From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1757296Ab2AEBp2 (ORCPT ); Wed, 4 Jan 2012 20:45:28 -0500 Received: from mail-gx0-f174.google.com ([209.85.161.174]:64668 "EHLO mail-gx0-f174.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1751190Ab2AEBpY (ORCPT ); Wed, 4 Jan 2012 20:45:24 -0500 Date: Thu, 5 Jan 2012 02:45:20 +0100 From: Frederic Weisbecker To: "Paul E. McKenney" Cc: Sasha Levin , linux-kernel Subject: Re: [PATCH] rcu: Improve detection of illegal synchronize_rcu() call from RCU read side Message-ID: <20120105014518.GD1143@somewhere> References: <1324901803.31721.4.camel@lappy> <20111226163148.GC2435@linux.vnet.ibm.com> <20111226163734.GF28309@somewhere.redhat.com> <20111226195656.GD2435@linux.vnet.ibm.com> <20120104190336.GC1143@somewhere> <20120104213035.GF2448@linux.vnet.ibm.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20120104213035.GF2448@linux.vnet.ibm.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 Wed, Jan 04, 2012 at 01:30:35PM -0800, Paul E. McKenney wrote: > On Wed, Jan 04, 2012 at 08:03:39PM +0100, Frederic Weisbecker wrote: > > Actually for the case of RCU, the wait_for_completion() called by synchronize_rcu() > > has a might_sleep() call that triggers a warning in this case. > > > > But in the case of SMP with 1 online CPU, the rcu_blocking_is_gp() > > checks returns right away on rcutree. So probably we need this? > > I modified this to push the might_sleep() down into the > rcu_blocking_is_gp() function, queued the result, and retained your > Signed-off-by. (Please let me know if there is any problem with this.) > > This does work for TREE_PREEMPT_RCU and for synchronize_rcu_bh() in > TREE_RCU, but not for synchronize_sched() in TREE_RCU. This is because > rcu_read_lock() and rcu_read_unlock() are no-ops in the TREE_RCU case. Not sure about that. This calls preempt_disable() which, in any case with CONFIG_DEBUG_ATOMIC_SLEEP, handles the preempt count. And that even if !CONFIG_PREEMPT. > > So I queued up a separate patch using rcu_lockdep_assert() to check for > illegal RCU grace period within the same-type RCU read-side critical > section, including for SRCU. This is also a partial solution, as it > does not handle things like this: > > void foo(void) > { > mutex_lock(&my_mutex); > . . . > synchronize_srcu(&my_srcu); > . . . > mutex_unlock(&my_mutex); > } > > void bar(void) > { > int idx; > > idx = rcu_read_lock(&m_srcu); > . . . > mutex_lock(&my_mutex); > . . . > mutex_unlock(&my_mutex); > . . . > srcu_read_unlock(&m_srcu, idx); > } > > This can be extended into a chain of locks and a chain of SRCU instances. > For an example of the latter, consider an SRCU-A read-side critical > section containing an SRCU-B grace period, an SRCU-B read-side critical > section containing an SRCU-C grace period, and so on, with the SRCU-Z > read-side critical section containing an RCU-A grace period. Heh! Indeed... > But it > is OK to hold a mutex across one SRCU read-side critical section while > acquiring that same mutex within another same-flavor SRCU read-side > critical section. So the analogy with reader-writer locking only goes > so far. > > At the moment, a full solution seems to require some surgery on lockdep > itself, but perhaps there is a better way. Ok. > > > rcutiny seems to be fine with the cond_resched() call, but srcu needs > > a special treatment. > > For the moment, I just applied rcu_lockdep_assert() everywhere -- zero > cost on non-lockdep kernels, and fully handles all of the RCU simple > self-deadlock cases. So, for RCU I'm not sure this is useful given the might_sleep() things. But for srcu it is. Thanks.