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=-3.8 required=3.0 tests=BAYES_00,DKIMWL_WL_HIGH, DKIM_SIGNED,DKIM_VALID,HEADER_FROM_DIFFERENT_DOMAINS,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 A8504C433B4 for ; Tue, 13 Apr 2021 08:17:41 +0000 (UTC) Received: from desiato.infradead.org (desiato.infradead.org [90.155.92.199]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by mail.kernel.org (Postfix) with ESMTPS id D408761369 for ; Tue, 13 Apr 2021 08:17:40 +0000 (UTC) DMARC-Filter: OpenDMARC Filter v1.3.2 mail.kernel.org D408761369 Authentication-Results: mail.kernel.org; dmarc=none (p=none dis=none) header.from=infradead.org Authentication-Results: mail.kernel.org; spf=none smtp.mailfrom=linux-riscv-bounces+linux-riscv=archiver.kernel.org@lists.infradead.org DKIM-Signature: v=1; a=rsa-sha256; q=dns/txt; c=relaxed/relaxed; d=lists.infradead.org; s=desiato.20200630; h=Sender:Content-Transfer-Encoding :Content-Type:List-Subscribe:List-Help:List-Post:List-Archive: List-Unsubscribe:List-Id:In-Reply-To:MIME-Version:References:Message-ID: Subject:Cc:To:From:Date:Reply-To:Content-ID:Content-Description:Resent-Date: Resent-From:Resent-Sender:Resent-To:Resent-Cc:Resent-Message-ID:List-Owner; bh=y+3tCfy/xgji3Nohj+AJVxwvpZlBxwZvZ8FFW/TqlYU=; b=ZJDfAIto6e6lAp6lnAuFMg3Jx FX2LVNytf3mg2mYOaPokcfNSVBb1SU4t63PyWEtuEcQTyXQC2QXBNeiIVBhcqsDgmSAmv8fEC/Lss CMjP2bG3Js21UuuN39flRAD0PaJ6/oUzL2AL6+qzDnJegBiEX5xbijU1/DK1oSLOBl1ZWCPgK4LKi 4PH4jJjGcIjkjzeFUAlyX5ugD01x/0Sy8qnb5Xss4RAg89oPoCu/mmFtwsLlRSaDT4kDuPRBZAGdl In0M1DnMgSRLrawBOf65aX9tX02I2F0pW4YYTzcuyy2eOh+lOXDk8YS3ydpaSrviRIDWJ+AHP8vt2 2fifDcx6w==; Received: from localhost ([::1] helo=desiato.infradead.org) by desiato.infradead.org with esmtp (Exim 4.94 #2 (Red Hat Linux)) id 1lWEET-008ZRs-0H; Tue, 13 Apr 2021 08:17:13 +0000 Received: from j217100.upc-j.chello.nl ([24.132.217.100] helo=noisy.programming.kicks-ass.net) by desiato.infradead.org with esmtpsa (Exim 4.94 #2 (Red Hat Linux)) id 1lWEEM-008ZRN-DN; Tue, 13 Apr 2021 08:17:09 +0000 Received: from hirez.programming.kicks-ass.net (hirez.programming.kicks-ass.net [192.168.1.225]) (using TLSv1.3 with cipher TLS_AES_256_GCM_SHA384 (256/256 bits) key-exchange X25519 server-signature RSA-PSS (2048 bits)) (Client did not present a certificate) by noisy.programming.kicks-ass.net (Postfix) with ESMTPS id D990D3001F6; Tue, 13 Apr 2021 10:17:05 +0200 (CEST) Received: by hirez.programming.kicks-ass.net (Postfix, from userid 1000) id 6667E20224209; Tue, 13 Apr 2021 10:17:05 +0200 (CEST) Date: Tue, 13 Apr 2021 10:17:05 +0200 From: Peter Zijlstra To: Christoph =?iso-8859-1?Q?M=FCllner?= Cc: Palmer Dabbelt , Anup Patel , Guo Ren , linux-riscv , Linux Kernel Mailing List , Guo Ren , catalin.marinas@arm.com, will.deacon@arm.com, Arnd Bergmann Subject: Re: [PATCH] riscv: locks: introduce ticket-based spinlock implementation Message-ID: References: MIME-Version: 1.0 Content-Disposition: inline In-Reply-To: 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 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); > } A possibly better might be: if (ticket == (u16)val) 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. _______________________________________________ linux-riscv mailing list linux-riscv@lists.infradead.org http://lists.infradead.org/mailman/listinfo/linux-riscv