From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: X-Spam-Checker-Version: SpamAssassin 3.4.0 (2014-02-07) on aws-us-west-2-korg-lkml-1.web.codeaurora.org Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by smtp.lore.kernel.org (Postfix) with ESMTP id 97EEEC433EF for ; Wed, 1 Jun 2022 14:47:39 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1347768AbiFAOrj (ORCPT ); Wed, 1 Jun 2022 10:47:39 -0400 Received: from lindbergh.monkeyblade.net ([23.128.96.19]:58604 "EHLO lindbergh.monkeyblade.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S233844AbiFAOrh (ORCPT ); Wed, 1 Jun 2022 10:47:37 -0400 Received: from ams.source.kernel.org (ams.source.kernel.org [IPv6:2604:1380:4601:e00::1]) by lindbergh.monkeyblade.net (Postfix) with ESMTPS id 8102A50E28 for ; Wed, 1 Jun 2022 07:47:36 -0700 (PDT) Received: from smtp.kernel.org (relay.kernel.org [52.25.139.140]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by ams.source.kernel.org (Postfix) with ESMTPS id 49891B81AFD for ; Wed, 1 Jun 2022 14:47:35 +0000 (UTC) Received: by smtp.kernel.org (Postfix) with ESMTPSA id 86A10C385A5; Wed, 1 Jun 2022 14:47:32 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=linuxfoundation.org; s=korg; t=1654094853; bh=UCmfceCi1c2wNQpeJkRvdWAk+ZSmVjhjMrENuP10L5A=; h=Date:From:To:Cc:Subject:References:In-Reply-To:From; b=GXrj01hslRUccgMqGpq/ze9iN0HpOS0VxEzSA1O8U6/wJwmNtmc0LI23jGW6A+H2O LPBN/t+kNyzQK4OcBsMNIOqZNRXT2/4xwZkfQL6WirSqP1hKnGgp/b2ieOrjIgaLwv 0anzsDAh7BX/J2ewNYOdVEPl3M1+YT/g4QvGAAjk= Date: Wed, 1 Jun 2022 16:47:26 +0200 From: Greg KH To: cael Cc: Jiri Slaby , linux-serial Subject: Re: tty: fix a possible hang on tty device Message-ID: References: MIME-Version: 1.0 Content-Type: text/plain; charset=iso-8859-1 Content-Disposition: inline Content-Transfer-Encoding: 8bit In-Reply-To: Precedence: bulk List-ID: X-Mailing-List: linux-serial@vger.kernel.org On Wed, Jun 01, 2022 at 09:39:27PM +0800, cael wrote: > From: cael > Subject: [PATCH v2] tty: fix a possible hang on tty device > > We have met a hang on pty device, the reader was blocking > at epoll on master side, the writer was sleeping at wait_woken > inside n_tty_write on slave side, and the write buffer on > tty_port was full, we found that the reader and writer would > never be woken again and block forever. > > The problem was caused by a race between reader and kworker: > n_tty_read(reader): n_tty_receive_buf_common(kworker): > |room = N_TTY_BUF_SIZE - (ldata->read_head - tail) > |room <= 0 > copy_from_read_buf()| > n_tty_kick_worker() | > |ldata->no_room = true > > After writing to slave device, writer wakes up kworker to flush > data on tty_port to reader, and the kworker finds that reader > has no room to store data so room <= 0 is met. At this moment, > reader consumes all the data on reader buffer and call > n_tty_kick_worker to check ldata->no_room which is false and > reader quits reading. Then kworker sets ldata->no_room=true > and quits too. > > If write buffer is not full, writer will wake kworker to flush data > again after following writes, but if writer buffer is full and writer > goes to sleep, kworker will never be woken again and tty device is > blocked. > > This problem can be solved with a check for read buffer size inside > n_tty_receive_buf_common, if read buffer is empty and ldata->no_room > is true, a call to n_tty_kick_worker is necessary to keep flushing > data to reader. > > Signed-off-by: cael > Reviewed-by: Ilpo Järvinen > > --- > diff --git a/drivers/tty/n_tty.c b/drivers/tty/n_tty.c > index efc72104c840..21241ea7cdb9 100644 > --- a/drivers/tty/n_tty.c > +++ b/drivers/tty/n_tty.c > @@ -201,8 +201,8 @@ static void n_tty_kick_worker(struct