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Violators will be prosecuted; (version=TLSv1/SSLv3 cipher=AES256-GCM-SHA384 bits=256/256) Wed, 8 Aug 2018 09:03:03 -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 w78D32C715401344 (version=TLSv1/SSLv3 cipher=DHE-RSA-AES256-GCM-SHA384 bits=256 verify=FAIL); Wed, 8 Aug 2018 13:03:02 GMT Received: from b01ledav003.gho.pok.ibm.com (unknown [127.0.0.1]) by IMSVA (Postfix) with ESMTP id 0DE4BB2068; Wed, 8 Aug 2018 09:02:26 -0400 (EDT) Received: from b01ledav003.gho.pok.ibm.com (unknown [127.0.0.1]) by IMSVA (Postfix) with ESMTP id E75DDB205F; Wed, 8 Aug 2018 09:02:25 -0400 (EDT) Received: from paulmck-ThinkPad-W541 (unknown [9.80.212.138]) by b01ledav003.gho.pok.ibm.com (Postfix) with ESMTP; Wed, 8 Aug 2018 09:02:25 -0400 (EDT) Received: by paulmck-ThinkPad-W541 (Postfix, from userid 1000) id 35BB816C0722; Wed, 8 Aug 2018 06:03:02 -0700 (PDT) Date: Wed, 8 Aug 2018 06:03:02 -0700 From: "Paul E. McKenney" To: Steven Rostedt Cc: Joel Fernandes , Joel Fernandes , LKML , "Cc: Android Kernel" , Boqun Feng , Byungchul Park , Ingo Molnar , Masami Hiramatsu , Mathieu Desnoyers , Namhyung Kim , Peter Zijlstra , Thomas Glexiner , Tom Zanussi Subject: Re: [PATCH v12 3/3] tracing: Centralize preemptirq tracepoints and unify their usage Reply-To: paulmck@linux.vnet.ibm.com References: <20180807194515.4e549c1a@gandalf.local.home> <6D0A3FD6-2190-4CC0-A3C0-7B3759E73243@google.com> <20180807204820.50b83c6d@vmware.local.home> <20180807215522.04114097@vmware.local.home> <20180807222856.3ede96e7@vmware.local.home> <20180808084629.3290d1d6@gandalf.local.home> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20180808084629.3290d1d6@gandalf.local.home> User-Agent: Mutt/1.5.21 (2010-09-15) X-TM-AS-GCONF: 00 x-cbid: 18080813-0052-0000-0000-0000031B6A7D X-IBM-SpamModules-Scores: X-IBM-SpamModules-Versions: BY=3.00009507; HX=3.00000242; KW=3.00000007; PH=3.00000004; SC=3.00000266; SDB=6.01070961; UDB=6.00551345; IPR=6.00850480; MB=3.00022588; MTD=3.00000008; XFM=3.00000015; UTC=2018-08-08 13:03:06 X-IBM-AV-DETECTION: SAVI=unused REMOTE=unused XFE=unused x-cbparentid: 18080813-0053-0000-0000-00005DA82F7E Message-Id: <20180808130302.GJ24813@linux.vnet.ibm.com> X-Proofpoint-Virus-Version: vendor=fsecure engine=2.50.10434:,, definitions=2018-08-08_04:,, 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=643 adultscore=0 classifier=spam adjust=0 reason=mlx scancount=1 engine=8.0.1-1807170000 definitions=main-1808080135 Sender: linux-kernel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Wed, Aug 08, 2018 at 08:46:29AM -0400, Steven Rostedt wrote: > On Tue, 7 Aug 2018 20:53:54 -0700 > Joel Fernandes wrote: > > > > > When I talked to Paul few months ago about SRCU from NMI context, he > > > mentioned the per-cpu memory operations during srcu_read_lock can be > > > NMI interrupted, that's why we added that warning. > > > > So I looked more closely, __srcu_read_lock on 2 different handles may > > still be doing a this_cpu_inc on the same location.. > > (sp->sda->srcu_lock_count). :-( > > > > Paul any ideas on how to solve this? > > > > It does start to seem like a show stopper :-( > > What's wrong with a this_cpu_inc()? It's atomic for the CPU. Although > it wont be atomic for the capture of the idx. But I also don't see > interrupts being disabled, thus an NMI is no different than any > interrupt doing the same thing, right? On architectures without increment-memory instructions, if you take an NMI between the load from sp->sda->srcu_lock_count and the later store, you lose a count. Note that both __srcu_read_lock() and __srcu_read_unlock() do increments of different locations, so you cannot rely on the usual "NMI fixes up before exit" semantics you get when incrementing and decrementing the same location. Thanx, Paul