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, 24 Feb 2014 14:37:02 -0800 Message-ID: <20140224223701.GC8264@linux.vnet.ibm.com> References: <20140223003933.GQ4250@linux.vnet.ibm.com> <20140223063426.GT4250@linux.vnet.ibm.com> <20140224172110.GO8264@linux.vnet.ibm.com> <20140224185341.GU8264@linux.vnet.ibm.com> Reply-To: paulmck@linux.vnet.ibm.com Mime-Version: 1.0 Content-Type: text/plain; charset=us-ascii Return-path: Received: from e33.co.us.ibm.com ([32.97.110.151]:60814 "EHLO e33.co.us.ibm.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1752302AbaBXWhH (ORCPT ); Mon, 24 Feb 2014 17:37:07 -0500 Received: from /spool/local by e33.co.us.ibm.com with IBM ESMTP SMTP Gateway: Authorized Use Only! Violators will be prosecuted for from ; Mon, 24 Feb 2014 15:37:06 -0700 Content-Disposition: inline In-Reply-To: Sender: linux-arch-owner@vger.kernel.org List-ID: To: Linus Torvalds Cc: Torvald Riegel , 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, Feb 24, 2014 at 11:54:46AM -0800, Linus Torvalds wrote: > On Mon, Feb 24, 2014 at 10:53 AM, Paul E. McKenney > wrote: > > > > Good points. How about the following replacements? > > > > 3. Adding or subtracting an integer to/from a chained pointer > > results in another chained pointer in that same pointer chain. > > The results of addition and subtraction operations that cancel > > the chained pointer's value (for example, "p-(long)p" where "p" > > is a pointer to char) are implementation defined. > > > > 4. Bitwise operators ("&", "|", "^", and I suppose also "~") > > applied to a chained pointer and an integer for the purposes > > of alignment and pointer translation results in another > > chained pointer in that same pointer chain. Other uses > > of bitwise operators on chained pointers (for example, > > "p|~0") are implementation defined. > > Quite frankly, I think all of this language that is about the actual > operations is irrelevant and wrong. > > It's not going to help compiler writers, and it sure isn't going to > help users that read this. > > Why not just talk about "value chains" and that any operations that > restrict the value range severely end up breaking the chain. There is > no point in listing the operations individually, because every single > operation *can* restrict things. Listing individual operations and > depdendencies is just fundamentally wrong. > > For example, let's look at this obvious case: > > int q,*p = atomic_read(&pp, consume); > .. nothing modifies 'p' .. > q = *p; > > and there are literally *zero* operations that modify the value > change, so obviously the two operations are ordered, right? > > Wrong. > > What if the "nothing modifies 'p'" part looks like this: > > if (p != &myvariable) > return; > > and now any sane compiler will happily optimize "q = *p" into "q = > myvariable", and we're all done - nothing invalid was ever Yes, the compiler could do that. But it would still be required to carry a dependency from the memory_order_consume read to the "*p", which it could do by compiling "q = *p" rather than "q = myvariable" on the one hand or by emitting a memory-barrier instruction on the other. This was the point of #12: 12. A memory_order_consume load carries a dependency to any dereference operator (unary "*", "[]", and "->") in the resulting pointer chain. Thanx, Paul