From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1754330AbbFRJPY (ORCPT ); Thu, 18 Jun 2015 05:15:24 -0400 Received: from casper.infradead.org ([85.118.1.10]:40123 "EHLO casper.infradead.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1752955AbbFRJPP (ORCPT ); Thu, 18 Jun 2015 05:15:15 -0400 Date: Thu, 18 Jun 2015 11:15:05 +0200 From: Peter Zijlstra To: "Paul E. McKenney" Cc: umgwanakikbuti@gmail.com, mingo@elte.hu, ktkhai@parallels.com, rostedt@goodmis.org, tglx@linutronix.de, juri.lelli@gmail.com, pang.xunlei@linaro.org, oleg@redhat.com, wanpeng.li@linux.intel.com, linux-kernel@vger.kernel.org, Al Viro , Linus Torvalds Subject: Re: [PATCH 11/18] seqcount: Introduce raw_write_seqcount_barrier() Message-ID: <20150618091505.GI19282@twins.programming.kicks-ass.net> References: <20150611124636.448700267@infradead.org> <20150611124743.374180021@infradead.org> <20150611153341.GK3913@linux.vnet.ibm.com> <20150611214557.GA4249@linux.vnet.ibm.com> <20150617122924.GP3644@twins.programming.kicks-ass.net> <20150617145712.GZ3913@linux.vnet.ibm.com> <20150617154926.GE19282@twins.programming.kicks-ass.net> <20150617163731.GD3913@linux.vnet.ibm.com> <20150617171140.GG19282@twins.programming.kicks-ass.net> <20150617180214.GJ3913@linux.vnet.ibm.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20150617180214.GJ3913@linux.vnet.ibm.com> User-Agent: Mutt/1.5.21 (2012-12-30) Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Wed, Jun 17, 2015 at 11:02:14AM -0700, Paul E. McKenney wrote: > On Wed, Jun 17, 2015 at 07:11:40PM +0200, Peter Zijlstra wrote: > > On Wed, Jun 17, 2015 at 09:37:31AM -0700, Paul E. McKenney wrote: > > > The point of std::atomic<> (and of the equivalent C11 syntax) is to > > > force the compiler to suppress optimizations that are unsafe for shared > > > variables. We get more or less the same effect with volatile, protests > > > from compiler people notwithstanding. > > > > > > I often tell the compiler guys that they have to expect make -some- > > > concessions for being 30 years late to the concurrency party, but > > > it nevertheless makes sense to future-proof our code where it is > > > reasonable to do so. > > > > Right, so in that regards I would request the compiler option (and or > > #pragma) that disables all the out-of-thin-air nonsense. > > OK. What is the form of the #pragma? If it focuses on a specific > access, we are likely to get a lot of pushback. I didn't have anything specific in mind; other than #pragma no_speculative_stores_ever Which would forbid all these retarded 'optimizations' for the entire translation unit. > > Because while they hide behind their undefined behaviour, the fact is > > that all of their machines for the past 30 odd years have been relying > > on this 'undefined' behaviour to work. This being the machines they've > > been typing their useless specs on :-) > > Maybe I can scare them into doing all their work on UP systems. ;-) > > Interestingly enough, LLVM is taking a slightly different approach. > Rather than invoke undefined behavior, they say that data races result > in random bits being loaded. Not that it makes much difference to the > health and well-being of the software, mind you... I'm not sure I follow that argument. > > I doubt there's a single OS kernel (that supports SMP configurations) > > that does not rely on a whole host of 'undefined' behaviour. > > An alternative approach would be a compiler switch (or similar) that > changed the default atomic access from SC to relaxed. Then shared > variables could be marked atomic, and normal C code could be used to > access them, but without the compiler emitting memory barriers all over > the place (yes, even on x86). See, I don;'t think that is a realistic approach. Who is going to audit our ~16 million lines of code to mark all shared variables? Or all the other existing code bases that rely on this behaviour?