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Violators will be prosecuted; (version=TLSv1/SSLv3 cipher=AES256-GCM-SHA384 bits=256/256) Wed, 3 Apr 2019 14:19:19 +0100 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 x33DJIC916515084 (version=TLSv1/SSLv3 cipher=DHE-RSA-AES256-GCM-SHA384 bits=256 verify=OK); Wed, 3 Apr 2019 13:19:18 GMT Received: from b01ledav003.gho.pok.ibm.com (unknown [127.0.0.1]) by IMSVA (Postfix) with ESMTP id AEC5CB2065; Wed, 3 Apr 2019 13:19:18 +0000 (GMT) Received: from b01ledav003.gho.pok.ibm.com (unknown [127.0.0.1]) by IMSVA (Postfix) with ESMTP id 79337B2066; Wed, 3 Apr 2019 13:19:18 +0000 (GMT) Received: from paulmck-ThinkPad-W541 (unknown [9.80.202.55]) by b01ledav003.gho.pok.ibm.com (Postfix) with ESMTP; Wed, 3 Apr 2019 13:19:18 +0000 (GMT) Received: by paulmck-ThinkPad-W541 (Postfix, from userid 1000) id 2344016C0EB6; Wed, 3 Apr 2019 06:19:18 -0700 (PDT) Date: Wed, 3 Apr 2019 06:19:18 -0700 From: "Paul E. McKenney" To: Joel Fernandes Cc: Mathieu Desnoyers , rcu@vger.kernel.org, linux-kernel , Ingo Molnar , Lai Jiangshan , dipankar , Andrew Morton , Josh Triplett , Thomas Gleixner , Peter Zijlstra , rostedt , David Howells , Eric Dumazet , fweisbec , Oleg Nesterov , linux-nvdimm@lists.01.org, dri-devel@lists.freedesktop.org, amd-gfx@lists.freedesktop.org Subject: Re: [PATCH RFC tip/core/rcu 0/4] Forbid static SRCU use in modules Reply-To: paulmck@linux.ibm.com References: <20190402142816.GA13084@linux.ibm.com> <886051277.1395.1554218080591.JavaMail.zimbra@efficios.com> <20190402152334.GC4102@linux.ibm.com> <20190402184054.GA18150@google.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20190402184054.GA18150@google.com> User-Agent: Mutt/1.5.21 (2010-09-15) X-TM-AS-GCONF: 00 x-cbid: 19040313-2213-0000-0000-0000036FE736 X-IBM-SpamModules-Scores: X-IBM-SpamModules-Versions: BY=3.00010867; HX=3.00000242; KW=3.00000007; PH=3.00000004; SC=3.00000284; SDB=6.01183738; UDB=6.00619751; IPR=6.00964489; MB=3.00026276; MTD=3.00000008; XFM=3.00000015; UTC=2019-04-03 13:19:24 X-IBM-AV-DETECTION: SAVI=unused REMOTE=unused XFE=unused x-cbparentid: 19040313-2214-0000-0000-00005DE35D65 Message-Id: <20190403131918.GD4102@linux.ibm.com> X-Proofpoint-Virus-Version: vendor=fsecure engine=2.50.10434:,, definitions=2019-04-03_08:,, 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=8.0.1-1810050000 definitions=main-1904030091 Sender: linux-kernel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Tue, Apr 02, 2019 at 02:40:54PM -0400, Joel Fernandes wrote: > On Tue, Apr 02, 2019 at 08:23:34AM -0700, Paul E. McKenney wrote: > > On Tue, Apr 02, 2019 at 11:14:40AM -0400, Mathieu Desnoyers wrote: > > > ----- On Apr 2, 2019, at 10:28 AM, paulmck paulmck@linux.ibm.com wrote: > > > > > > > Hello! > > > > > > > > This series prohibits use of DEFINE_SRCU() and DEFINE_STATIC_SRCU() > > > > by loadable modules. The reason for this prohibition is the fact > > > > that using these two macros within modules requires that the size of > > > > the reserved region be increased, which is not something we want to > > > > be doing all that often. Instead, loadable modules should define an > > > > srcu_struct and invoke init_srcu_struct() from their module_init function > > > > and cleanup_srcu_struct() from their module_exit function. Note that > > > > modules using call_srcu() will also need to invoke srcu_barrier() from > > > > their module_exit function. > > > > > > This arbitrary API limitation seems weird. > > > > > > Isn't there a way to allow modules to use DEFINE_SRCU and DEFINE_STATIC_SRCU > > > while implementing them with dynamic allocation under the hood ? > > > > Although call_srcu() already has initialization hooks, some would > > also be required in srcu_read_lock(), and I am concerned about adding > > memory allocation at that point, especially given the possibility > > of memory-allocation failure. And the possibility that the first > > srcu_read_lock() happens in an interrupt handler or similar. > > > > Or am I missing a trick here? > > Hi Paul, > > Which 'reserved region' are you referring to? Isn't this region also > used for non-module cases in which case the same problem applies to > non-modules? The percpu/module reservation discussed in this thread: https://lore.kernel.org/lkml/c72402f2-967e-cd56-99d8-9139c9e7f267@google.com/T/#mbcacf60ddc0b3fd6e831a3ea71efc90da359a3bf For non-modules, global per-CPU variables are statically allocated. For modules, they must be dynamically allocated at modprobe time, and their size is set by PERCPU_MODULE_RESERVE. Thanx, Paul