From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S933540AbdCHC0S (ORCPT ); Tue, 7 Mar 2017 21:26:18 -0500 Received: from mx0a-001b2d01.pphosted.com ([148.163.156.1]:49070 "EHLO mx0a-001b2d01.pphosted.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S933455AbdCHC0P (ORCPT ); Tue, 7 Mar 2017 21:26:15 -0500 Date: Tue, 7 Mar 2017 18:26:03 -0800 From: "Paul E. McKenney" To: Boqun Feng Cc: Dmitry Vyukov , josh@joshtriplett.org, Steven Rostedt , Mathieu Desnoyers , jiangshanlai@gmail.com, LKML , syzkaller Subject: Re: rcu: WARNING in rcu_seq_end Reply-To: paulmck@linux.vnet.ibm.com References: <20170306100741.GJ30506@linux.vnet.ibm.com> <20170306230800.GK30506@linux.vnet.ibm.com> <20170307142740.uh2nnaw44albn3t2@tardis> <20170307152715.GM30506@linux.vnet.ibm.com> <20170307230513.7mwnnhuqqbgsecm6@tardis> <20170307233154.GV30506@linux.vnet.ibm.com> <20170308013913.4si2ldnomfktzedq@tardis> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20170308013913.4si2ldnomfktzedq@tardis> User-Agent: Mutt/1.5.21 (2010-09-15) X-TM-AS-GCONF: 00 x-cbid: 17030802-0052-0000-0000-0000018F10FF X-IBM-SpamModules-Scores: X-IBM-SpamModules-Versions: BY=3.00006742; HX=3.00000240; KW=3.00000007; PH=3.00000004; SC=3.00000206; SDB=6.00831483; UDB=6.00407950; IPR=6.00609075; BA=6.00005194; NDR=6.00000001; ZLA=6.00000005; ZF=6.00000009; ZB=6.00000000; ZP=6.00000000; ZH=6.00000000; ZU=6.00000002; MB=3.00014550; XFM=3.00000012; UTC=2017-03-08 02:26:06 X-IBM-AV-DETECTION: SAVI=unused REMOTE=unused XFE=unused x-cbparentid: 17030802-0053-0000-0000-00004EEA1411 Message-Id: <20170308022602.GA30506@linux.vnet.ibm.com> X-Proofpoint-Virus-Version: vendor=fsecure engine=2.50.10432:,, definitions=2017-03-08_01:,, signatures=0 X-Proofpoint-Spam-Details: rule=outbound_notspam policy=outbound score=0 spamscore=0 suspectscore=0 malwarescore=0 phishscore=0 adultscore=0 bulkscore=0 classifier=spam adjust=0 reason=mlx scancount=1 engine=8.0.1-1702020001 definitions=main-1703080018 Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Wed, Mar 08, 2017 at 09:39:13AM +0800, Boqun Feng wrote: > On Tue, Mar 07, 2017 at 03:31:54PM -0800, Paul E. McKenney wrote: > > On Wed, Mar 08, 2017 at 07:05:13AM +0800, Boqun Feng wrote: > > > On Tue, Mar 07, 2017 at 07:27:15AM -0800, Paul E. McKenney wrote: > > > > On Tue, Mar 07, 2017 at 03:43:42PM +0100, Dmitry Vyukov wrote: > > > > > On Tue, Mar 7, 2017 at 3:27 PM, Boqun Feng wrote: > > > > > > On Tue, Mar 07, 2017 at 08:05:19AM +0100, Dmitry Vyukov wrote: > > > > > > [...] > > > > > >> >> > > > > > >> >> What is that mutex? And what locks/unlocks provide synchronization? I > > > > > >> >> see that one uses exp_mutex and another -- exp_wake_mutex. > > > > > >> > > > > > > >> > Both of them. > > > > > >> > > > > > > >> > ->exp_mutex is acquired by the task requesting the grace period, and > > > > > >> > the counter's first increment is done by that task under that mutex. > > > > > >> > This task then schedules a workqueue, which drives forward the grace > > > > > >> > period. Upon grace-period completion, the workqueue handler does the > > > > > >> > second increment (the one that your patch addressed). The workqueue > > > > > >> > handler then acquires ->exp_wake_mutex and wakes the task that holds > > > > > >> > ->exp_mutex (along with all other tasks waiting for this grace period), > > > > > >> > and that task releases ->exp_mutex, which allows the next grace period to > > > > > >> > start (and the first increment for that next grace period to be carried > > > > > >> > out under that lock). The workqueue handler releases ->exp_wake_mutex > > > > > >> > after finishing its wakeups. > > > > > >> > > > > > >> Then we need the following for the case when task requesting the grace > > > > > >> period does not block, right? > > > > > > > > > > > > Won't be necessary I think, as the smp_mb() in rcu_seq_end() and the > > > > > > smp_mb__before_atomic() in sync_exp_work_done() already provide the > > > > > > required ordering, no? > > > > > > > > > > smp_mb() is probably fine, but smp_mb__before_atomic() is release not > > > > > acquire. If we want to play that game, then I guess we also need > > > > > > The point is that smp_mb__before_atomic() + atomic_long_inc() will > > > guarantee a smp_mb() before or right along with the atomic operation, > > > and that's enough because rcu_seq_done() followed by a smp_mb() will > > > give it a acquire-like behavior. > > > > Given current architectures, true enough, from what I can see. > > > > However, let's take a look at atomic_ops.rst: > > > > > > If a