From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S932413Ab2ASPyw (ORCPT ); Thu, 19 Jan 2012 10:54:52 -0500 Received: from mx1.redhat.com ([209.132.183.28]:40071 "EHLO mx1.redhat.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S932281Ab2ASPyt (ORCPT ); Thu, 19 Jan 2012 10:54:49 -0500 Date: Thu, 19 Jan 2012 10:54:45 -0500 From: Vivek Goyal To: Tejun Heo Cc: axboe@kernel.dk, ctalbott@google.com, rni@google.com, linux-kernel@vger.kernel.org Subject: Re: [PATCH 01/12] blkcg: obtaining blkg should be enclosed inside rcu_read_lock() Message-ID: <20120119155445.GB10908@redhat.com> References: <1326935490-11827-1-git-send-email-tj@kernel.org> <1326935490-11827-2-git-send-email-tj@kernel.org> <20120119100729.GA2649@redhat.com> <20120119153938.GA5198@google.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20120119153938.GA5198@google.com> User-Agent: Mutt/1.5.21 (2010-09-15) Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Thu, Jan 19, 2012 at 07:39:38AM -0800, Tejun Heo wrote: > Hello, Vivek. > > On Thu, Jan 19, 2012 at 05:07:29AM -0500, Vivek Goyal wrote: > > On Wed, Jan 18, 2012 at 05:11:19PM -0800, Tejun Heo wrote: > > > When looking up or creating blkg's, both blk-throttle and cfq-iosched > > > drops rcu_read_lock() right after lookup is complete. This isn't > > > safe. Refcnt isn't incremented at that point and rcu lock is the only > > > thing holding the blkg. It shouldn't be dropped until after refcnt is > > > incremented by the caller. > > > > throtl_get_tg() and cfq_get_cfqg() are called with queue lock held and > > tg and cfqg are protected by queue lock as they can not go away as long > > as queue lock is held. > > Ah, right. > > > I had used rcu read lock to access blkcg pointer here. That's why when > > we are done with accessing blkcg, we drop rcu read lock and return back > > to caller with group pointer, which is aready holding either a queue > > lock or rcu read lock to protect returned group pointer. > > > > So if we are protecting blkcg using rcu, then it should make sense to > > take that lock inside throtl_get_tg() and cfq_get_cfqg() respectively and > > it should not be left to the caller? > > No, no matter whatever synchronization scheme is in use, the code is > seriously screwed up if it's doing something like, > > lock(); > a = lookup(); > unlock(); > return a; > > You should *NEVER* be doing that. I guess ioc_lookup_icq() is doing something similar. We call it under queue lock. Take rcu lock inside for sanity of radix tree and then release rcu lock and return icq. Thanks Vivek