tty_struct *tty) > struct n_tty_data *ldata = tty->disc_data; > > /* Did the input worker stop? Restart it */ > - if (unlikely(ldata->no_room)) { > - ldata->no_room = 0; > + if (unlikely(READ_ONCE(ldata->no_room))) { > + WRITE_ONCE(ldata->no_room, 0); > > WARN_RATELIMIT(tty->port->itty == NULL, > "scheduling with invalid itty\n"); > @@ -1632,7 +1632,7 @@ n_tty_receive_buf_common(struct tty_struct *tty, > const unsigned char *cp, > if (overflow && room < 0) > ldata->read_head--; > room = overflow; > - ldata->no_room = flow && !room; > + WRITE_ONCE(ldata->no_room, flow && !room); > } else > overflow = 0; > > @@ -1663,6 +1663,21 @@ n_tty_receive_buf_common(struct tty_struct > *tty, const unsigned char *cp, > } else > n_tty_check_throttle(tty); > > + if (READ_ONCE(ldata->no_room)) { > + /* > + * Reader ensures that read_tail is updated before > checking no_room, > + * make sure that no_room is set before reading read_tail here. > + * Now no_room is visible by reader, the race needs to > be handled is > + * that reader has passed checkpoint for no_room and > reader buffer > + * is empty, if so n_tty_kick_worker will not be > called by reader, > + * instead, this function is called here. > + * barrier is paired with smp_mb() in n_tty_read() > + */ > + smp_mb(); > + if (!chars_in_buffer(tty)) > + n_tty_kick_worker(tty); > + } > + > up_read(&tty->termios_rwsem); > > return rcvd; > @@ -2180,8 +2195,14 @@ static ssize_t n_tty_read(struct tty_struct > *tty, struct file *file, > if (time) > timeout = time; > } > - if (tail != ldata->read_tail) > + if (tail != ldata->read_tail) { > + /* > + * Make sure no_room is not read before setting read_tail, > + * paired with smp_mb() in n_tty_receive_buf_common() > + */ > > + smp_mb(); > n_tty_kick_worker(tty); > + } > up_read(&tty->termios_rwsem); > > remove_wait_queue(&tty->read_wait, &wait); > -- > 2.27.0 Hi, This is the friendly patch-bot of Greg Kroah-Hartman. You have sent him a patch that has triggered this response. He used to manually respond to these common problems, but in order to save his sanity (he kept writing the same thing over and over, yet to different people), I was created. Hopefully you will not take offence and will fix the problem in your patch and resubmit it so that it can be accepted into the Linux kernel tree. You are receiving this message because of the following common error(s) as indicated below: - Your patch is malformed (tabs converted to spaces, linewrapped, etc.) and can not be applied. Please read the file, Documentation/email-clients.txt in order to fix this. - You did not write a descriptive Subject: for the patch, allowing Greg, and everyone else, to know what this patch is all about. Please read the section entitled "The canonical patch format" in the kernel file, Documentation/SubmittingPatches for what a proper Subject: line should look like. - It looks like you did not use your "real" name for the patch on either the Signed-off-by: line, or the From: line (both of which have to match). Please read the kernel file, Documentation/SubmittingPatches for how to do this correctly. - This looks like a new version of a previously submitted patch, but you did not list below the --- line any changes from the previous version. Please read the section entitled "The canonical patch format" in the kernel file, Documentation/SubmittingPatches for what needs to be done here to properly describe this. If you wish to discuss this problem further, or you have questions about how to resolve this issue, please feel free to respond to this email and Greg will reply once he has dug out from the pending patches received from other developers. thanks, greg k-h's patch email bot