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=-5.8 required=3.0 tests=BAYES_00,DKIM_SIGNED, DKIM_VALID,DKIM_VALID_AU,HEADER_FROM_DIFFERENT_DOMAINS,MAILING_LIST_MULTI, SPF_HELO_NONE,SPF_PASS 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 1B2B1C433E4 for ; Mon, 29 Mar 2021 12:51:01 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id C98F26195C for ; Mon, 29 Mar 2021 12:51:00 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S231478AbhC2Mu2 (ORCPT ); Mon, 29 Mar 2021 08:50:28 -0400 Received: from lindbergh.monkeyblade.net ([23.128.96.19]:51984 "EHLO lindbergh.monkeyblade.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S231460AbhC2MuK (ORCPT ); Mon, 29 Mar 2021 08:50:10 -0400 Received: from casper.infradead.org (casper.infradead.org [IPv6:2001:8b0:10b:1236::1]) by lindbergh.monkeyblade.net (Postfix) with ESMTPS id 63782C061574; Mon, 29 Mar 2021 05:50:10 -0700 (PDT) DKIM-Signature: v=1; a=rsa-sha256; q=dns/txt; c=relaxed/relaxed; d=infradead.org; s=casper.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=1918iT4uzotqNOl2NIVcMWIyYjsHFO6ajOdXvBHTmz0=; b=OsUkcjaEC5QtmphwWlRuqGUNXm sguIJD8uR8Q6kUa/CzuVxzzaza4CR5bOBGKP4wnkUapwhIyaIa816SR+QuolvtGEeRoMiWR2U9eOr Y1eGuQ39WXYD2PlMMJRwOtlSCLtcRu4QL+9EJgRmP4v9eAwl7tq2lUyzErBosRYBRiXGA6p4r3piE l3JMoIDOkzIUC+gtjIaBZ6l+MT9NDe1uo3vldrRNQoHOB8kZQIUDYfehHZXB4CZYq4g4KuqP6ffRZ BR6rRc+bXfkJyW0+ypDZRJmZNSqGj/yhXMvIYXGocXid1OhDKbYDx235eu3aXlyeJx62QZv3dzYgb mTqTXNMg==; Received: from j217100.upc-j.chello.nl ([24.132.217.100] helo=noisy.programming.kicks-ass.net) by casper.infradead.org with esmtpsa (Exim 4.94 #2 (Red Hat Linux)) id 1lQrKo-001aXI-L9; Mon, 29 Mar 2021 12:49:37 +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 CFD843011FE; Mon, 29 Mar 2021 14:49:31 +0200 (CEST) Received: by hirez.programming.kicks-ass.net (Postfix, from userid 1000) id BA3C42BB1A697; Mon, 29 Mar 2021 14:49:31 +0200 (CEST) Date: Mon, 29 Mar 2021 14:49:31 +0200 From: Peter Zijlstra To: Guo Ren Cc: linux-riscv , Linux Kernel Mailing List , linux-csky@vger.kernel.org, linux-arch , Guo Ren , Will Deacon , Ingo Molnar , Waiman Long , Arnd Bergmann , Anup Patel Subject: Re: [PATCH v4 3/4] locking/qspinlock: Add ARCH_USE_QUEUED_SPINLOCKS_XCHG32 Message-ID: References: <1616868399-82848-1-git-send-email-guoren@kernel.org> <1616868399-82848-4-git-send-email-guoren@kernel.org> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: Precedence: bulk List-ID: X-Mailing-List: linux-csky@vger.kernel.org On Mon, Mar 29, 2021 at 08:01:41PM +0800, Guo Ren wrote: > u32 a = 0x55aa66bb; > u16 *ptr = &a; > > CPU0 CPU1 > ========= ========= > xchg16(ptr, new) while(1) > WRITE_ONCE(*(ptr + 1), x); > > When we use lr.w/sc.w implement xchg16, it'll cause CPU0 deadlock. Then I think your LL/SC is broken. That also means you really don't want to build super complex locking primitives on top, because that live-lock will percolate through. Step 1 would be to get your architecute fixed such that it can provide fwd progress guarantees for LL/SC. Otherwise there's absolutely no point in building complex systems with it.