From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1751561Ab3KRXmd (ORCPT ); Mon, 18 Nov 2013 18:42:33 -0500 Received: from merlin.infradead.org ([205.233.59.134]:53934 "EHLO merlin.infradead.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1751064Ab3KRXma (ORCPT ); Mon, 18 Nov 2013 18:42:30 -0500 Date: Tue, 19 Nov 2013 00:42:09 +0100 From: Peter Zijlstra To: Andrew Morton Cc: NeilBrown , Ingo Molnar , Thomas Gleixner , lkml , "Dr. H. Nikolaus Schaller" , Marek Belisko , Mark Brown Subject: Re: [PATCH/RFC] wait_for_completion_timeout() considered harmful. Message-ID: <20131118234209.GO16796@laptop.programming.kicks-ass.net> References: <20131117080603.2a0d3b6d@notabene.brown> <20131118152746.937b2b7971d7a4bba4ef996d@linux-foundation.org> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20131118152746.937b2b7971d7a4bba4ef996d@linux-foundation.org> User-Agent: Mutt/1.5.21 (2012-12-30) Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Mon, Nov 18, 2013 at 03:27:46PM -0800, Andrew Morton wrote: > On Sun, 17 Nov 2013 08:06:03 +1100 NeilBrown wrote: > > > It would be reasonable to assume that > > > > wait_for_completion_timeout(&wm8350->auxadc_done, msecs_to_jiffies(5)); > > > > would wait at least 5 msecs for the auxadc_done to complete. But it does not. > > With a HZ of 200 or less, msecs_to_jiffies(5) has value '1', and so this > > will only wait until the next "timer tick", which could happen immediately. > > > > This can lead to incorrect results - and has done so in out-of-tree patches > > for drivers/misc/bmp085.c which uses a very similar construct to enable interrupt > > based result collection. > > > > The documentation for several *_timeout* functions claim they will wait for > > "timeout jiffies" to have elapsed where this is not the case. They will > > actually wait for "timeout" jiffies to have started implying an elapsed time > > between (timeout-1) and (timeout). > > > > This patch corrects some of this documentation, and adds a collection of > > wait_for_completion*_msecs() > > interfaces which wait at least the given number of milliseconds for the > > completion (or a signal). > > Mutter. wait_for_x(..., 5ms) should wait for a minimum of 5ms, no matter > what. > > So I'd suggest we make that happen, rather than adding some new interfaces? Yeah, also the completion interface is just one of many that's buggered this way. I briefly talked to Thomas about this earlier today and we need to fix this at a lower level -- the quick 'n dirty solution is to add 1 jiffy down in the timer-wheel when we enqueue these things. And yes, we very much don't want to create new interfaces with similar but slightly different semantics, that's just asking for trouble.