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[209.132.180.67]) by mx.google.com with ESMTP id 44si11870885plc.134.2019.04.08.11.33.45; Mon, 08 Apr 2019 11:34:00 -0700 (PDT) Received-SPF: pass (google.com: best guess record for domain of linux-kernel-owner@vger.kernel.org designates 209.132.180.67 as permitted sender) client-ip=209.132.180.67; Authentication-Results: mx.google.com; dkim=pass header.i=@efficios.com header.s=default header.b=Z9uNo4Gd; spf=pass (google.com: best guess record for domain of linux-kernel-owner@vger.kernel.org designates 209.132.180.67 as permitted sender) smtp.mailfrom=linux-kernel-owner@vger.kernel.org; dmarc=pass (p=NONE sp=NONE dis=NONE) header.from=efficios.com Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1727222AbfDHRYv (ORCPT + 99 others); Mon, 8 Apr 2019 13:24:51 -0400 Received: from mail.efficios.com ([167.114.142.138]:51878 "EHLO mail.efficios.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1727064AbfDHRYv (ORCPT ); Mon, 8 Apr 2019 13:24:51 -0400 Received: from localhost (ip6-localhost [IPv6:::1]) by mail.efficios.com (Postfix) with ESMTP id 2AA751D3C11; Mon, 8 Apr 2019 13:24:49 -0400 (EDT) Received: from mail.efficios.com ([IPv6:::1]) by localhost (mail02.efficios.com [IPv6:::1]) (amavisd-new, port 10032) with ESMTP id uI0pIdqPP207; Mon, 8 Apr 2019 13:24:48 -0400 (EDT) Received: from localhost (ip6-localhost [IPv6:::1]) by mail.efficios.com (Postfix) with ESMTP id 741671D3C07; Mon, 8 Apr 2019 13:24:48 -0400 (EDT) DKIM-Filter: OpenDKIM Filter v2.10.3 mail.efficios.com 741671D3C07 DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=efficios.com; s=default; t=1554744288; bh=1M+OWNGn6B0X5UNLNWK6wBdxLxyal0HepysHXtGipeI=; h=Date:From:To:Message-ID:MIME-Version; b=Z9uNo4GdvT9LW2U+RhHvtGAHb5tWgMNSOnbnjkPhhUGBWIFFg7YQLNtkNVzekyrgf aXj3DrhS61NJPKZeCj4Tea9sqOiEGGwO+FeiRlDCnFe3qUEaD5BQOKdA4kPaEmQGvP FveqWH8WDOG5n7xTfJXmzHluB6KpqQyUW90WeKezrIg7+j9C5D+ngT7J2PzhZf879K eMJdyVzwDthqLofLr/X792WXn85MS5Yw6V7cMyF//BUsNAYp7dzIDZ/RBQhqXUU4iB 1J1EQA8VGndscpWNxtr7Cvh46XvMEEo9RFEzu/f19T89B4K0hTJmRrBiF49/PdrojV tNHmMHWDFIieA== X-Virus-Scanned: amavisd-new at efficios.com Received: from mail.efficios.com ([IPv6:::1]) by localhost (mail02.efficios.com [IPv6:::1]) (amavisd-new, port 10026) with ESMTP id ggu6RhjFBjjO; Mon, 8 Apr 2019 13:24:48 -0400 (EDT) Received: from mail02.efficios.com (mail02.efficios.com [167.114.142.138]) by mail.efficios.com (Postfix) with ESMTP id 4BDD81D3C00; Mon, 8 Apr 2019 13:24:48 -0400 (EDT) Date: Mon, 8 Apr 2019 13:24:47 -0400 (EDT) From: Mathieu Desnoyers To: paulmck Cc: "Joel Fernandes, Google" , rcu , 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 , dri-devel , amd-gfx Message-ID: <1489474416.1465.1554744287985.JavaMail.zimbra@efficios.com> In-Reply-To: <20190408154616.GO14111@linux.ibm.com> References: <20190402142816.GA13084@linux.ibm.com> <1632568795.549.1554669696728.JavaMail.zimbra@efficios.com> <20190407210718.GA6656@localhost> <20190408022728.GF14111@linux.ibm.com> <1504296005.857.1554728734661.JavaMail.zimbra@efficios.com> <20190408142230.GJ14111@linux.ibm.com> <1447252022.1166.1554734972823.JavaMail.zimbra@efficios.com> <20190408154616.GO14111@linux.ibm.com> Subject: Re: [PATCH RFC tip/core/rcu 0/4] Forbid static SRCU use in modules MIME-Version: 1.0 Content-Type: text/plain; charset=utf-8 Content-Transfer-Encoding: 7bit X-Originating-IP: [167.114.142.138] X-Mailer: Zimbra 8.8.12_GA_3794 (ZimbraWebClient - FF66 (Linux)/8.8.12_GA_3794) Thread-Topic: Forbid static SRCU use in modules Thread-Index: auZInwGCZ+N5gceWBx181eqISfjl2w== Sender: linux-kernel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org ----- On Apr 8, 2019, at 11:46 AM, paulmck paulmck@linux.ibm.com wrote: > On Mon, Apr 08, 2019 at 10:49:32AM -0400, Mathieu Desnoyers wrote: >> ----- On Apr 8, 2019, at 10:22 AM, paulmck paulmck@linux.ibm.com wrote: >> >> > On Mon, Apr 08, 2019 at 09:05:34AM -0400, Mathieu Desnoyers wrote: >> >> ----- On Apr 7, 2019, at 10:27 PM, paulmck paulmck@linux.ibm.com wrote: >> >> >> >> > On Sun, Apr 07, 2019 at 09:07:18PM +0000, Joel Fernandes wrote: >> >> >> On Sun, Apr 07, 2019 at 04:41:36PM -0400, Mathieu Desnoyers wrote: >> >> >> > >> >> >> > ----- On Apr 7, 2019, at 3:32 PM, Joel Fernandes, Google joel@joelfernandes.org >> >> >> > wrote: >> >> >> > >> >> >> > > On Sun, Apr 07, 2019 at 03:26:16PM -0400, Mathieu Desnoyers wrote: >> >> >> > >> ----- On Apr 7, 2019, at 9:59 AM, paulmck paulmck@linux.ibm.com wrote: >> >> >> > >> >> >> >> > >> > On Sun, Apr 07, 2019 at 06:39:41AM -0700, Paul E. McKenney wrote: >> >> >> > >> >> On Sat, Apr 06, 2019 at 07:06:13PM -0400, Joel Fernandes wrote: >> >> >> > >> > >> >> >> > >> > [ . . . ] >> >> >> > >> > >> >> >> > >> >> > > diff --git a/include/asm-generic/vmlinux.lds.h >> >> >> > >> >> > > b/include/asm-generic/vmlinux.lds.h >> >> >> > >> >> > > index f8f6f04c4453..c2d919a1566e 100644 >> >> >> > >> >> > > --- a/include/asm-generic/vmlinux.lds.h >> >> >> > >> >> > > +++ b/include/asm-generic/vmlinux.lds.h >> >> >> > >> >> > > @@ -338,6 +338,10 @@ >> >> >> > >> >> > > KEEP(*(__tracepoints_ptrs)) /* Tracepoints: pointer array */ \ >> >> >> > >> >> > > __stop___tracepoints_ptrs = .; \ >> >> >> > >> >> > > *(__tracepoints_strings)/* Tracepoints: strings */ \ >> >> >> > >> >> > > + . = ALIGN(8); \ >> >> >> > >> >> > > + __start___srcu_struct = .; \ >> >> >> > >> >> > > + *(___srcu_struct_ptrs) \ >> >> >> > >> >> > > + __end___srcu_struct = .; \ >> >> >> > >> >> > > } \ >> >> >> > >> >> > >> >> >> > >> >> > This vmlinux linker modification is not needed. I tested without it and srcu >> >> >> > >> >> > torture works fine with rcutorture built as a module. Putting further prints >> >> >> > >> >> > in kernel/module.c verified that the kernel is able to find the srcu structs >> >> >> > >> >> > just fine. You could squash the below patch into this one or apply it on top >> >> >> > >> >> > of the dev branch. >> >> >> > >> >> >> >> >> > >> >> Good point, given that otherwise FORTRAN named common blocks would not >> >> >> > >> >> work. >> >> >> > >> >> >> >> >> > >> >> But isn't one advantage of leaving that stuff in the RO_DATA_SECTION() >> >> >> > >> >> macro that it can be mapped read-only? Or am I suffering from excessive >> >> >> > >> >> optimism? >> >> >> > >> > >> >> >> > >> > And to answer the other question, in the case where I am suffering from >> >> >> > >> > excessive optimism, it should be a separate commit. Please see below >> >> >> > >> > for the updated original commit thus far. >> >> >> > >> > >> >> >> > >> > And may I have your Tested-by? >> >> >> > >> >> >> >> > >> Just to confirm: does the cleanup performed in the modules going >> >> >> > >> notifier end up acting as a barrier first before freeing the memory ? >> >> >> > >> If not, is it explicitly stated that a barrier must be issued before >> >> >> > >> module unload ? >> >> >> > >> >> >> >> > > >> >> >> > > You mean rcu_barrier? It is mentioned in the documentation that this is the >> >> >> > > responsibility of the module writer to prevent delays for all modules. >> >> >> > >> >> >> > It's a srcu barrier yes. Considering it would be a barrier specific to the >> >> >> > srcu domain within that module, I don't see how it would cause delays for >> >> >> > "all" modules if we implicitly issue the barrier on module unload. What >> >> >> > am I missing ? >> >> >> >> >> >> Yes you are right. I thought of this after I just sent my email. I think it >> >> >> makes sense for srcu case to do and could avoid a class of bugs. >> >> > >> >> > If there are call_srcu() callbacks outstanding, the module writer still >> >> > needs the srcu_barrier() because otherwise callbacks arrive after >> >> > the module text has gone, which will be disappoint the CPU when it >> >> > tries fetching instructions that are no longer mapped. If there are >> >> > no call_srcu() callbacks from that module, then there is no need for >> >> > srcu_barrier() either way. >> >> > >> >> > So if an srcu_barrier() is needed, the module developer needs to >> >> > supply it. >> >> >> >> When you say "callbacks arrive after the module text has gone", >> >> I think you assume that free_module() is invoked before the >> >> MODULE_STATE_GOING notifiers are called. But it's done in the >> >> opposite order: going notifiers are called first, and then >> >> free_module() is