From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from mail-pl1-f198.google.com (mail-pl1-f198.google.com [209.85.214.198]) by kanga.kvack.org (Postfix) with ESMTP id 5BF0F8E0095 for ; Tue, 11 Dec 2018 12:11:22 -0500 (EST) Received: by mail-pl1-f198.google.com with SMTP id a9so11013342pla.2 for ; Tue, 11 Dec 2018 09:11:22 -0800 (PST) Received: from mail-sor-f41.google.com (mail-sor-f41.google.com. [209.85.220.41]) by mx.google.com with SMTPS id 67sor22829968pgb.68.2018.12.11.09.11.20 for (Google Transport Security); Tue, 11 Dec 2018 09:11:20 -0800 (PST) Content-Type: text/plain; charset=us-ascii Mime-Version: 1.0 (Mac OS X Mail 12.1 \(3445.101.1\)) Subject: Re: Should this_cpu_read() be volatile? From: Nadav Amit In-Reply-To: <20181210085532.GG5289@hirez.programming.kicks-ass.net> Date: Tue, 11 Dec 2018 09:11:17 -0800 Content-Transfer-Encoding: 7bit Message-Id: <058624AF-3933-4C44-A137-E33FC5180B86@gmail.com> References: <20181203224920.GQ10377@bombadil.infradead.org> <20181206102559.GG13538@hirez.programming.kicks-ass.net> <55B665E1-3F64-4D87-B779-D1B4AFE719A9@gmail.com> <20181207084550.GA2237@hirez.programming.kicks-ass.net> <20181208105220.GF5289@hirez.programming.kicks-ass.net> <5DE00B41-835C-4E68-B192-2A3C7ACB4392@gmail.com> <20181210085532.GG5289@hirez.programming.kicks-ass.net> Sender: owner-linux-mm@kvack.org List-ID: To: Peter Zijlstra Cc: Matthew Wilcox , Vlastimil Babka , Linux-MM , LKML , X86 ML , Ingo Molnar , Thomas Gleixner , Andy Lutomirski > On Dec 10, 2018, at 12:55 AM, Peter Zijlstra wrote: > > On Sun, Dec 09, 2018 at 04:57:43PM -0800, Nadav Amit wrote: >>> On Dec 8, 2018, at 2:52 AM, Peter Zijlstra wrote: > >>> My patch proposed here: >>> >>> https://marc.info/?l=linux-mm&m=154409548410209 >>> >>> would actually fix that one I think, preempt_count() uses >>> raw_cpu_read_4() which will loose the volatile with that patch. > >> I tested the patch you referenced, and it certainly improves the situation >> for reads, but there are still small and big issues lying around. > > I'm sure :-(, this has been 'festering' for a long while it seems. And > esp. on x86 specific code, where for a long time we all assumed the > various per-cpu APIs were in fact the same (which turns out to very much > not be true). > >> The biggest one is that (I think) smp_processor_id() should apparently use >> __this_cpu_read(). > > Agreed, and note that this will also improve code generation on !x86. > > However, I'm not sure the current !debug definition: > > #define smp_processor_id() raw_smp_processor_id() > > is actually correct. Where raw_smp_processor_id() must be > this_cpu_read() to avoid CSE, we actually want to allow CSE on > smp_processor_id() etc.. Yes. That makes sense. > >> There are all kind of other smaller issues, such as set_irq_regs() and >> get_irq_regs(), which should run with disabled interrupts. They affect the >> generated code in do_IRQ() and others. >> >> But beyond that, there are so many places in the code that use >> this_cpu_read() while IRQs are guaranteed to be disabled. For example >> arch/x86/mm/tlb.c is full with this_cpu_read/write() and almost(?) all >> should be running with interrupts disabled. Having said that, in my build >> only flush_tlb_func_common() was affected. > > This all feels like something static analysis could help with; such > tools would also make sense for !x86 where the difference between the > various per-cpu accessors is even bigger. If something like that existed, it could also allow to get rid of local_irq_save() (and use local_irq_disable() instead).