From mboxrd@z Thu Jan 1 00:00:00 1970 From: "Paul E. McKenney" Subject: Re: [RFC][PATCH 0/5] arch: atomic rework Date: Mon, 3 Mar 2014 11:20:26 -0800 Message-ID: <20140303192026.GO11910@linux.vnet.ibm.com> References: <20140224185341.GU8264@linux.vnet.ibm.com> <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> Reply-To: paulmck@linux.vnet.ibm.com Mime-Version: 1.0 Content-Type: text/plain; charset=us-ascii Return-path: List-Unsubscribe: List-Archive: List-Post: List-Help: Sender: gcc-owner@gcc.gnu.org Content-Disposition: inline In-Reply-To: <1393872908.28840.11660.camel@triegel.csb> 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" List-Id: linux-arch.vger.kernel.org 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.) > I don't think the wording is flawed. We could raise the requirement of > having more than one value left for r1 to having more than N with N > 1 > values left, but the fundamental problem remains in that a compiler > could try to generate a (big) switch statement. > > Instead, I think that this indicates that the value_dep_preserving type > modifier would be useful: It would tell the compiler that it shouldn't > transform this into a branch in this case, yet allow that optimization > for all other code. Understood! BTW, my current task is generating examples using the value_dep_preserving type for RCU-protected array indexes. Thanx, Paul From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from e39.co.us.ibm.com ([32.97.110.160]:55269 "EHLO e39.co.us.ibm.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1753942AbaCCTUb (ORCPT ); Mon, 3 Mar 2014 14:20:31 -0500 Received: from /spool/local by e39.co.us.ibm.com with IBM ESMTP SMTP Gateway: Authorized Use Only! Violators will be prosecuted for from ; Mon, 3 Mar 2014 12:20:30 -0700 Date: Mon, 3 Mar 2014 11:20:26 -0800 From: "Paul E. McKenney" Subject: Re: [RFC][PATCH 0/5] arch: atomic rework Message-ID: <20140303192026.GO11910@linux.vnet.ibm.com> Reply-To: paulmck@linux.vnet.ibm.com References: <20140224185341.GU8264@linux.vnet.ibm.com> <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> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <1393872908.28840.11660.camel@triegel.csb> Sender: linux-arch-owner@vger.kernel.org List-ID: 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" Message-ID: <20140303192026.qNaAdlZR4TeZ8ATCsJmfQV7qbzXcO5wTZw2S-1qSqWM@z> 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.) > I don't think the wording is flawed. We could raise the requirement of > having more than one value left for r1 to having more than N with N > 1 > values left, but the fundamental problem remains in that a compiler > could try to generate a (big) switch statement. > > Instead, I think that this indicates that the value_dep_preserving type > modifier would be useful: It would tell the compiler that it shouldn't > transform this into a branch in this case, yet allow that optimization > for all other code. Understood! BTW, my current task is generating examples using the value_dep_preserving type for RCU-protected array indexes. Thanx, Paul