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 7AC13C76195 for ; Fri, 17 Mar 2023 07:26:20 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S230227AbjCQH0T (ORCPT ); Fri, 17 Mar 2023 03:26:19 -0400 Received: from lindbergh.monkeyblade.net ([23.128.96.19]:52184 "EHLO lindbergh.monkeyblade.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S230299AbjCQH0S (ORCPT ); Fri, 17 Mar 2023 03:26:18 -0400 Received: from mail-pl1-x62c.google.com (mail-pl1-x62c.google.com [IPv6:2607:f8b0:4864:20::62c]) by lindbergh.monkeyblade.net (Postfix) with ESMTPS id EC5AF5CC0E; Fri, 17 Mar 2023 00:26:06 -0700 (PDT) Received: by mail-pl1-x62c.google.com with SMTP id le6so4422931plb.12; Fri, 17 Mar 2023 00:26:06 -0700 (PDT) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=gmail.com; s=20210112; t=1679037966; h=content-transfer-encoding:mime-version:references:in-reply-to :message-id:date:subject:cc:to:from:from:to:cc:subject:date :message-id:reply-to; bh=8wd3NuPx9Xrv4wZq4NfbNl6nVymad+l2rLb284yv9Ic=; b=HUjRru7K1q2V0+LjN+cklA8NqIRrkFoDlNfw/60TK6/mPZlsIuFSou0nVzrFhQuVcd zcI6aHbxSGO1i9BeWFHbqVzJvntGV9utE/pBioy90Z5zz30zngpM4lxHPJ2NN9MwH94f mgJdkwT34Z2oAldIfYkeBrSxtVoLscp9/nSjdfEBj8k+vYhVe8Gj4/PASgovy2hVgsdO ajyo6qU/2UDOEaLaDslVTL5ARDGT4Cnf+ioc0HmHok14Nf1MJ7nL4FyCDbEgmJp9AHCV tifpj0BVEAEs7rPiORXk/YE9BISrbu88xKqv2CR54pDuSu4Iu0DnYiPMe1lJSi9ASV5C 6ejQ== X-Google-DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=1e100.net; s=20210112; t=1679037966; h=content-transfer-encoding:mime-version:references:in-reply-to :message-id:date:subject:cc:to:from:x-gm-message-state:from:to:cc :subject:date:message-id:reply-to; bh=8wd3NuPx9Xrv4wZq4NfbNl6nVymad+l2rLb284yv9Ic=; b=NAEzm9jIio4cfdrVXAtCbSaLyGRAIXdTGWehIbqk1uxLMdv86lXSNNJD6ntRIZyzL3 bm3h9eO76KE/7qmYGsIAJdgXnwjQtz0fMD6SY7kQMUDyexWjlJPLfv7WSxpTwSQfASc6 bJJplYlqqVKYu/1kChHZwW8qfefPyMMApYJJHUr5SMaLU3dweHX8zcVy8aMi6+9yQv6g nB6BuTVi1veyxypotq4StzxqqqZhztqjLIa97DjQO/aYW1UgL2BtU5A4BMaB/+tR1s9Y F3k5QaCuF9+VnAPqWthaftpu8P056rWsi0gcBJEUIUiuatqgxsAdO+CFfDwf+kGeJO7d 9kCA== X-Gm-Message-State: AO0yUKUKDIGMDvWQV4moHmRzgC4vwMvCfCTTFp7WfiLUbghFhDX5cSxX vUA+QAROPsguDmi5XxXGJ1I= X-Google-Smtp-Source: AK7set9EMjDY66tpoZFnVEj+GJ5EPoyN01MmGI08BZt6c2374pdBpeHlL+AHkx5hraoUMxGhMWTAEw== X-Received: by 2002:a17:90a:1903:b0:23b:2963:ec94 with SMTP id 3-20020a17090a190300b0023b2963ec94mr7319033pjg.29.1679037966170; Fri, 17 Mar 2023 00:26:06 -0700 (PDT) Received: from localhost.localdomain ([43.155.90.222]) by smtp.googlemail.com with ESMTPSA id t10-20020a170902b20a00b001a0403f6a97sm848406plr.202.2023.03.17.00.26.03 (version=TLS1_2 cipher=ECDHE-ECDSA-AES128-GCM-SHA256 bits=128/128); Fri, 17 Mar 2023 00:26:05 -0700 (PDT) From: juanfengpy@gmail.com X-Google-Original-From: caelli@tencent.com To: jirislaby@kernel.org, gregkh@linuxfoundation.org Cc: ilpo.jarvinen@linux.intel.com, benbjiang@tencent.com, robinlai@tencent.com, linux-serial@vger.kernel.org, juanfengpy@gmail.com, Hui Li , Subject: [PATCH v8] tty: fix hang on tty device with no_room set Date: Fri, 17 Mar 2023 15:25:46 +0800 Message-Id: <1679037946-18156-1-git-send-email-caelli@tencent.com> X-Mailer: git-send-email 1.8.3.1 In-Reply-To: <35358148-3d46-40e5-aa1e-84e7fb6dbb6f@kernel.org> References: <35358148-3d46-40e5-aa1e-84e7fb6dbb6f@kernel.org> MIME-Version: 1.0 Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 8bit Precedence: bulk List-ID: X-Mailing-List: stable@vger.kernel.org From: Hui Li 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 blocked forever. The problem was caused by a race between reader and kworker: n_tty_read(reader): n_tty_receive_buf_common(kworker): copy_from_read_buf()| |room = N_TTY_BUF_SIZE - (ldata->read_head - tail) |room <= 0 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 calls 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 write 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. Cc: Fixes: 42458f41d08f ("n_tty: Ensure reader restarts worker for next reader") Reviewed-by: Ilpo Järvinen Signed-off-by: Hui Li --- Patch changelogs between v1 and v2: -add barrier inside n_tty_read and n_tty_receive_buf_common; -comment why barrier is needed; -access to ldata->no_room is changed with READ_ONCE and WRITE_ONCE; Patch changelogs between v2 and v3: -in function n_tty_receive_buf_common, add unlikely to check ldata->no_room, eg: if (unlikely(ldata->no_room)), and READ_ONCE is removed here to get locality; -change comment for barrier to show the race condition to make comment easier to understand; Patch changelogs between v3 and v4: -change subject from 'tty: fix a possible hang on tty device' to 'tty: fix hang on tty device with no_room set' to make subject more obvious; Patch changelogs between v4 and v5: -name is changed from cael to caelli, li is added as the family name and caelli is the fullname. Patch changelogs between v5 and v6: -change from and Signed-off-by, from 'caelli ' to 'caelli ', later one is my corporate address. Patch changelogs between v6 and v7: -change name from caelli to 'Hui Li', which is my name in chinese. -the comment for barrier is improved, and a Fixes and Reviewed-by tags is added. Patch changelogs between v7 and v8: -Simplify the comments for barriers. drivers/tty/n_tty.c | 25 +++++++++++++++++++++---- 1 file changed, 21 insertions(+), 4 deletions(-) diff --git a/drivers/tty/n_tty.c b/drivers/tty/n_tty.c index c8f56c9b1a1c..4dff2f34e2d0 100644 --- a/drivers/tty/n_tty.c +++ b/drivers/tty/n_tty.c @@ -204,8 +204,8 @@ 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"); @@ -1698,7 +1698,7 @@ 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; @@ -1729,6 +1729,17 @@ n_tty_receive_buf_common(struct tty_struct *tty, const unsigned char *cp, } else n_tty_check_throttle(tty); + if (unlikely(ldata->no_room)) { + /* + * Barrier here is to ensure to read the latest read_tail in + * chars_in_buffer() and to make sure that read_tail is not loaded + * before ldata->no_room is set. + */ + smp_mb(); + if (!chars_in_buffer(tty)) + n_tty_kick_worker(tty); + } + up_read(&tty->termios_rwsem); return rcvd; @@ -2282,8 +2293,14 @@ static ssize_t n_tty_read(struct tty_struct *tty, struct file *file, if (time) timeout = time; } - if (old_tail != ldata->read_tail) + if (old_tail != ldata->read_tail) { + /* + * Make sure no_room is not read in n_tty_kick_worker() + * before setting ldata->read_tail in copy_from_read_buf(). + */ + smp_mb(); n_tty_kick_worker(tty); + } up_read(&tty->termios_rwsem); remove_wait_queue(&tty->read_wait, &wait); -- 2.27.0