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Violators will be prosecuted; (version=TLSv1/SSLv3 cipher=AES256-GCM-SHA384 bits=256/256) Sat, 23 Jun 2018 13:54:32 -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 w5NHsVuQ14352750 (version=TLSv1/SSLv3 cipher=DHE-RSA-AES256-GCM-SHA384 bits=256 verify=FAIL); Sat, 23 Jun 2018 17:54:31 GMT Received: from b01ledav003.gho.pok.ibm.com (unknown [127.0.0.1]) by IMSVA (Postfix) with ESMTP id 93F35B2064; Sat, 23 Jun 2018 13:54:29 -0400 (EDT) Received: from b01ledav003.gho.pok.ibm.com (unknown [127.0.0.1]) by IMSVA (Postfix) with ESMTP id 58BB4B2066; Sat, 23 Jun 2018 13:54:29 -0400 (EDT) Received: from paulmck-ThinkPad-W541 (unknown [9.85.148.28]) by b01ledav003.gho.pok.ibm.com (Postfix) with ESMTP; Sat, 23 Jun 2018 13:54:29 -0400 (EDT) Received: by paulmck-ThinkPad-W541 (Postfix, from userid 1000) id A35AB16C41B5; Sat, 23 Jun 2018 10:56:34 -0700 (PDT) Date: Sat, 23 Jun 2018 10:56:34 -0700 From: "Paul E. McKenney" To: Joel Fernandes Cc: Steven Rostedt , Byungchul Park , Byungchul Park , jiangshanlai@gmail.com, josh@joshtriplett.org, Mathieu Desnoyers , linux-kernel@vger.kernel.org, kernel-team@lge.com, luto@kernel.org Subject: Re: [RFC 2/2] rcu: Remove ->dynticks_nmi_nesting from struct rcu_dynticks Reply-To: paulmck@linux.vnet.ibm.com References: <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> <20180622181916.GA13628@joelaf.mtv.corp.google.com> <20180622205813.GV3593@linux.vnet.ibm.com> <20180622170042.4adfbe21@gandalf.local.home> <20180622211600.GX3593@linux.vnet.ibm.com> <20180623154839.GA12939@joelaf.mtv.corp.google.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20180623154839.GA12939@joelaf.mtv.corp.google.com> User-Agent: Mutt/1.5.21 (2010-09-15) X-TM-AS-GCONF: 00 x-cbid: 18062317-0052-0000-0000-0000030315D4 X-IBM-SpamModules-Scores: X-IBM-SpamModules-Versions: BY=3.00009247; HX=3.00000241; KW=3.00000007; PH=3.00000004; SC=3.00000265; SDB=6.01051252; UDB=6.00538817; IPR=6.00829908; MB=3.00021827; MTD=3.00000008; XFM=3.00000015; UTC=2018-06-23 17:54:35 X-IBM-AV-DETECTION: SAVI=unused REMOTE=unused XFE=unused x-cbparentid: 18062317-0053-0000-0000-00005D1D939B Message-Id: <20180623175634.GI3593@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-1806230213 Sender: linux-kernel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Sat, Jun 23, 2018 at 08:48:39AM -0700, Joel Fernandes wrote: > On Fri, Jun 22, 2018 at 02:16:00PM -0700, Paul E. McKenney wrote: > > On Fri, Jun 22, 2018 at 05:00:42PM -0400, Steven Rostedt wrote: > > > On Fri, 22 Jun 2018 13:58:13 -0700 > > > "Paul E. McKenney" wrote: > > > > > > > Something like this: > > > > > > > > IRQ entered > > > > > > > > And never exited. Ever. I actually saw this in 2011. > > > > > > I still believe this was actually a bug. And perhaps you made the RCU > > > code robust enough to handle this bug ;-) > > > > Welcome to my world! > > > > But I recall it being used in several places, so if it was a bug, it > > was an intentional bug. Probably the worst kind. > > > > Sort of like nested NMIs and interrupts within NMI handlers. ;-) > > > > > > Or something like this: > > > > > > > > IRQ exited > > > > > > > > Without a corresponding IRQ enter. > > > > > > > > The current code handles both of these situations, at least assuming > > > > that the interrupt entry/exit happens during a non-idle period. > > > > > > > > > > So why this function-call structure? Well, you see, NMI handlers can > > > > > > take what appear to RCU to be normal interrupts... > > > > > > > > > > > > (And I just added that fun fact to Requirements.html.) > > > > > > > > > > Yes, I'll definitely go through all the interrupt requirements in the doc and > > > > > thanks for referring me to it. > > > > > > > > My concern may well be obsolete. It would be good if it was! ;-) > > > > > > I'd love to mandate that irq_enter() must be paired with irq_exit(). I > > > don't really see any rationale for it to be otherwise. If there is a > > > case, perhaps it needs to be fixed. > > > > Given that the usermode helpers now look to be common code using > > workqueues, kthreads, and calls to do_execve(), it might well be that > > the days of half-interrupts are behind us. > > > > But how to actually validate this? My offer of adding a WARN_ON_ONCE() > > and waiting a few years still stands, but perhaps you have a better > > approach. > > Hi Paul, I am Ok with adding a warning for a couple of releases if you and > others are Ok with it, how about something like this? Feel free to use the > diff as a starting point or a different approach if you/others prefer > something else. Thanks. A few years rather than a few releases, but yes. ;-) The checks would need to go just before the "crowbar" stores. I will put something together after Byungchul's patches in this area have had time to burn in for a few days. Thanx, Paul > ---8<----------------------- > > diff --git a/kernel/rcu/tree.c b/kernel/rcu/tree.c > index 8788ddbc0d13..176de74f5027 100644 > --- a/kernel/rcu/tree.c > +++ b/kernel/rcu/tree.c > @@ -767,6 +767,21 @@ void rcu_idle_enter(void) > */ > void rcu_user_enter(void) > { > + struct rcu_dynticks *rdtp = this_cpu_ptr(&rcu_dynticks); > + > + /* > + * Add warning to ensure: no known instances of entering userspace from > + * IRQ/NMI handlers exist today. Currently special crowbarring of the > + * dynticks_nmi_nesting and maintaining of this separate counter when > + * dynticks_nesting exists, is done in order to handle entering of > + * userspace from an IRQ context and never returning. Lets track it for > + * a couple of kernel releases and then if the warning doesn't occur, > + * we can try to simplify the code and combine/eliminate the counters. > + * See: http://lkml.kernel.org/r/20180622181422.GT3593@linux.vnet.ibm.com > + */ > + WARN_ON_ONCE(rdtp->dynticks_nmi_nesting && > + rdtp->dynticks_nmi_nesting != DYNTICK_IRQ_NONIDLE); > + > lockdep_assert_irqs_disabled(); > rcu_eqs_enter(true); > } >