invoked. >> >> >> >> So AFAIU it would be safe to issue the srcu_barrier() from the module >> >> going notifier. >> >> >> >> Or am I missing something ? >> > >> > We do seem to be talking past each other. ;-) >> > >> > This has nothing to do with the order of events at module-unload time. >> > >> > So please let me try again. >> > >> > If a given srcu_struct in a module never has call_srcu() invoked, there >> > is no need to invoke rcu_barrier() at any time, whether at module-unload >> > time or not. Adding rcu_barrier() in this case adds overhead and latency >> > for no good reason. >> >> Not if we invoke srcu_barrier() for that specific domain. If >> call_srcu was never invoked for a srcu domain, I don't see why >> srcu_barrier() should be more expensive than a simple check that >> the domain does not have any srcu work queued. > > But that simple check does involve a cache miss for each possible CPU (not > just each online CPU), so it is non-trivial, especially on large systems. > >> > If a given srcu_struct in a module does have at least one call_srcu() >> > invoked, it is already that module's responsibility to make sure that >> > the code sticks around long enough for the callback to be invoked. >> >> I understand that when users do explicit dynamic allocation/cleanup of >> srcu domains, they indeed need to take care of doing explicit srcu_barrier(). >> However, if they do static definition of srcu domains, it would be nice >> if we can handle the barriers under the hood. > > All else being equal, of course. But... > >> > This means that correct SRCU users that invoke call_srcu() already >> > have srcu_barrier() at module-unload time. Incorrect SRCU users, with >> > reasonable probability, now get a WARN_ON() at module-unload time, with >> > the per-CPU state getting leaked. Before this change, they would (also >> > with reasonable probability) instead get an instruction-fetch fault when >> > the SRCU callback was invoked after the completion of the module unload. >> > Furthermore, in all cases where they would previously have gotten the >> > instruction-fetch fault, they now get the WARN_ON(), like this: >> > >> > if (WARN_ON(rcu_segcblist_n_cbs(&sdp->srcu_cblist))) >> > return; /* Forgot srcu_barrier(), so just leak it! */ >> > >> > So this change already represents an improvement in usability. >> >> Considering that we can do a srcu_barrier() for the specific domain, >> and that it should add no noticeable overhead if there is no queued >> callbacks, I don't see a good reason for leaving the srcu_barrier >> invocation to the user rather than implicitly doing it from the >> module going notifier. > > Now, I could automatically add an indicator of whether or not a > call_srcu() had happened, but then again, that would either add a > call_srcu() scalability bottleneck or again require a scan of all possible > CPUs... to figure out if it was necessary to scan all possible CPUs. > > Or is scanning all possible CPUs down in the noise in this case? Or > am I missing a trick that would reduce the overhead? Module unloading implicitly does a synchronize_rcu (for RCU-sched), and a stop_machine. So I would be tempted to say that overhead of iteration over all CPUs might not matter that much considering the rest. About notifying that a call_srcu has happened for the srcu domain in a scalable fashion, let's see... We could have a flag "call_srcu_used" for each call_srcu domain. Whenever call_srcu is invoked, it would load that flag. It sets it on first use. The idea here is to only use that flag when srcu_barrier is performed right before the srcu domain cleanup (it could become part of that cleanup). Else, using it in all srcu_barrier() might be tricky, because we may then need to add memory barriers or locking to the call_srcu fast-path, which is an overhead we try to avoid. However, if we only use that flag as part of the srcu domain cleanup, it's already prohibited to invoke call_srcu concurrently with the cleanup of the same domain, so I don't think we would need any memory barriers in call_srcu. Thoughts ? Thanks, Mathieu -- Mathieu Desnoyers EfficiOS Inc. http://www.efficios.com