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=-2.3 required=3.0 tests=DKIM_INVALID,DKIM_SIGNED, HEADER_FROM_DIFFERENT_DOMAINS,MAILING_LIST_MULTI,SPF_PASS,USER_AGENT_MUTT autolearn=ham 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 06744C282DA for ; Fri, 19 Apr 2019 19:31:03 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [209.132.180.67]) by mail.kernel.org (Postfix) with ESMTP id C20FA21479 for ; Fri, 19 Apr 2019 19:31:02 +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="jfK9Qs3q" Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1728048AbfDSTbB (ORCPT ); Fri, 19 Apr 2019 15:31:01 -0400 Received: from merlin.infradead.org ([205.233.59.134]:52654 "EHLO merlin.infradead.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1726331AbfDSTa6 (ORCPT ); Fri, 19 Apr 2019 15:30:58 -0400 DKIM-Signature: v=1; a=rsa-sha256; q=dns/txt; c=relaxed/relaxed; d=infradead.org; s=merlin.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:Resent-Date: Resent-From:Resent-Sender:Resent-To:Resent-Cc:Resent-Message-ID:List-Id: List-Help:List-Unsubscribe:List-Subscribe:List-Post:List-Owner:List-Archive; bh=EeK/UKGIlkcrqy9X052/pZ9M/vDBfGLns9hKVfQ8IQQ=; b=jfK9Qs3qnJDlHfY7Orm1Fyq3a gb4IbW9elW1czyt1dp25Z02CHzCyRLRDGOUQD9Sh6ZBltx3Y/iFY1bfEkcTSLVyUaH0aN1r7qEMq7 IPUeglqHLzMEWszL2aj2mZLXy5EEDMgdyBoZKv7bt/cRLs1Yh4/lk+8zaCz3AF181P///tm/AXtyL nUGR5udDJKK6taam8rUe3j6hsPhRCxQ9bwomXNORdHGi446XX6SMRBqwlFsRQ1HVXzYg+5gi/8rGZ VGP6Mra0JlRQtOvihaTNnvuTf25XJRXyfk21CLtoSd7bUiJl+ky2ktiCJJksd2sT7cXy3GXh7mueQ 1vuLSvOKQ==; Received: from j217100.upc-j.chello.nl ([24.132.217.100] helo=hirez.programming.kicks-ass.net) by merlin.infradead.org with esmtpsa (Exim 4.90_1 #2 (Red Hat Linux)) id 1hHTMR-0005n8-Ul; Fri, 19 Apr 2019 13:15:24 +0000 Received: by hirez.programming.kicks-ass.net (Postfix, from userid 1000) id 997EB29BF0BFE; Fri, 19 Apr 2019 15:15:22 +0200 (CEST) Date: Fri, 19 Apr 2019 15:15:22 +0200 From: Peter Zijlstra To: Waiman Long Cc: Ingo Molnar , Will Deacon , Thomas Gleixner , linux-kernel@vger.kernel.org, x86@kernel.org, Davidlohr Bueso , Linus Torvalds , Tim Chen , huang ying Subject: Re: [PATCH v4 14/16] locking/rwsem: Guard against making count negative Message-ID: <20190419131522.GW14281@hirez.programming.kicks-ass.net> References: <20190413172259.2740-1-longman@redhat.com> <20190413172259.2740-15-longman@redhat.com> <20190418135151.GB12232@hirez.programming.kicks-ass.net> <20190418144036.GE12232@hirez.programming.kicks-ass.net> <4cbd3c18-c9c0-56eb-4e01-ee355a69057a@redhat.com> <20190419102647.GP7905@worktop.programming.kicks-ass.net> <20190419120207.GO4038@hirez.programming.kicks-ass.net> <20190419130304.GV14281@hirez.programming.kicks-ass.net> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20190419130304.GV14281@hirez.programming.kicks-ass.net> 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 Fri, Apr 19, 2019 at 03:03:04PM +0200, Peter Zijlstra wrote: > On Fri, Apr 19, 2019 at 02:02:07PM +0200, Peter Zijlstra wrote: > > On Fri, Apr 19, 2019 at 12:26:47PM +0200, Peter Zijlstra wrote: > > > I thought of a horrible horrible alternative: > > > > Hurm, that's broken as heck. Let me try again. > > So I can't make that scheme work, it all ends up wanting to have > cmpxchg(). > > Do we have a performance comparison somewhere of xadd vs cmpxchg > readers? I tried looking in the old threads, but I can't seem to locate > it. > > We need new instructions :/ Or more clever than I can muster just now. In particular, an (unsigned) saturation arithmetic variant of XADD would be very nice to have at this point.