From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from psmtp.com (na3sys010amx196.postini.com [74.125.245.196]) by kanga.kvack.org (Postfix) with SMTP id D7D886B004D for ; Wed, 20 Jun 2012 21:37:36 -0400 (EDT) Message-ID: <4FE27AE8.2080906@kernel.org> Date: Thu, 21 Jun 2012 10:37:44 +0900 From: Minchan Kim MIME-Version: 1.0 Subject: Re: [PATCH] mm: offlining memory may block forever References: In-Reply-To: Content-Type: text/plain; charset=ISO-8859-1 Content-Transfer-Encoding: 7bit Sender: owner-linux-mm@kvack.org List-ID: To: Aaditya Kumar Cc: KOSAKI Motohiro , linux-kernel@vger.kernel.org, linux-mm@kvack.org, stable@kernel.org, kosaki.motohiro@jp.fujitsu.com, gregkh@linuxfoundation.org, Mel Gorman , KAMEZAWA Hiroyuki , Michal Hocko , tim.bird@am.sony.com, frank.rowand@am.sony.com, takuzo.ohara@ap.sony.com, kan.iibuchi@jp.sony.com, aaditya.kumar@ap.sony.com On 06/21/2012 01:23 AM, Aaditya Kumar wrote: > Offlining memory may block forever, waiting for kswapd() to wake up because > kswapd() does not check the event kthread->should_stop before sleeping. > > The proper pattern, from Documentation/memory-barriers.txt, is: > --- waker --- > event_indicated = 1; > wake_up_process(event_daemon); > > --- sleeper --- > for (;;) { > set_current_state(TASK_UNINTERRUPTIBLE); > if (event_indicated) > break; > schedule(); > } > > set_current_state() may be wrapped by: > prepare_to_wait(); > > In the kswapd() case, event_indicated is kthread->should_stop. > --- offlining memory (waker) --- > kswapd_stop() > kthread_stop() > kthread->should_stop = 1 > wake_up_process() > wait_for_completion() > > > --- kswapd_try_to_sleep (sleeper) --- > kswapd_try_to_sleep() > prepare_to_wait() > . > . > schedule() > . > . > finish_wait() > > The schedule() needs to be protected by a test of kthread->should_stop, > which is wrapped by kthread_should_stop(). > > Reproducer: > Do heavy file I/O in background. > Do a memory offline/online in a tight loop > > > Signed-off-by: Aaditya Kumar Reviewed-by: Minchan Kim Nitpick: We can remove kthread_should_stop check earlier in kswapd_try_to_sleep. But it's no biggie. And I hope you change patch title Title : Fix loss of kswapd wakeup in kswapd_stop Description: Offlining memory may block forever because blah, blah, blah. -- Kind regards, Minchan Kim -- 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/ . Don't email: email@kvack.org