From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S932757AbaCDVfO (ORCPT ); Tue, 4 Mar 2014 16:35:14 -0500 Received: from e37.co.us.ibm.com ([32.97.110.158]:39486 "EHLO e37.co.us.ibm.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1754973AbaCDVfJ (ORCPT ); Tue, 4 Mar 2014 16:35:09 -0500 Date: Tue, 4 Mar 2014 13:35:04 -0800 From: "Paul E. McKenney" To: Torvald Riegel Cc: Linus Torvalds , Will Deacon , Peter Zijlstra , Ramana Radhakrishnan , David Howells , "linux-arch@vger.kernel.org" , "linux-kernel@vger.kernel.org" , "akpm@linux-foundation.org" , "mingo@kernel.org" , "gcc@gcc.gnu.org" Subject: Re: [RFC][PATCH 0/5] arch: atomic rework Message-ID: <20140304213504.GA9809@linux.vnet.ibm.com> Reply-To: paulmck@linux.vnet.ibm.com References: <1393515453.28840.9961.camel@triegel.csb> <20140227190611.GU8264@linux.vnet.ibm.com> <20140227205312.GX8264@linux.vnet.ibm.com> <20140301005047.GA14777@linux.vnet.ibm.com> <1393872908.28840.11660.camel@triegel.csb> <20140303192026.GO11910@linux.vnet.ibm.com> <1393879579.28840.11949.camel@triegel.csb> <20140304190032.GY11910@linux.vnet.ibm.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20140304190032.GY11910@linux.vnet.ibm.com> User-Agent: Mutt/1.5.21 (2010-09-15) X-TM-AS-MML: disable X-Content-Scanned: Fidelis XPS MAILER x-cbid: 14030421-7164-0000-0000-00000007EDE3 Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Tue, Mar 04, 2014 at 11:00:32AM -0800, Paul E. McKenney wrote: > On Mon, Mar 03, 2014 at 09:46:19PM +0100, Torvald Riegel wrote: > > xagsmtp2.20140303204700.3556@vmsdvma.vnet.ibm.com > > X-Xagent-Gateway: vmsdvma.vnet.ibm.com (XAGSMTP2 at VMSDVMA) > > > > On Mon, 2014-03-03 at 11:20 -0800, Paul E. McKenney wrote: > > > On Mon, Mar 03, 2014 at 07:55:08PM +0100, Torvald Riegel wrote: > > > > xagsmtp2.20140303190831.9500@uk1vsc.vnet.ibm.com > > > > X-Xagent-Gateway: uk1vsc.vnet.ibm.com (XAGSMTP2 at UK1VSC) > > > > > > > > On Fri, 2014-02-28 at 16:50 -0800, Paul E. McKenney wrote: > > > > > +o Do not use the results from the boolean "&&" and "||" when > > > > > + dereferencing. For example, the following (rather improbable) > > > > > + code is buggy: > > > > > + > > > > > + int a[2]; > > > > > + int index; > > > > > + int force_zero_index = 1; > > > > > + > > > > > + ... > > > > > + > > > > > + r1 = rcu_dereference(i1) > > > > > + r2 = a[r1 && force_zero_index]; /* BUGGY!!! */ > > > > > + > > > > > + The reason this is buggy is that "&&" and "||" are often compiled > > > > > + using branches. While weak-memory machines such as ARM or PowerPC > > > > > + do order stores after such branches, they can speculate loads, > > > > > + which can result in misordering bugs. > > > > > + > > > > > +o Do not use the results from relational operators ("==", "!=", > > > > > + ">", ">=", "<", or "<=") when dereferencing. For example, > > > > > + the following (quite strange) code is buggy: > > > > > + > > > > > + int a[2]; > > > > > + int index; > > > > > + int flip_index = 0; > > > > > + > > > > > + ... > > > > > + > > > > > + r1 = rcu_dereference(i1) > > > > > + r2 = a[r1 != flip_index]; /* BUGGY!!! */ > > > > > + > > > > > + As before, the reason this is buggy is that relational operators > > > > > + are often compiled using branches. And as before, although > > > > > + weak-memory machines such as ARM or PowerPC do order stores > > > > > + after such branches, but can speculate loads, which can again > > > > > + result in misordering bugs. > > > > > > > > Those two would be allowed by the wording I have recently proposed, > > > > AFAICS. r1 != flip_index would result in two possible values (unless > > > > there are further constraints due to the type of r1 and the values that > > > > flip_index can have). > > > > > > And I am OK with the value_dep_preserving type providing more/better > > > guarantees than we get by default from current compilers. > > > > > > One question, though. Suppose that the code did not want a value > > > dependency to be tracked through a comparison operator. What does > > > the developer do in that case? (The reason I ask is that I have > > > not yet found a use case in the Linux kernel that expects a value > > > dependency to be tracked through a comparison.) > > > > Hmm. I suppose use an explicit cast to non-vdp before or after the > > comparison? > > That should work well assuming that things like "if", "while", and "?:" > conditions are happy to take a vdp. This assumes that p->a only returns > vdp if field "a" is declared vdp, otherwise we have vdps running wild > through the program. ;-) > > The other thing that can happen is that a vdp can get handed off to > another synchronization mechanism, for example, to reference counting: > > p = atomic_load_explicit(&gp, memory_order_consume); > if (do_something_with(p->a)) { > /* fast path protected by RCU. */ > return 0; > } > if (atomic_inc_not_zero(&p->refcnt) { > /* slow path protected by reference counting. */ > return do_something_else_with((struct foo *)p); /* CHANGE */ > } > /* Needed slow path, but raced with deletion. */ > return -EAGAIN; > > I am guessing that the cast ends the vdp. Is that the case? And here is a more elaborate example from the Linux kernel: struct md_rdev value_dep_preserving *rdev; /* CHANGE */ rdev = rcu_dereference(conf->mirrors[disk].rdev); if (r1_bio->bios[disk] == IO_BLOCKED || rdev == NULL || test_bit(Unmerged, &rdev->flags) || test_bit(Faulty, &rdev->flags)) continue; The fact that the "rdev == NULL" returns vdp does not force the "||" operators to be evaluated arithmetically because the entire function is an "if" condition, correct? Thanx, Paul