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,HEADER_FROM_DIFFERENT_DOMAINS, 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 162A7C433ED for ; Mon, 26 Apr 2021 02:07:13 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id C5853611F0 for ; Mon, 26 Apr 2021 02:07:12 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S231608AbhDZCHw (ORCPT ); Sun, 25 Apr 2021 22:07:52 -0400 Received: from us-smtp-delivery-124.mimecast.com ([216.205.24.124]:47613 "EHLO us-smtp-delivery-124.mimecast.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S231502AbhDZCHw (ORCPT ); Sun, 25 Apr 2021 22:07:52 -0400 DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=redhat.com; s=mimecast20190719; t=1619402831; 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=soanG+9qRJI3J9HhDX0QCMYbAkfnS1p7/A5cxZozGaU=; b=D7HHvAHpl78wNM+VJOO2eejFdWaXYQyIaBFRDTL8fMZXtKyP9JzV5Qj0pPXecpB8VZ4d8K Iy/YqTZpMlL6WII00qS4q16bjE4qaTYpg3EbOSIFY3zlicHvVFX5b6rcbbBaZoTR5l3T6H FK6l3dniKVz1oDu4B4t0tVgWeakfFRs= 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-121-hNEplzNfP-S6ZRygBUhJHw-1; Sun, 25 Apr 2021 22:07:09 -0400 X-MC-Unique: hNEplzNfP-S6ZRygBUhJHw-1 Received: from smtp.corp.redhat.com (int-mx01.intmail.prod.int.phx2.redhat.com [10.5.11.11]) (using TLSv1.2 with cipher AECDH-AES256-SHA (256/256 bits)) (No client certificate requested) by mimecast-mx01.redhat.com (Postfix) with ESMTPS id 55984343A4; Mon, 26 Apr 2021 02:07:07 +0000 (UTC) Received: from T590 (ovpn-12-48.pek2.redhat.com [10.72.12.48]) by smtp.corp.redhat.com (Postfix) with ESMTPS id 172169CA0; Mon, 26 Apr 2021 02:06:57 +0000 (UTC) Date: Mon, 26 Apr 2021 10:07:03 +0800 From: Ming Lei To: Bart Van Assche Cc: linux-nvme@lists.infradead.org, linux-scsi@vger.kernel.org, Jens Axboe , linux-block@vger.kernel.org, "Martin K . Petersen" , Christoph Hellwig , Khazhy Kumykov , Shin'ichiro Kawasaki , Hannes Reinecke , John Garry , David Jeffery Subject: Re: [PATCH 8/8] blk-mq: clear stale request in tags->rq[] before freeing one request pool Message-ID: References: <20210425085753.2617424-1-ming.lei@redhat.com> <20210425085753.2617424-9-ming.lei@redhat.com> <69764ff2-f339-0dc0-aac0-a1f9f4b30d53@acm.org> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <69764ff2-f339-0dc0-aac0-a1f9f4b30d53@acm.org> X-Scanned-By: MIMEDefang 2.79 on 10.5.11.11 Precedence: bulk List-ID: X-Mailing-List: linux-scsi@vger.kernel.org On Sun, Apr 25, 2021 at 06:50:48PM -0700, Bart Van Assche wrote: > On 4/25/21 5:49 PM, Ming Lei wrote: > > On Sun, Apr 25, 2021 at 01:42:59PM -0700, Bart Van Assche wrote: > >> Using cmpxchg() on set->tags[] is only safe if all other set->tags[] > >> accesses are changed into WRITE_ONCE() or READ_ONCE(). > > > > Why? > > > > Semantic of cmpxchg() is to modify value pointed by the address if its > > old value is same with passed 'rq'. That is exactly what we need. > > > > writting 'void *' is always atomic. if someone has touched > > '->rqs[tag]', cmpxchg() won't modify the value. > > WRITE_ONCE() supports data types that have the same size as char, short, > int, long and long long. That includes void *. If writes to these data > types would always be atomic then we wouldn't need the WRITE_ONCE() > macro. OK, then we don't need WRITE_ONCE(), since WRITE on tags->rqs[i] is always atomic. Thanks, Ming