From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from psmtp.com (na3sys010amx168.postini.com [74.125.245.168]) by kanga.kvack.org (Postfix) with SMTP id CE7D86B002C for ; Tue, 6 Mar 2012 08:15:28 -0500 (EST) Message-ID: <4F560DA8.5040302@parallels.com> Date: Tue, 6 Mar 2012 17:14:16 +0400 From: Glauber Costa MIME-Version: 1.0 Subject: Re: [RFC PATCH] checkpatch: Warn on use of yield() 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> In-Reply-To: <1331037942.11248.307.camel@twins> Content-Type: text/plain; charset="ISO-8859-1"; format=flowed Content-Transfer-Encoding: 7bit Sender: owner-linux-mm@kvack.org List-ID: To: Peter Zijlstra Cc: Joe Perches , Andrew Morton , Mel Gorman , Miao Xie , Christoph Lameter , linux-mm@kvack.org, linux-kernel@vger.kernel.org, Theodore Ts'o On 03/06/2012 04:45 PM, Peter Zijlstra wrote: > On Mon, 2012-03-05 at 18:01 -0800, Joe Perches wrote: > >> +# check for use of yield() >> + if ($line =~ /\byield\s*\(\s*\)/ { >> + WARN("YIELD", >> + "yield() is deprecated, consider cpu_relax()\n" . $herecurr); >> + } > > Its not deprecated as such, its just a very dangerous and ill considered > API. > > cpu_relax() is not a good substitute suggestion in that its still a busy > wait and prone to much of the same problems. > > The case at hand was a life-lock due to expecting that yield() would run > another process which it needed in order to complete. Yield() does not > provide that guarantee. > > Looking at fs/ext4/mballoc.c, we have this gem: > > > /* > * Yield the CPU here so that we don't get soft lockup > * in non preempt case. > */ > yield(); > > This is of course complete crap as well.. I suspect they want > cond_resched() there. And: > > /* let others to free the space */ > yield(); > > Like said, yield() doesn't guarantee anything like running anybody else, > does it rely on that? Or is it optimistic? > > Another fun user: > > void tasklet_kill(struct tasklet_struct *t) > { > if (in_interrupt()) > printk("Attempt to kill tasklet from interrupt\n"); > > while (test_and_set_bit(TASKLET_STATE_SCHED,&t->state)) { > do { > yield(); > } while (test_bit(TASKLET_STATE_SCHED,&t->state)); > } > tasklet_unlock_wait(t); > clear_bit(TASKLET_STATE_SCHED,&t->state); > } > > The only reason that doesn't explode is because running tasklets is > non-preemptible, However since they're non-preemptible they shouldn't > run long and you might as well busy spin. If they can run long, yield() > isn't your biggest problem. > > mm/memory_hotplug.c has two yield() calls in offline_pages() and I've no > idea what they're trying to achieve. > > But really, yield() is basically _always_ the wrong thing. The right > thing can be: > > cond_resched(); wait_event(); or something entirely different. > > So instead of suggesting an alternative, I would suggest thinking about > the actual problem in order to avoid the non-thinking solutions the > checkpatch brigade is so overly fond of :/ > > Maybe something like: > > "yield() is dangerous and wrong, rework your code to not use it." > > That at least requires some sort of thinking and doesn't suggest blind > substitution. > Can't we point people to some Documentation file that explains the alternatives? -- To unsubscribe, send a message with 'unsubscribe linux-mm' in the body to majordomo@kvack.org. For more info on Linux MM, see: http://www.linux-mm.org/ . Fight unfair telecom internet charges in Canada: sign http://stopthemeter.ca/ Don't email: email@kvack.org