From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1756953Ab2CFSBD (ORCPT ); Tue, 6 Mar 2012 13:01:03 -0500 Received: from perches-mx.perches.com ([206.117.179.246]:53276 "EHLO labridge.com" rhost-flags-OK-OK-OK-FAIL) by vger.kernel.org with ESMTP id S1753887Ab2CFSBB (ORCPT ); Tue, 6 Mar 2012 13:01:01 -0500 Message-ID: <1331056859.2140.7.camel@joe2Laptop> Subject: Re: [RFC PATCH] checkpatch: Warn on use of yield() From: Joe Perches To: Peter Zijlstra Cc: Andrew Morton , Mel Gorman , Miao Xie , Christoph Lameter , linux-mm@kvack.org, linux-kernel@vger.kernel.org, "Theodore Ts'o" Date: Tue, 06 Mar 2012 10:00:59 -0800 In-Reply-To: <1331056466.11248.327.camel@twins> References: <20120302112358.GA3481@suse.de> <1330723262.11248.233.camel@twins> <20120305121804.3b4daed4.akpm@linux-foundation.org> <1330999280.10358.3.camel@joe2Laptop> <1331037942.11248.307.camel@twins> <1331055666.2140.3.camel@joe2Laptop> <1331056466.11248.327.camel@twins> Content-Type: text/plain; charset="UTF-8" X-Mailer: Evolution 3.2.2- Content-Transfer-Encoding: 7bit Mime-Version: 1.0 Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Tue, 2012-03-06 at 18:54 +0100, Peter Zijlstra wrote: > On Tue, 2012-03-06 at 09:41 -0800, Joe Perches wrote: > > Perhaps the kernel-doc comments in sched/core.c > > should/could be expanded/updated. > > Something like this? > > --- > kernel/sched/core.c | 20 ++++++++++++++++++-- > 1 files changed, 18 insertions(+), 2 deletions(-) > > diff --git a/kernel/sched/core.c b/kernel/sched/core.c > index 2963fbb..a05a0f7 100644 > --- a/kernel/sched/core.c > +++ b/kernel/sched/core.c > @@ -4577,8 +4577,24 @@ EXPORT_SYMBOL(__cond_resched_softirq); > /** > * yield - yield the current processor to other threads. > * > - * This is a shortcut for kernel-space yielding - it marks the > - * thread runnable and calls sys_sched_yield(). > + * Do not ever use this function, there's a 99% chance you're doing it wrong. > + * > + * The scheduler is at all times free to pick the calling task as the most > + * eligible task to run, if removing the yield() call from your code breaks > + * it, its already broken. > + * > + * Typical broken usage is: > + * > + * while (!event) > + * yield(); > + * > + * where one assumes that yield() will let 'the other' process run that will > + * make event true. If the current task is a SCHED_FIFO task that will never > + * happen. Never use yield() as a progress guarantee!! > + * > + * If you want to use yield() to wait for something, use wait_event(). > + * If you want to use yield() to be 'nice' for others, use cond_resched(). > + * If you still want to use yield(), do not! > */ Yes. I'll update the checkpatch message to say something like: "Using yield() is generally wrong. See yield() kernel-doc (sched/core.c)"