From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1757573Ab2AECBY (ORCPT ); Wed, 4 Jan 2012 21:01:24 -0500 Received: from e31.co.us.ibm.com ([32.97.110.149]:57857 "EHLO e31.co.us.ibm.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1751923Ab2AECBV (ORCPT ); Wed, 4 Jan 2012 21:01:21 -0500 Date: Wed, 4 Jan 2012 18:01:08 -0800 From: "Paul E. McKenney" To: Frederic Weisbecker Cc: Sasha Levin , linux-kernel Subject: Re: [PATCH] rcu: Improve detection of illegal synchronize_rcu() call from RCU read side Message-ID: <20120105020108.GQ2448@linux.vnet.ibm.com> Reply-To: paulmck@linux.vnet.ibm.com 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> <20120105014518.GD1143@somewhere> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20120105014518.GD1143@somewhere> User-Agent: Mutt/1.5.21 (2010-09-15) x-cbid: 12010502-7282-0000-0000-000005639E7F Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Thu, Jan 05, 2012 at 02:45:20AM +0100, Frederic Weisbecker wrote: > 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. Ah, of course! I keep forgetting that CONFIG_DEBUG_ATOMIC_SLEEP selects CONFIG_PREEMPT_COUNT. > > 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. One nice thing about the lockdep approach is that it tracks where the conflicting RCU read-side critical section started. But I am planning for these to be 3.4 material, so we do have some time to refine them. Thanx, Paul