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Violators will be prosecuted; (version=TLSv1/SSLv3 cipher=AES256-GCM-SHA384 bits=256/256) Fri, 22 Jun 2018 12:10:04 -0400 Received: from b01ledav003.gho.pok.ibm.com (b01ledav003.gho.pok.ibm.com [9.57.199.108]) by b01cxnp23033.gho.pok.ibm.com (8.14.9/8.14.9/NCO v10.0) with ESMTP id w5MGA3pf9503052 (version=TLSv1/SSLv3 cipher=DHE-RSA-AES256-GCM-SHA384 bits=256 verify=FAIL); Fri, 22 Jun 2018 16:10:03 GMT Received: from b01ledav003.gho.pok.ibm.com (unknown [127.0.0.1]) by IMSVA (Postfix) with ESMTP id 89A82B2068; Fri, 22 Jun 2018 12:10:02 -0400 (EDT) Received: from b01ledav003.gho.pok.ibm.com (unknown [127.0.0.1]) by IMSVA (Postfix) with ESMTP id 51965B2065; Fri, 22 Jun 2018 12:10:02 -0400 (EDT) Received: from paulmck-ThinkPad-W541 (unknown [9.70.82.159]) by b01ledav003.gho.pok.ibm.com (Postfix) with ESMTP; Fri, 22 Jun 2018 12:10:02 -0400 (EDT) Received: by paulmck-ThinkPad-W541 (Postfix, from userid 1000) id 8522C16C429C; Fri, 22 Jun 2018 09:12:05 -0700 (PDT) Date: Fri, 22 Jun 2018 09:12:05 -0700 From: "Paul E. McKenney" To: Andy Lutomirski Cc: joel@joelfernandes.org, max.byungchul.park@gmail.com, Byungchul Park , Lai Jiangshan , Josh Triplett , Steven Rostedt , Mathieu Desnoyers , LKML , kernel-team@lge.com Subject: Re: [RFC 2/2] rcu: Remove ->dynticks_nmi_nesting from struct rcu_dynticks Reply-To: paulmck@linux.vnet.ibm.com References: <1529484440-20634-1-git-send-email-byungchul.park@lge.com> <1529484440-20634-2-git-send-email-byungchul.park@lge.com> <20180620145814.GQ3593@linux.vnet.ibm.com> <20180620164902.GW3593@linux.vnet.ibm.com> <20180622055659.GA255098@joelaf.mtv.corp.google.com> <20180622132843.GN3593@linux.vnet.ibm.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: User-Agent: Mutt/1.5.21 (2010-09-15) X-TM-AS-GCONF: 00 x-cbid: 18062216-0040-0000-0000-00000443ED4A X-IBM-SpamModules-Scores: X-IBM-SpamModules-Versions: BY=3.00009239; HX=3.00000241; KW=3.00000007; PH=3.00000004; SC=3.00000265; SDB=6.01050739; UDB=6.00538508; IPR=6.00829709; MB=3.00021808; MTD=3.00000008; XFM=3.00000015; UTC=2018-06-22 16:10:06 X-IBM-AV-DETECTION: SAVI=unused REMOTE=unused XFE=unused x-cbparentid: 18062216-0041-0000-0000-0000084A0026 Message-Id: <20180622161205.GQ3593@linux.vnet.ibm.com> X-Proofpoint-Virus-Version: vendor=fsecure engine=2.50.10434:,, definitions=2018-06-22_03:,, 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=8.0.1-1806210000 definitions=main-1806220180 Sender: linux-kernel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Fri, Jun 22, 2018 at 07:19:13AM -0700, Andy Lutomirski wrote: > On Fri, Jun 22, 2018 at 6:26 AM Paul E. McKenney > wrote: > > > > On Thu, Jun 21, 2018 at 10:56:59PM -0700, Joel Fernandes wrote: > > > Hi Paul, > > > > > > On Wed, Jun 20, 2018 at 09:49:02AM -0700, Paul E. McKenney wrote: > > > > On Thu, Jun 21, 2018 at 01:05:22AM +0900, Byungchul Park wrote: > > > > > On Wed, Jun 20, 2018 at 11:58 PM, Paul E. McKenney > > > > > wrote: > > > > > > On Wed, Jun 20, 2018 at 05:47:20PM +0900, Byungchul Park wrote: > > > > > >> Hello folks, > > > > > >> > > > > > >> I'm careful in saying that ->dynticks_nmi_nesting can be removed but I > > > > > >> think it's possible since the only thing we are interested in with > > > > > >> regard to ->dynticks_nesting or ->dynticks_nmi_nesting is whether rcu is > > > > > >> idle or not. > > > > > > > > > > > > Please keep in mind that NMIs cannot be masked, which means that the > > > > > > rcu_nmi_enter() and rcu_nmi_exit() pair can be invoked at any point in > > > > > > the process, between any consecutive pair of instructions. The saving > > > > > > > > And yes, I should have looked at this patch more closely before replying. > > > > But please see below. > > > > > > > > > I believe I understand what NMI is and why you introduced > > > > > ->dynticks_nmi_nesting. Or am I missing something? > > > > > > > > Perhaps the fact that there are architectures that can enter interrupt > > > > handlers and never leave them when the CPU is non-idle. One example of > > > > this is the usermode upcalls in the comment that you removed. > > > > > > I spent some time tonight and last night trying to understand this concept of > > > never leaving an interrupt, I hope you don't mind me asking this dumb > > > question... perhaps I will learn something : Could you let me know how is it > > > possible that an interrupt never exits? > > > > > > Typically an interrupt never exiting sounds like a hard-lockup. This is how > > > hardlock detector works: Since regular interrupts in linux can't nest, the > > > hardlockup detector checks if hrtimer interrupts are being handled and if > > > not, then it throws a splat, panics the kernel etc. So I am a bit troubled by > > > this interrupt never exiting concept.. > > > > > > Further since an interrupt is an atomic context, it cannot sleep or schedule > > > into usermode so how are these upcalls handled from the interrupt? > > > > It has been some years since I traced the code flow, but what happened > > back then is that it switches itself from an interrupt handler to not > > without actually returning from the interrupt. This can only happen when > > interrupting a non-idle process, thankfully, and RCU's dyntick-idle code > > relies on this restriction. If I remember correctly, the code ends up > > executing in the context of the interrupted process, but it has been some > > years, so please apply appropriate skepticism. > > ... > > > > > I have never seen NMIs be unpaired or improperly nested. However, > > given that rcu_irq_enter() invokes rcu_nmi_enter() and rcu_irq_exit() > > invokes rcu_nmi_exit(), it is definitely the case that rcu_nmi_enter() > > and rcu_nmi_exit() need to deal with unpaired and improperly nested > > invocations. > > This is very strange. There are certainly cases in x86 where an > interrupt-ish code path can become less interrupt-ish without > returning (killing a task that overflows a kernel stack is an > example), but the RCU calls should still nest correctly. Do you know > the history of this requirement? I believe that they are called "usermode helpers", and are (were?) used on a number of architectures to implement system calls from within the kernel. Thanx, Paul