disc = tty_ldisc_ref(tty); if (disc == NULL) /* !TTY_LDISC */ return; spin_lock_irqsave(&tty->buf.lock, flags); - + mthread = 0; if (!test_and_set_bit(TTY_FLUSHING, &tty->flags)) { struct tty_buffer *head; while ((head = tty->buf.head) != NULL) { @@ -445,6 +446,12 @@ static void flush_to_ldisc(struct work_s spin_lock_irqsave(&tty->buf.lock, flags); } clear_bit(TTY_FLUSHING, &tty->flags); + } else { + mthread = 1; + printk(KERN_WARNING "Tty %s FLUSHING CPU %d.\n", tty->name, percpu_read(cpu_number)); + } + if (mthread) { + printk(KERN_WARNING "Tty %s FLUSHING multithreaded CPU %d.\n", tty->name, percpu_read(cpu_number)); } /* We may have a deferred request to flush the input buffer, And get in my syslog: Nov 7 21:34:13 serh kernel: [ 76.323848] Tty ptm0 FLUSHING CPU 0. Nov 7 21:34:13 serh kernel: [ 76.323850] Tty ptm0 FLUSHING multithreaded CPU 0. Nov 7 21:34:13 serh kernel: [ 76.323856] Tty ptm0 FLUSHING CPU 0. Nov 7 21:34:13 serh kernel: [ 76.323857] Tty ptm0 FLUSHING multithreaded CPU 0. Nov 7 21:34:13 serh kernel: [ 76.323861] Tty ptm0 FLUSHING multithreaded CPU 1. Nov 7 21:34:13 serh kernel: [ 76.336022] Tty ptm0 FLUSHING CPU 0. Nov 7 21:34:13 serh kernel: [ 76.336024] Tty ptm0 FLUSHING multithreaded CPU 0. Nov 7 21:34:13 serh kernel: [ 76.336030] Tty ptm0 FLUSHING multithreaded CPU 1. Nov 7 21:34:13 serh kernel: [ 76.353134] Tty ptm0 FLUSHING CPU 0. Nov 7 21:34:13 serh kernel: [ 76.353136] Tty ptm0 FLUSHING multithreaded CPU 0. Nov 7 21:34:13 serh kernel: [ 76.353143] Tty ptm0 FLUSHING CPU 0. Nov 7 21:34:13 serh kernel: [ 76.353145] Tty ptm0 FLUSHING multithreaded CPU 0. Nov 7 21:34:13 serh kernel: [ 76.353148] Tty ptm0 FLUSHING multithreaded CPU 1. ...... ......