From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1753247AbaI2QyZ (ORCPT ); Mon, 29 Sep 2014 12:54:25 -0400 Received: from casper.infradead.org ([85.118.1.10]:39028 "EHLO casper.infradead.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1751601AbaI2QyY (ORCPT ); Mon, 29 Sep 2014 12:54:24 -0400 Date: Mon, 29 Sep 2014 18:54:18 +0200 From: Peter Zijlstra To: Kirill Tkhai Cc: Sasha Levin , mingo@kernel.org, torvalds@linux-foundation.org, paulmck@linux.vnet.ibm.com, linux-kernel@vger.kernel.org Subject: Re: [PATCH] sched: Use RCU read lock on all calls to dl_bw_of() Message-ID: <20140929165418.GQ5430@worktop> References: <1412003970-25277-1-git-send-email-sasha.levin@oracle.com> <1412009027.20287.17.camel@tkhai> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <1412009027.20287.17.camel@tkhai> User-Agent: Mutt/1.5.22.1 (2013-10-16) Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Mon, Sep 29, 2014 at 08:43:47PM +0400, Kirill Tkhai wrote: > Thanks for your report. It looks like your fix is not enough, because > we check for rcu_read_lock_sched_held() in dl_bw_of(). It still warns > even if rcu_read_lock() is held. > > I used rcu_read_lock_sched_held() because we free root_domain using > call_rcu_sched(). So, it's necessary to held rcu_read_lock_sched(), > and my initial commit has this problem too. > > It looks like we should fix it in a way like this: > > [PATCH]sched: Use dl_bw_of() under rcu_read_lock_sched() > > rq->rd is freed using call_rcu_sched(), and it's accessed with preemption > disabled in the most cases. > > So in other places we should use rcu_read_lock_sched() to access it to fit > the scheme: > > rcu_read_lock_sched() or preempt_disable() <==> call_rcu_sched(). Hmm, sad that. I cannot remember why that is rcu_sched, I suspect because we rely on it someplace but I cannot remember where. We could of course do a double take on that and use call_rcu after call_rcu_sched(), such that either or both are sufficient. I would very much prefer not to add extra preempt_disable()s if possible.