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Violators will be prosecuted; (version=TLSv1/SSLv3 cipher=AES256-GCM-SHA384 bits=256/256) Mon, 19 Aug 2019 03:29:26 +0100 Received: from b01ledav003.gho.pok.ibm.com (b01ledav003.gho.pok.ibm.com [9.57.199.108]) by b01cxnp22034.gho.pok.ibm.com (8.14.9/8.14.9/NCO v10.0) with ESMTP id x7J2TPGJ51249522 (version=TLSv1/SSLv3 cipher=DHE-RSA-AES256-GCM-SHA384 bits=256 verify=OK); Mon, 19 Aug 2019 02:29:25 GMT Received: from b01ledav003.gho.pok.ibm.com (unknown [127.0.0.1]) by IMSVA (Postfix) with ESMTP id 6FE0BB2064; Mon, 19 Aug 2019 02:29:25 +0000 (GMT) Received: from b01ledav003.gho.pok.ibm.com (unknown [127.0.0.1]) by IMSVA (Postfix) with ESMTP id 3BB24B2065; Mon, 19 Aug 2019 02:29:25 +0000 (GMT) Received: from paulmck-ThinkPad-W541 (unknown [9.85.201.199]) by b01ledav003.gho.pok.ibm.com (Postfix) with ESMTP; Mon, 19 Aug 2019 02:29:25 +0000 (GMT) Received: by paulmck-ThinkPad-W541 (Postfix, from userid 1000) id EB73D16C13AF; Sun, 18 Aug 2019 19:29:27 -0700 (PDT) Date: Sun, 18 Aug 2019 19:29:27 -0700 From: "Paul E. McKenney" To: Joel Fernandes Cc: linux-kernel@vger.kernel.org, Josh Triplett , Lai Jiangshan , Mathieu Desnoyers , rcu@vger.kernel.org, Steven Rostedt Subject: Re: [RFC v2] rcu/tree: Try to invoke_rcu_core() if in_irq() during unlock Reply-To: paulmck@linux.ibm.com References: <20190818214948.GA134430@google.com> <20190818221210.GP28441@linux.ibm.com> <20190818223230.GA143857@google.com> <20190818223511.GB143857@google.com> <20190818233135.GQ28441@linux.ibm.com> <20190818233839.GA160903@google.com> <20190819012153.GR28441@linux.ibm.com> <20190819014143.GB160903@google.com> <20190819014623.GC160903@google.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20190819014623.GC160903@google.com> User-Agent: Mutt/1.5.21 (2010-09-15) X-TM-AS-GCONF: 00 x-cbid: 19081902-2213-0000-0000-000003BD00A5 X-IBM-SpamModules-Scores: X-IBM-SpamModules-Versions: BY=3.00011613; HX=3.00000242; KW=3.00000007; PH=3.00000004; SC=3.00000287; SDB=6.01248860; UDB=6.00659224; IPR=6.01030386; MB=3.00028226; MTD=3.00000008; XFM=3.00000015; UTC=2019-08-19 02:29:28 X-IBM-AV-DETECTION: SAVI=unused REMOTE=unused XFE=unused x-cbparentid: 19081902-2214-0000-0000-00005FB215AD Message-Id: <20190819022927.GS28441@linux.ibm.com> X-Proofpoint-Virus-Version: vendor=fsecure engine=2.50.10434:,, definitions=2019-08-19_01:,, signatures=0 X-Proofpoint-Spam-Details: rule=outbound_notspam policy=outbound score=0 priorityscore=1501 malwarescore=0 suspectscore=0 phishscore=0 bulkscore=0 spamscore=0 clxscore=1015 lowpriorityscore=0 mlxscore=0 impostorscore=0 mlxlogscore=999 adultscore=0 classifier=spam adjust=0 reason=mlx scancount=1 engine=8.0.1-1906280000 definitions=main-1908190026 Sender: linux-kernel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Sun, Aug 18, 2019 at 09:46:23PM -0400, Joel Fernandes wrote: > On Sun, Aug 18, 2019 at 09:41:43PM -0400, Joel Fernandes wrote: > > On Sun, Aug 18, 2019 at 06:21:53PM -0700, Paul E. McKenney wrote: > [snip] > > > > > Also, your commit log's point #2 is "in_irq() implies in_interrupt() > > > > > which implies raising softirq will not do any wake ups." This mention > > > > > of softirq seems a bit odd, given that we are going to wake up a rcuc > > > > > kthread. Of course, this did nothing to quell my suspicions. ;-) > > > > > > > > Yes, I should delete this #2 from the changelog since it is not very relevant > > > > (I feel now). My point with #2 was that even if were to raise a softirq > > > > (which we are not), a scheduler wakeup of ksoftirqd is impossible in this > > > > path anyway since in_irq() implies in_interrupt(). > > > > > > Please! Could you also add a first-principles explanation of why > > > the added condition is immune from scheduler deadlocks? > > > > Sure I can add an example in the change log, however I was thinking of this > > example which you mentioned: > > https://lore.kernel.org/lkml/20190627173831.GW26519@linux.ibm.com/ > > > > previous_reader() > > { > > rcu_read_lock(); > > do_something(); /* Preemption happened here. */ > > local_irq_disable(); /* Cannot be the scheduler! */ > > do_something_else(); > > rcu_read_unlock(); /* Must defer QS, task still queued. */ > > do_some_other_thing(); > > local_irq_enable(); > > } > > > > current_reader() /* QS from previous_reader() is still deferred. */ > > { > > local_irq_disable(); /* Might be the scheduler. */ > > do_whatever(); > > rcu_read_lock(); > > do_whatever_else(); > > rcu_read_unlock(); /* Must still defer reporting QS. */ > > do_whatever_comes_to_mind(); > > local_irq_enable(); > > } > > > > One modification of the example could be, previous_reader() could also do: > > previous_reader() > > { > > rcu_read_lock(); > > do_something_that_takes_really_long(); /* causes need_qs in > > the unlock_special_union to be set */ > > local_irq_disable(); /* Cannot be the scheduler! */ > > do_something_else(); > > rcu_read_unlock(); /* Must defer QS, task still queued. */ > > do_some_other_thing(); > > local_irq_enable(); > > } > > The point you were making in that thread being, current_reader() -> > rcu_read_unlock() -> rcu_read_unlock_special() would not do any wakeups > because previous_reader() sets the deferred_qs bit. > > Anyway, I will add all of this into the changelog. Examples are good, but what makes it so that there are no examples of its being unsafe? And a few questions along the way, some quick quiz, some more serious. Would it be safe if it checked in_interrupt() instead of in_irq()? If not, should the in_interrupt() in the "if" condition preceding the added "else if" be changed to in_irq()? Would it make sense to add an "|| !irqs_were_disabled" do your new "else if" condition? Would the body of the "else if" actually be executed in current mainline? In an attempt to be at least a little constructive, I am doing some testing of this patch overnight, along with a WARN_ON_ONCE() to see if that invoke_rcu_core() is ever reached. Thanx, Paul