From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: X-Spam-Checker-Version: SpamAssassin 3.4.0 (2014-02-07) on aws-us-west-2-korg-lkml-1.web.codeaurora.org X-Spam-Level: X-Spam-Status: No, score=-2.0 required=3.0 tests=DKIM_INVALID,DKIM_SIGNED, HEADER_FROM_DIFFERENT_DOMAINS,MAILING_LIST_MULTI,SPF_HELO_NONE,SPF_PASS, USER_AGENT_SANE_1 autolearn=no autolearn_force=no version=3.4.0 Received: from mail.kernel.org (mail.kernel.org [198.145.29.99]) by smtp.lore.kernel.org (Postfix) with ESMTP id B9C1DC2BA83 for ; Thu, 13 Feb 2020 08:58:56 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [209.132.180.67]) by mail.kernel.org (Postfix) with ESMTP id 80C2620848 for ; Thu, 13 Feb 2020 08:58:56 +0000 (UTC) Authentication-Results: mail.kernel.org; dkim=fail reason="signature verification failed" (2048-bit key) header.d=infradead.org header.i=@infradead.org header.b="XGX91zDZ" Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1729557AbgBMI6z (ORCPT ); Thu, 13 Feb 2020 03:58:55 -0500 Received: from bombadil.infradead.org ([198.137.202.133]:59546 "EHLO bombadil.infradead.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1726545AbgBMI6z (ORCPT ); Thu, 13 Feb 2020 03:58:55 -0500 DKIM-Signature: v=1; a=rsa-sha256; q=dns/txt; c=relaxed/relaxed; d=infradead.org; s=bombadil.20170209; h=In-Reply-To:Content-Type:MIME-Version :References:Message-ID:Subject:Cc:To:From:Date:Sender:Reply-To: Content-Transfer-Encoding:Content-ID:Content-Description; bh=cSINIGEzeX/CUP3TaQ2VxJhKGWx9x1IfWDs8UzeGCQk=; b=XGX91zDZAYlOdqN6Bf6OTpZ9RI maPP2pzK+Xxc8QgxZr9iOxVj179dx3RZHw9+NlH9381EkLOZ1x7szqFSOTgqmyhzQqdSK+UmwvJsp gEBe6Rtc/hOHNTSg2oprmZ5S9JgKGaj+uttUE9gRxsuGUqH9OZkp+jYAEmaWqWBHhA7GwF2HXWrFn W4I6NFtTkhE5qLymQ+cc7sstNfrd2B9mAfygRcPobbQuVOS8ovdAq1Qp2rGsV2Mik0sevbLsH15v8 c4ZMkpxOzCz6u3yNJ9hLbDWAZuMZTQTtsr/YQrQxNGMYOEz2+2LLLVXmQrfeD1aUboRhdoVLmB1mV 5N7XHSuA==; Received: from j217100.upc-j.chello.nl ([24.132.217.100] helo=noisy.programming.kicks-ass.net) by bombadil.infradead.org with esmtpsa (Exim 4.92.3 #3 (Red Hat Linux)) id 1j2AKi-000634-04; Thu, 13 Feb 2020 08:58:52 +0000 Received: from hirez.programming.kicks-ass.net (hirez.programming.kicks-ass.net [192.168.1.225]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (Client did not present a certificate) by noisy.programming.kicks-ass.net (Postfix) with ESMTPS id 442E33012D8; Thu, 13 Feb 2020 09:57:00 +0100 (CET) Received: by hirez.programming.kicks-ass.net (Postfix, from userid 1000) id C74072040AD15; Thu, 13 Feb 2020 09:58:49 +0100 (CET) Date: Thu, 13 Feb 2020 09:58:49 +0100 From: Peter Zijlstra To: Michal Simek Cc: linux-kernel@vger.kernel.org, monstr@monstr.eu, git@xilinx.com, arnd@arndb.de, Stefan Asserhall load and store , Boqun Feng , Will Deacon , paulmck@kernel.org Subject: Re: [PATCH 7/7] microblaze: Do atomic operations by using exclusive ops Message-ID: <20200213085849.GL14897@hirez.programming.kicks-ass.net> References: <20200212155500.GB14973@hirez.programming.kicks-ass.net> <4b46b33e-14ad-7097-f0db-2915ac772f15@xilinx.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <4b46b33e-14ad-7097-f0db-2915ac772f15@xilinx.com> User-Agent: Mutt/1.10.1 (2018-07-13) Sender: linux-kernel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Thu, Feb 13, 2020 at 09:06:24AM +0100, Michal Simek wrote: > On 12. 02. 20 16:55, Peter Zijlstra wrote: > > On Wed, Feb 12, 2020 at 04:42:29PM +0100, Michal Simek wrote: > >> +static inline void atomic_set(atomic_t *v, int i) > >> +{ > >> + int result, tmp; > >> + > >> + __asm__ __volatile__ ( > >> + /* load conditional address in %2 to %0 */ > >> + "1: lwx %0, %2, r0;\n" > >> + /* attempt store */ > >> + " swx %3, %2, r0;\n" > >> + /* checking msr carry flag */ > >> + " addic %1, r0, 0;\n" > >> + /* store failed (MSR[C] set)? try again */ > >> + " bnei %1, 1b;\n" > >> + /* Outputs: result value */ > >> + : "=&r" (result), "=&r" (tmp) > >> + /* Inputs: counter address */ > >> + : "r" (&v->counter), "r" (i) > >> + : "cc", "memory" > >> + ); > >> +} > >> +#define atomic_set atomic_set > > > > Uuuuhh.. *what* ?!? > > > > Are you telling me your LL/SC implementation is so bugger that > > atomic_set() being a WRITE_ONCE() does not in fact work? > > Just keep in your mind that this code was written long time ago and > there could be a lot of things/technique used at that time by IIRC > powerpc and I hope that review process will fix these things and I > really appreciation your comments. I don't think I've ever seen Power do this, but I've not checked the git history. > Stefan is the right person to say if we really need to use exclusive > loads/stores instructions or use what I see in include/linux/compiler.h. > > Please correct me if I am wrong. > WRITE_ONCE is __write_once_size which is normal write in C which I > expect will be converted in asm to non exclusive writes. And barrier is > called only for cases above 8bytes. > > READ_ONCE is normal read follow by barrier all the time. Right: WRITE_ONCE() is something like: *(volatile typeof(var)*)(&(var)) = val; And should translate to just a regular store; the volatile just tells the C compiler it should not do funny things with it. READ_ONCE() is something like: val = *(volatile typeof(var)*)(&(var)); And should translate to just a regular load; the volatile again tells the compiler to not be funny about it. No memory barriers what so ever, not even a compiler barrier as such. The thing is, your bog standard LL/SC _SHOULD_ fail the SC if someone else does a regular store to the same variable. See the example in Documentation/atomic_t.txt. That is, a competing SW/SWI should result in the interconnect responding with something other than EXOKAY, the SWX should fail and MSR[C] <- 1. > Also is there any testsuite I should run to verify all these atomics > operations? That would really help but I haven't seen any tool (but also > didn't try hard to find it out). Will, Paul; can't this LKMM thing generate kernel modules to run? And do we have a 'nice' collection of litmus tests that cover atomic_t ? The one in atomic_t.txt should cover this one at least.