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Violators will be prosecuted; (version=TLSv1/SSLv3 cipher=AES256-GCM-SHA384 bits=256/256) Fri, 22 Jun 2018 17:13:58 -0400 Received: from b01ledav003.gho.pok.ibm.com (b01ledav003.gho.pok.ibm.com [9.57.199.108]) by b01cxnp23032.gho.pok.ibm.com (8.14.9/8.14.9/NCO v10.0) with ESMTP id w5MLDvmR13107536 (version=TLSv1/SSLv3 cipher=DHE-RSA-AES256-GCM-SHA384 bits=256 verify=FAIL); Fri, 22 Jun 2018 21:13:57 GMT Received: from b01ledav003.gho.pok.ibm.com (unknown [127.0.0.1]) by IMSVA (Postfix) with ESMTP id AB386B2065; Fri, 22 Jun 2018 17:13:56 -0400 (EDT) Received: from b01ledav003.gho.pok.ibm.com (unknown [127.0.0.1]) by IMSVA (Postfix) with ESMTP id 84E9EB2064; Fri, 22 Jun 2018 17:13:56 -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 17:13:56 -0400 (EDT) Received: by paulmck-ThinkPad-W541 (Postfix, from userid 1000) id 4F02416C3BD3; Fri, 22 Jun 2018 14:16:00 -0700 (PDT) Date: Fri, 22 Jun 2018 14:16:00 -0700 From: "Paul E. McKenney" To: Steven Rostedt Cc: Joel Fernandes , 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: <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> <20180622181916.GA13628@joelaf.mtv.corp.google.com> <20180622205813.GV3593@linux.vnet.ibm.com> <20180622170042.4adfbe21@gandalf.local.home> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20180622170042.4adfbe21@gandalf.local.home> User-Agent: Mutt/1.5.21 (2010-09-15) X-TM-AS-GCONF: 00 x-cbid: 18062221-0064-0000-0000-0000031F200D X-IBM-SpamModules-Scores: X-IBM-SpamModules-Versions: BY=3.00009242; HX=3.00000241; KW=3.00000007; PH=3.00000004; SC=3.00000265; SDB=6.01050840; UDB=6.00538568; IPR=6.00829811; MB=3.00021811; MTD=3.00000008; XFM=3.00000015; UTC=2018-06-22 21:14:01 X-IBM-AV-DETECTION: SAVI=unused REMOTE=unused XFE=unused x-cbparentid: 18062221-0065-0000-0000-000039AF004A Message-Id: <20180622211600.GX3593@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=874 adultscore=0 classifier=spam adjust=0 reason=mlx scancount=1 engine=8.0.1-1806210000 definitions=main-1806220234 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 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. Thanx, Paul