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=-6.0 required=3.0 tests=BAYES_00,DKIMWL_WL_HIGH, DKIM_SIGNED,DKIM_VALID,DKIM_VALID_AU,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 7CF39C433ED for ; Wed, 14 Apr 2021 02:27:15 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id 4C84760E09 for ; Wed, 14 Apr 2021 02:27:15 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S232775AbhDNC1e (ORCPT ); Tue, 13 Apr 2021 22:27:34 -0400 Received: from mail.kernel.org ([198.145.29.99]:48584 "EHLO mail.kernel.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S231379AbhDNC1c (ORCPT ); Tue, 13 Apr 2021 22:27:32 -0400 Received: by mail.kernel.org (Postfix) with ESMTPSA id 68A21613D0 for ; 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Tue, 13 Apr 2021 19:27:10 -0700 (PDT) MIME-Version: 1.0 References: In-Reply-To: From: Guo Ren Date: Wed, 14 Apr 2021 10:26:57 +0800 X-Gmail-Original-Message-ID: Message-ID: Subject: Re: [PATCH] riscv: locks: introduce ticket-based spinlock implementation To: Peter Zijlstra Cc: =?UTF-8?Q?Christoph_M=C3=BCllner?= , Palmer Dabbelt , Anup Patel , linux-riscv , Linux Kernel Mailing List , Guo Ren , Catalin Marinas , Will Deacon , Arnd Bergmann Content-Type: text/plain; charset="UTF-8" Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Thx Peter, On Tue, Apr 13, 2021 at 4:17 PM Peter Zijlstra wrote: > > On Tue, Apr 13, 2021 at 10:03:01AM +0200, Peter Zijlstra wrote: > > > For ticket locks you really only needs atomic_fetch_add() and > > smp_store_release() and an architectural guarantees that the > > atomic_fetch_add() has fwd progress under contention and that a sub-word > > store (through smp_store_release()) will fail the SC. > > > > Then you can do something like: > > > > void lock(atomic_t *lock) > > { > > u32 val = atomic_fetch_add(1<<16, lock); /* SC, gives us RCsc */ > > u16 ticket = val >> 16; > > > > for (;;) { > > if (ticket == (u16)val) > > break; > > cpu_relax(); > > val = atomic_read_acquire(lock); > > } Should it be? for (;;) { if (ticket == (u16)val) { __atomic_acquire_fence(); break; } > > A possibly better might be: > > if (ticket == (u16)val) > return; Should it be? if (ticket == (u16)val) { __atomic_acquire_fence(); return; } > > atomic_cond_read_acquire(lock, ticket == (u16)VAL); > > Since that allows architectures to use WFE like constructs. > > > } > > > > void unlock(atomic_t *lock) > > { > > u16 *ptr = (u16 *)lock + (!!__BIG_ENDIAN__); > > u32 val = atomic_read(lock); > > > > smp_store_release(ptr, (u16)val + 1); > > } > > > > That's _almost_ as simple as a test-and-set :-) It isn't quite optimal > > on x86 for not being allowed to use a memop on unlock, since its being > > forced into a load-store because of all the volatile, but whatever. -- Best Regards Guo Ren ML: https://lore.kernel.org/linux-csky/ 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=-4.0 required=3.0 tests=BAYES_00,DKIMWL_WL_HIGH, DKIM_SIGNED,DKIM_VALID,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 50262C433ED for ; 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Tue, 13 Apr 2021 19:27:12 -0700 (PDT) X-Gm-Message-State: AOAM531dfPU+/qA8PqwUAMszLRYwOqJhM32XfmEYpFtBkEuwIkdI7Pa7 L3IN3yTxnG8YXoQeL+VoEvsM3hMARH4M6RNq+zc= X-Google-Smtp-Source: ABdhPJy7VEgmZ+iMoGsQksNaDynISBuYMg7ywSL8k0F1mmkVV8QOLDpxORYNJaXvx2rEWbV2DYsQkOmJebE39XOxVM4= X-Received: by 2002:ac2:4e84:: with SMTP id o4mr1971646lfr.557.1618367230801; Tue, 13 Apr 2021 19:27:10 -0700 (PDT) MIME-Version: 1.0 References: In-Reply-To: From: Guo Ren Date: Wed, 14 Apr 2021 10:26:57 +0800 X-Gmail-Original-Message-ID: Message-ID: Subject: Re: [PATCH] riscv: locks: introduce ticket-based spinlock implementation To: Peter Zijlstra Cc: =?UTF-8?Q?Christoph_M=C3=BCllner?= , Palmer Dabbelt , Anup Patel , linux-riscv , Linux Kernel Mailing List , Guo Ren , Catalin Marinas , Will Deacon , Arnd Bergmann X-CRM114-Version: 20100106-BlameMichelson ( TRE 0.8.0 (BSD) ) MR-646709E3 X-CRM114-CacheID: sfid-20210413_192712_817805_AAB883DC X-CRM114-Status: GOOD ( 17.20 ) X-BeenThere: linux-riscv@lists.infradead.org X-Mailman-Version: 2.1.34 Precedence: list List-Id: List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Content-Type: text/plain; charset="us-ascii" Content-Transfer-Encoding: 7bit Sender: "linux-riscv" Errors-To: linux-riscv-bounces+linux-riscv=archiver.kernel.org@lists.infradead.org Thx Peter, On Tue, Apr 13, 2021 at 4:17 PM Peter Zijlstra wrote: > > On Tue, Apr 13, 2021 at 10:03:01AM +0200, Peter Zijlstra wrote: > > > For ticket locks you really only needs atomic_fetch_add() and > > smp_store_release() and an architectural guarantees that the > > atomic_fetch_add() has fwd progress under contention and that a sub-word > > store (through smp_store_release()) will fail the SC. > > > > Then you can do something like: > > > > void lock(atomic_t *lock) > > { > > u32 val = atomic_fetch_add(1<<16, lock); /* SC, gives us RCsc */ > > u16 ticket = val >> 16; > > > > for (;;) { > > if (ticket == (u16)val) > > break; > > cpu_relax(); > > val = atomic_read_acquire(lock); > > } Should it be? for (;;) { if (ticket == (u16)val) { __atomic_acquire_fence(); break; } > > A possibly better might be: > > if (ticket == (u16)val) > return; Should it be? if (ticket == (u16)val) { __atomic_acquire_fence(); return; } > > atomic_cond_read_acquire(lock, ticket == (u16)VAL); > > Since that allows architectures to use WFE like constructs. > > > } > > > > void unlock(atomic_t *lock) > > { > > u16 *ptr = (u16 *)lock + (!!__BIG_ENDIAN__); > > u32 val = atomic_read(lock); > > > > smp_store_release(ptr, (u16)val + 1); > > } > > > > That's _almost_ as simple as a test-and-set :-) It isn't quite optimal > > on x86 for not being allowed to use a memop on unlock, since its being > > forced into a load-store because of all the volatile, but whatever. -- Best Regards Guo Ren ML: https://lore.kernel.org/linux-csky/ _______________________________________________ linux-riscv mailing list linux-riscv@lists.infradead.org http://lists.infradead.org/mailman/listinfo/linux-riscv