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=-2.4 required=3.0 tests=DKIM_SIGNED,DKIM_VALID, DKIM_VALID_AU,HEADER_FROM_DIFFERENT_DOMAINS,MAILING_LIST_MULTI,SPF_HELO_NONE, SPF_PASS,USER_AGENT_SANE_1 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 CB55CC432C0 for ; Mon, 18 Nov 2019 16:28:54 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [209.132.180.67]) by mail.kernel.org (Postfix) with ESMTP id 9E7CB218AE for ; Mon, 18 Nov 2019 16:28:54 +0000 (UTC) Authentication-Results: mail.kernel.org; dkim=pass (1024-bit key) header.d=redhat.com header.i=@redhat.com header.b="h5JJ+Ws9" Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1726729AbfKRQ2x (ORCPT ); Mon, 18 Nov 2019 11:28:53 -0500 Received: from us-smtp-2.mimecast.com ([207.211.31.81]:35358 "EHLO us-smtp-delivery-1.mimecast.com" rhost-flags-OK-OK-OK-FAIL) by vger.kernel.org with ESMTP id S1726314AbfKRQ2x (ORCPT ); Mon, 18 Nov 2019 11:28:53 -0500 DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=redhat.com; s=mimecast20190719; t=1574094532; 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: content-transfer-encoding:content-transfer-encoding: in-reply-to:in-reply-to:references:references; bh=rbgAKR+UZDyd5dRmL+L1xv5GWoM1EVRNJf8t4zbNXts=; b=h5JJ+Ws9JZXTIGEpeQefJIAKqY0nN781gtUoeSa3tf0YUqsYndyQ1way0yfQFKXvKkHjRi 0FOf71NbMEXs34qPzb6LkeJ7Y2HeSrf/UOVPYfnNIT8K+Byl8TImNXZm8Vwso9lCSzRUxR 9jXyBqO7NSdtauLb4SJqsARz6sBFJNo= 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-41-YZuYxyFoNkWDRwJZsfaCKQ-1; Mon, 18 Nov 2019 11:28:51 -0500 Received: from smtp.corp.redhat.com (int-mx07.intmail.prod.int.phx2.redhat.com [10.5.11.22]) (using TLSv1.2 with cipher AECDH-AES256-SHA (256/256 bits)) (No client certificate requested) by mimecast-mx01.redhat.com (Postfix) with ESMTPS id 9196C8C451C; Mon, 18 Nov 2019 16:28:49 +0000 (UTC) Received: from dhcp-27-174.brq.redhat.com (unknown [10.43.17.44]) by smtp.corp.redhat.com (Postfix) with SMTP id F3AC4100EBB8; Mon, 18 Nov 2019 16:28:38 +0000 (UTC) Received: by dhcp-27-174.brq.redhat.com (nbSMTP-1.00) for uid 1000 oleg@redhat.com; Mon, 18 Nov 2019 17:28:49 +0100 (CET) Date: Mon, 18 Nov 2019 17:28:38 +0100 From: Oleg Nesterov To: Peter Zijlstra Cc: mingo@kernel.org, will@kernel.org, tglx@linutronix.de, linux-kernel@vger.kernel.org, bigeasy@linutronix.de, juri.lelli@redhat.com, williams@redhat.com, bristot@redhat.com, longman@redhat.com, dave@stgolabs.net, jack@suse.com Subject: Re: [PATCH 4/5] locking/percpu-rwsem: Extract __percpu_down_read_trylock() Message-ID: <20191118162837.GA3025@redhat.com> References: <20191113102115.116470462@infradead.org> <20191113102855.868390100@infradead.org> MIME-Version: 1.0 In-Reply-To: <20191113102855.868390100@infradead.org> User-Agent: Mutt/1.5.24 (2015-08-30) X-Scanned-By: MIMEDefang 2.84 on 10.5.11.22 X-MC-Unique: YZuYxyFoNkWDRwJZsfaCKQ-1 X-Mimecast-Spam-Score: 0 Content-Type: text/plain; charset=WINDOWS-1252 Content-Transfer-Encoding: quoted-printable Content-Disposition: inline Sender: linux-kernel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Hi Peter, sorry for delay. I'll re-read this series tomorrow, but everything looks correct at first glance... Except one very minor problem in this patch, see below. On 11/13, Peter Zijlstra wrote: > > -bool __percpu_down_read(struct percpu_rw_semaphore *sem, bool try) > +static bool __percpu_down_read_trylock(struct percpu_rw_semaphore *sem) > { > =09__this_cpu_inc(*sem->read_count); > =20 > @@ -70,14 +70,21 @@ bool __percpu_down_read(struct percpu_rw > =09 * If !readers_block the critical section starts here, matched by the > =09 * release in percpu_up_write(). > =09 */ > -=09if (likely(!smp_load_acquire(&sem->readers_block))) > +=09if (likely(!atomic_read_acquire(&sem->readers_block))) I don't think this can be compiled ;) ->readers_block is "int" until the ne= xt patch makes it atomic_t and renames to ->block. And. I think __percpu_down_read_trylock() should do =09if (atomic_read(&sem->block)) =09=09return false; at the start, before __this_cpu_inc(read_count). Suppose that the pending writer sleeps in rcuwait_wait_event(readers_active= _check). If the new reader comes, it is better to not wake up that writer. Oleg.