From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1755500AbcEEAvB (ORCPT ); Wed, 4 May 2016 20:51:01 -0400 Received: from mail-pa0-f49.google.com ([209.85.220.49]:36117 "EHLO mail-pa0-f49.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1754654AbcEEAu7 (ORCPT ); Wed, 4 May 2016 20:50:59 -0400 Subject: Re: [BUG] drivers/tty: read() on a noncanonical blocking tty randomly fails when VMIN > received >= buf To: Julio Guerra References: <16504442-9e91-aaab-1f2b-9b4aa9c219f5@farjump.io> <572A80C0.4080704@hurleysoftware.com> <3319f492-9b1b-d006-e903-7de761a87a53@farjump.io> Cc: linux-serial@vger.kernel.org, linux-kernel@vger.kernel.org From: Peter Hurley Message-ID: <572A98F0.3040306@hurleysoftware.com> Date: Wed, 4 May 2016 17:50:56 -0700 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:38.0) Gecko/20100101 Thunderbird/38.7.2 MIME-Version: 1.0 In-Reply-To: <3319f492-9b1b-d006-e903-7de761a87a53@farjump.io> Content-Type: text/plain; charset=utf-8 Content-Transfer-Encoding: 8bit Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On 05/04/2016 04:27 PM, Julio Guerra wrote: >>> When a tty (here a slave pty) is set in noncanonical input and blocking read modes, a read() randomly blocks when: >>> "VMIN > kernel received >= user buffer size > 0". >>> >>> The standard says that read() should block until VMIN bytes are received [1][2]. Whether this is an implementation defined case not really specified by POSIX or not, it should not behave randomly (otherwise it really should be documented in termios manpage). >> >> This is not a bug. >> >> From the termios(3) man page: >> >> * MIN > 0; TIME == 0: read(2) blocks until the lesser of MIN bytes or the number of bytes requested are avail‐ >> able, and returns the lesser of these two values. >> > > This does not appear in my man... > > Anyway, how do you explain the random behavior then? A long standing bug in this read mode allows the asynchronous input processing thread to race with the read() thread and become confused about how much data remains. I fixed this in 4.6; when I run your test on 4.6, it consistently returns the full user buffer. Regards, Peter Hurley