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=-8.3 required=3.0 tests=BAYES_00,DKIMWL_WL_HIGH, DKIM_SIGNED,DKIM_VALID,DKIM_VALID_AU,HEADER_FROM_DIFFERENT_DOMAINS, MAILING_LIST_MULTI,SPF_HELO_NONE,SPF_PASS,URIBL_BLOCKED,USER_AGENT_SANE_1 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 19D08C43464 for ; Fri, 18 Sep 2020 10:48:36 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id B9C7D20848 for ; Fri, 18 Sep 2020 10:48:35 +0000 (UTC) Authentication-Results: mail.kernel.org; dkim=pass (1024-bit key) header.d=redhat.com header.i=@redhat.com header.b="Sq99/oHw" Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1726389AbgIRKse (ORCPT ); Fri, 18 Sep 2020 06:48:34 -0400 Received: from us-smtp-delivery-124.mimecast.com ([216.205.24.124]:42184 "EHLO us-smtp-delivery-124.mimecast.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1726115AbgIRKse (ORCPT ); Fri, 18 Sep 2020 06:48:34 -0400 DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=redhat.com; s=mimecast20190719; t=1600426112; h=from:from:reply-to:subject:subject:date:date:message-id:message-id: to:to:cc:cc:mime-version:mime-version:content-type:content-type: in-reply-to:in-reply-to:references:references; bh=R1chLQYyqvdYMEdkQJ5aLl7aSBAy8CC1WIB+Xh754Tw=; b=Sq99/oHwuOCqotrL/joQ50TIn17H9UUfCgiE/97SUxfmR1+TYUQ7etnfjP4vuZ83OMxdTZ ycTg4QFu207Ltp+fJrN5AggJMgqqrX/BS9KRFen7gMNq1nGtCJdNgbRwtye5YErWp0WVYw qLxGlXjIKIfEV2ksooxpi2AKI2PffGs= Received: from mimecast-mx01.redhat.com (mimecast-mx01.redhat.com [209.132.183.4]) (Using TLS) by relay.mimecast.com with ESMTP id us-mta-237-SrCELimjN-W__2WG3SIN9A-1; Fri, 18 Sep 2020 06:48:31 -0400 X-MC-Unique: SrCELimjN-W__2WG3SIN9A-1 Received: from smtp.corp.redhat.com (int-mx04.intmail.prod.int.phx2.redhat.com [10.5.11.14]) (using TLSv1.2 with cipher AECDH-AES256-SHA (256/256 bits)) (No client certificate requested) by mimecast-mx01.redhat.com (Postfix) with ESMTPS id 15A38640A0; Fri, 18 Sep 2020 10:48:29 +0000 (UTC) Received: from dhcp-27-174.brq.redhat.com (unknown [10.40.193.72]) by smtp.corp.redhat.com (Postfix) with SMTP id F32135DEBF; Fri, 18 Sep 2020 10:48:25 +0000 (UTC) Received: by dhcp-27-174.brq.redhat.com (nbSMTP-1.00) for uid 1000 oleg@redhat.com; Fri, 18 Sep 2020 12:48:28 +0200 (CEST) Date: Fri, 18 Sep 2020 12:48:24 +0200 From: Oleg Nesterov To: peterz@infradead.org Cc: Jan Kara , Boaz Harrosh , Hou Tao , Ingo Molnar , Will Deacon , Dennis Zhou , Tejun Heo , Christoph Lameter , linux-kernel@vger.kernel.org, linux-fsdevel@vger.kernel.org Subject: Re: [RFC PATCH] locking/percpu-rwsem: use this_cpu_{inc|dec}() for read_count Message-ID: <20200918104824.GA23469@redhat.com> References: <20200915150610.GC2674@hirez.programming.kicks-ass.net> <20200915153113.GA6881@redhat.com> <20200915155150.GD2674@hirez.programming.kicks-ass.net> <20200915160344.GH35926@hirez.programming.kicks-ass.net> <20200917120132.GA5602@redhat.com> <20200918090702.GB18920@quack2.suse.cz> <20200918100112.GN1362448@hirez.programming.kicks-ass.net> <20200918101216.GL35926@hirez.programming.kicks-ass.net> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20200918101216.GL35926@hirez.programming.kicks-ass.net> User-Agent: Mutt/1.5.24 (2015-08-30) X-Scanned-By: MIMEDefang 2.79 on 10.5.11.14 Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On 09/18, Peter Zijlstra wrote: > > On Fri, Sep 18, 2020 at 12:01:12PM +0200, peterz@infradead.org wrote: > > + u64 sum = per_cpu_sum(*(u64 *)sem->read_count); > > Moo, that doesn't work, we have to do two separate sums. Or we can re-introduce "atomic_t slow_read_ctr". percpu_up_read_irqsafe(sem) { preempt_disable(); atomic_dec_release(&sem->slow_read_ctr); if (!rcu_sync_is_idle(&sem->rss)) rcuwait_wake_up(&sem->writer); preempt_enable(); } readers_active_check(sem) { unsigned int sum = per_cpu_sum(*sem->read_count) + (unsigned int)atomic_read(&sem->slow_read_ctr); if (sum) return false; ... } Of course, this assumes that atomic_t->counter underflows "correctly", just like "unsigned int". But again, do we really want this? Oleg.