From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1752995AbaCGSeI (ORCPT ); Fri, 7 Mar 2014 13:34:08 -0500 Received: from mx1.redhat.com ([209.132.183.28]:44728 "EHLO mx1.redhat.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1751646AbaCGSeF (ORCPT ); Fri, 7 Mar 2014 13:34:05 -0500 Subject: Re: [RFC][PATCH 0/5] arch: atomic rework From: Torvald Riegel To: paulmck@linux.vnet.ibm.com 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" In-Reply-To: <20140305181537.GM3334@linux.vnet.ibm.com> References: <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> <20140304213504.GA9809@linux.vnet.ibm.com> <1394038499.28840.14979.camel@triegel.csb> <20140305181537.GM3334@linux.vnet.ibm.com> Content-Type: text/plain; charset="UTF-8" Date: Fri, 07 Mar 2014 19:33:25 +0100 Message-ID: <1394217205.28840.19576.camel@triegel.csb> Mime-Version: 1.0 Content-Transfer-Encoding: 7bit Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Wed, 2014-03-05 at 10:15 -0800, Paul E. McKenney wrote: > On Wed, Mar 05, 2014 at 05:54:59PM +0100, Torvald Riegel wrote: > > On Tue, 2014-03-04 at 13:35 -0800, Paul E. McKenney wrote: > > > 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? > > > > That's a good question, and one that as far as I understand currently, > > essentially boils down to whether we want to have tight restrictions on > > which operations are still vdp. > > > > If we look at the different combinations, then it seems we can't decide > > on whether we have a value-dependency just due to a vdp type: > > * non-vdp || vdp: vdp iff non-vdp == false > > * vdp || non-vdp: vdp iff non-vdp == false? > > * vdp || vdp: always vdp? (and dependency on both?) > > > > I'm not sure it makes sense to try to not make all of those > > vdp-by-default. The first and second case show that it's dependent on > > the specific execution anyway, and thus is already covered by the > > requirement that the value must still matter. The vdp type is just a > > way to prevent inappropriate compiler optimizations; it's not critical > > for correctness is we make more stuff vdp, yet it may prevent some > > optimizations in the affected expression. > > > > If the compiler knows that some vdp-typed evaluation will not have a > > value-dependency anyway, then it can just optimize this evaluation like > > non-vdp code. > > > > I guess not much would change for the code you posted, because we > > already have to evaluate || operands in order, I believe (e.g., don't > > access rdev->flags before doing the rdev == NULL check, modulo as-if). > > Do I understand your question correctly? > > Let me give an example for the other side: > > struct foo value_dep_preserving *p; > struct foo value_dep_preserving *q; > > p = rcu_dereference(gp); > q = rcu_dereference(gq); > return myarray[p || q]]; /* Linux kernel doesn't do this. */ > > If we wanted this to work (and I am not at all convinced that we do), > the compiler would have to force a data dependency through the "||". Yes. > But I would be just as happy to instead just say that boolean logical > operators ("||" and "&&") never return vdp values. I think those aren't actually the problem (or if they were, we'd need to think about & and | on 1-bit integers or bitfields as well), but ... > Ditto for the > relational operators ("==", "!=", ">", ">=", "<", and "<="). No one > seems to rely on value dependencies via these operators, after all, > and preserving value dependencies through them seems to require that > the compiler generate odd code. ... that any conversion from vdp to bool requires specialized handling by the compiler. That happens on implicit conversion (as in "p || q") and in the operators you mentioned. I don't see a reason why conversion to bool (or any other operator returning bool and taking vdp as operand) should be *always* non-vdp. But it seems it would be easier to misuse than other operators.