From mboxrd@z Thu Jan 1 00:00:00 1970 From: "Paul E. McKenney" Subject: Re: [RFC][PATCH 0/5] arch: atomic rework Date: Tue, 4 Mar 2014 11:00:32 -0800 Message-ID: <20140304190032.GY11910@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> Reply-To: paulmck@linux.vnet.ibm.com Mime-Version: 1.0 Content-Type: text/plain; charset=us-ascii Return-path: Received: from e31.co.us.ibm.com ([32.97.110.149]:59139 "EHLO e31.co.us.ibm.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1753999AbaCDTAi (ORCPT ); Tue, 4 Mar 2014 14:00:38 -0500 Received: from /spool/local by e31.co.us.ibm.com with IBM ESMTP SMTP Gateway: Authorized Use Only! Violators will be prosecuted for from ; Tue, 4 Mar 2014 12:00:38 -0700 Content-Disposition: inline In-Reply-To: <1393879579.28840.11949.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" 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? Thanx, Paul