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=-6.2 required=3.0 tests=BAYES_00,DKIMWL_WL_HIGH, DKIM_SIGNED,DKIM_VALID,DKIM_VALID_AU,MAILING_LIST_MULTI,SPF_HELO_NONE, SPF_PASS,URIBL_BLOCKED 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 59073C433E0 for ; Tue, 30 Mar 2021 03:15:13 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id 33A1C60C3E for ; Tue, 30 Mar 2021 03:15:13 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S230347AbhC3DOk (ORCPT ); Mon, 29 Mar 2021 23:14:40 -0400 Received: from mail.kernel.org ([198.145.29.99]:37708 "EHLO mail.kernel.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S229689AbhC3DO0 (ORCPT ); Mon, 29 Mar 2021 23:14:26 -0400 Received: by mail.kernel.org (Postfix) with ESMTPSA id EC07D61864; Tue, 30 Mar 2021 03:14:08 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=kernel.org; s=k20201202; t=1617074049; bh=6eSMLRiak11IODOB+CePI7a65DX9N8EkAy9ZzrCIP+E=; h=References:In-Reply-To:From:Date:Subject:To:Cc:From; b=KdMi5KiMduBWy6xItpcEY9isb6e1nAEdmm9B7NVKTZqjF7+XWdljKvaUejdZRj94U qX67Y1c/XOgi9ZKIGLXXCN4VvmqEwFf8du1gXCAXZD3bTU4HmKJ0MLlEWg/JIAwinH WXgA5tPnnDNdUb2WG2acEptutFjaiM2ZY08Tl7DYFPvkKg2VJ1N4OCgFtiCsoGXX7k WEF9v+hEtplZ766rLquAVZYq5UOENcf9DfrgULGkGDm8uLzhLQkFGtDGkJjmlALSGK aMMzRqLK9GmKpYm5j27NOW0PIxO+ssEnNA45LMd+ldPrTaoq7fPTZATXycZfHSAbc6 UkH9hUYA93fGQ== Received: by mail-lj1-f178.google.com with SMTP id u9so18196746ljd.11; Mon, 29 Mar 2021 20:14:08 -0700 (PDT) X-Gm-Message-State: AOAM533xojLaMbVQuvvFH3mkmvGx3B2EVx5uSVuhMeA4INfHaQmx6mS5 4sPhQIQFMgvUYOKnrNO7zXlqEbDThzhjTcj0ePU= X-Google-Smtp-Source: ABdhPJybBWMS4ER6LoDJh0lOBZ6phOaqhlXDyrhTcDXbHrJqobU5QYkYxbC743xlAK9DNTW0TUKWXN1k7NjP7snB74o= X-Received: by 2002:a05:651c:211e:: with SMTP id a30mr13812844ljq.18.1617074047272; Mon, 29 Mar 2021 20:14:07 -0700 (PDT) MIME-Version: 1.0 References: <1616868399-82848-1-git-send-email-guoren@kernel.org> <1616868399-82848-4-git-send-email-guoren@kernel.org> In-Reply-To: From: Guo Ren Date: Tue, 30 Mar 2021 11:13:55 +0800 X-Gmail-Original-Message-ID: Message-ID: Subject: Re: [PATCH v4 3/4] locking/qspinlock: Add ARCH_USE_QUEUED_SPINLOCKS_XCHG32 To: Peter Zijlstra 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 Content-Type: text/plain; charset="UTF-8" Precedence: bulk List-ID: X-Mailing-List: linux-csky@vger.kernel.org On Mon, Mar 29, 2021 at 8:50 PM Peter Zijlstra wrote: > > 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. Do you mean the below implementation has live-lock risk? +static __always_inline u32 xchg_tail(struct qspinlock *lock, u32 tail) +{ + u32 old, new, val = atomic_read(&lock->val); + + for (;;) { + new = (val & _Q_LOCKED_PENDING_MASK) | tail; + old = atomic_cmpxchg(&lock->val, val, new); + if (old == val) + break; + + val = old; + } + return old; +} > > 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. Quote Waiman's comment [1] on xchg16 optimization: "This optimization is needed to make the qspinlock achieve performance parity with ticket spinlock at light load." [1] https://lore.kernel.org/kvm/1429901803-29771-6-git-send-email-Waiman.Long@hp.com/ So for a non-xhg16 machine: - ticket-lock for small numbers of CPUs - qspinlock for large numbers of CPUs Okay, I'll put all of them into the next patch :P -- Best Regards Guo Ren ML: https://lore.kernel.org/linux-csky/