caller requires memory barrier semantics around an atomic_t > > operation which does not return a value, a set of interfaces are > > defined which accomplish this:: > > > > void smp_mb__before_atomic(void); > > void smp_mb__after_atomic(void); > > > > For example, smp_mb__before_atomic() can be used like so:: > > > > obj->dead = 1; > > smp_mb__before_atomic(); > > atomic_dec(&obj->ref_count); > > > > It makes sure that all memory operations preceding the atomic_dec() > > call are strongly ordered with respect to the atomic counter > > operation. In the above example, it guarantees that the assignment of > > "1" to obj->dead will be globally visible to other cpus before the > > atomic counter decrement. > > > > Without the explicit smp_mb__before_atomic() call, the > > implementation could legally allow the atomic counter update visible > > to other cpus before the "obj->dead = 1;" assignment. > > > > So the ordering is guaranteed against the atomic operation, not > > necessarily the stuff after it. But again, the implementations I know > > of do make the guarantee, hence my calling it a theoretical bug in the > > commit log. > > Fair enough ;-) It's me who misunderstood this part of document. > > However, the names of the barriers are smp_mb__{before,after}_atomic(), > so if they, semantically, only provide ordering for the corresponding > atomic ops rather than a full barrier, I would their names are > misleading ;-) Well, if you have both ordering before and after, then you have full ordering. > > > > > smp_mb__after_atomic() there. But it would be way easier to understand > > > > > > Adding smp_mb__after_atomic() would be pointless as it's the load of > > > ->expedited_sequence that we want to ensure having acquire behavior > > > rather than the atomic increment of @stat. > > > > Again, agreed given current code, but atomic_ops.rst doesn't guarantee > > ordering past the actual atomic operation itself. > > Neither does atomic_ops.rst guarantee the ordering between a load before > the atomic op and memory accesses after the atomic op, right? I.e. > atomic_ops.rst doesn't say no for reordering like this: > > r1 = READ_ONCE(a); ---------+ > atomic_long_inc(b); | > smp_mb__after_atomic(); | > WRITE_ONCE(c); | > {r1 = READ_ONCE(a)} <-------+ > > So it's still not an acquire for READ_ONCE(a), in our case "a" is > ->expedited_sequence. > > To me, we can either fix the atomic_ops.rst or, as I proposed, just > change smp_mb__before_atomic() to smp_mb(). Or have both an smp_mb__before_atomic() and an smp_mb__after_atomic(), as is the usual approach when you need full ordering. ;-) Thanx, Paul > Thoughts? > > Regards, > Boqun > > > Thanx, Paul > > > > > > > what's happens there and prove that it's correct, if we use > > > > > store_release/load_acquire. > > > > > > > > Fair point, how about the following? > > > > > > > > Thanx, Paul > > > > > > > > ------------------------------------------------------------------------ > > > > > > > > commit 6fd8074f1976596898e39f5b7ea1755652533906 > > > > Author: Paul E. McKenney > > > > Date: Tue Mar 7 07:21:23 2017 -0800 > > > > > > > > rcu: Add smp_mb__after_atomic() to sync_exp_work_done() > > > > > > > > The sync_exp_work_done() function needs to fully order the counter-check > > > > operation against anything happening after the corresponding grace period. > > > > This is a theoretical bug, as all current architectures either provide > > > > full ordering for atomic operation on the one hand or implement, > > > > however, a little future-proofing is a good thing. This commit > > > > therefore adds smp_mb__after_atomic() after the atomic_long_inc() > > > > in sync_exp_work_done(). > > > > > > > > Reported-by: Dmitry Vyukov > > > > Signed-off-by: Paul E. McKenney > > > > > > > > diff --git a/kernel/rcu/tree_exp.h b/kernel/rcu/tree_exp.h > > > > index 027e123d93c7..652071abd9b4 100644 > > > > --- a/kernel/rcu/tree_exp.h > > > > +++ b/kernel/rcu/tree_exp.h > > > > @@ -247,6 +247,7 @@ static bool sync_exp_work_done(struct rcu_state *rsp, atomic_long_t *stat, > > > > /* Ensure test happens before caller kfree(). */ > > > > smp_mb__before_atomic(); /* ^^^ */ > > > > atomic_long_inc(stat); > > > > + smp_mb__after_atomic(); /* ^^^ */ > > > > > > If we really care about future-proofing, I think it's more safe to > > > change smp_mb__before_atomic() to smp_mb() rather than adding > > > __after_atomic() barrier. Though I think both would be unnecessary ;-) > > > > > > Regards, > > > Boqun > > > > > > > return true; > > > > } > > > > return false; > > > > > > > >