From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1755533Ab2CFRyn (ORCPT ); Tue, 6 Mar 2012 12:54:43 -0500 Received: from merlin.infradead.org ([205.233.59.134]:50854 "EHLO merlin.infradead.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1751272Ab2CFRym convert rfc822-to-8bit (ORCPT ); Tue, 6 Mar 2012 12:54:42 -0500 Message-ID: <1331056466.11248.327.camel@twins> Subject: Re: [RFC PATCH] checkpatch: Warn on use of yield() From: Peter Zijlstra To: Joe Perches 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 18:54:26 +0100 In-Reply-To: <1331055666.2140.3.camel@joe2Laptop> 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> Content-Type: text/plain; charset=US-ASCII Content-Transfer-Encoding: 7BIT X-Mailer: Evolution 3.2.2- 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 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! */ void __sched yield(void) {