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=-0.7 required=3.0 tests=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 4FCADC3A59E for ; Mon, 2 Sep 2019 14:46:14 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [209.132.180.67]) by mail.kernel.org (Postfix) with ESMTP id 32ABE2087E for ; Mon, 2 Sep 2019 14:46:14 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1731697AbfIBOqL (ORCPT ); Mon, 2 Sep 2019 10:46:11 -0400 Received: from mx1.redhat.com ([209.132.183.28]:60948 "EHLO mx1.redhat.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1726382AbfIBOqL (ORCPT ); Mon, 2 Sep 2019 10:46:11 -0400 Received: from smtp.corp.redhat.com (int-mx08.intmail.prod.int.phx2.redhat.com [10.5.11.23]) (using TLSv1.2 with cipher AECDH-AES256-SHA (256/256 bits)) (No client certificate requested) by mx1.redhat.com (Postfix) with ESMTPS id 907FF10F23ED; Mon, 2 Sep 2019 14:46:10 +0000 (UTC) Received: from gondolin (dhcp-192-222.str.redhat.com [10.33.192.222]) by smtp.corp.redhat.com (Postfix) with ESMTP id 02F89196B2; Mon, 2 Sep 2019 14:46:06 +0000 (UTC) Date: Mon, 2 Sep 2019 16:46:04 +0200 From: Cornelia Huck To: Parav Pandit Cc: "alex.williamson@redhat.com" , Jiri Pirko , "kwankhede@nvidia.com" , "davem@davemloft.net" , "kvm@vger.kernel.org" , "linux-kernel@vger.kernel.org" , "netdev@vger.kernel.org" Subject: Re: [PATCH v2 1/6] mdev: Introduce sha1 based mdev alias Message-ID: <20190902164604.1d04614f.cohuck@redhat.com> In-Reply-To: References: <20190826204119.54386-1-parav@mellanox.com> <20190829111904.16042-1-parav@mellanox.com> <20190829111904.16042-2-parav@mellanox.com> <20190830111720.04aa54e9.cohuck@redhat.com> <20190830143927.163d13a7.cohuck@redhat.com> <20190830160223.332fd81f.cohuck@redhat.com> Organization: Red Hat GmbH MIME-Version: 1.0 Content-Type: text/plain; charset=US-ASCII Content-Transfer-Encoding: 7bit X-Scanned-By: MIMEDefang 2.84 on 10.5.11.23 X-Greylist: Sender IP whitelisted, not delayed by milter-greylist-4.6.2 (mx1.redhat.com [10.5.110.66]); Mon, 02 Sep 2019 14:46:10 +0000 (UTC) Sender: netdev-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: netdev@vger.kernel.org On Fri, 30 Aug 2019 15:45:13 +0000 Parav Pandit wrote: > > > > > > This detour via the local variable looks weird to me. Can you > > > > > > either create the alias directly in the mdev (would need to > > > > > > happen later in the function, but I'm not sure why you generate > > > > > > the alias before checking for duplicates anyway), or do an explicit copy? > > > > > Alias duplicate check is done after generating it, because > > > > > duplicate alias are > > > > not allowed. > > > > > The probability of collision is rare. > > > > > So it is speculatively generated without hold the lock, because > > > > > there is no > > > > need to hold the lock. > > > > > It is compared along with guid while mutex lock is held in single loop. > > > > > And if it is duplicate, there is no need to allocate mdev. > > > > > > > > > > It will be sub optimal to run through the mdev list 2nd time after > > > > > mdev > > > > creation and after generating alias for duplicate check. > > > > > > > > Ok, but what about copying it? I find this "set local variable to > > > > NULL after ownership is transferred" pattern a bit unintuitive. > > > > Copying it to the mdev (and then unconditionally freeing it) looks more > > obvious to me. > > > Its not unconditionally freed. > > > > That's not what I have been saying :( > > > Ah I see. You want to allocate alias memory twice; once inside mdev device and another one in _create() function. > _create() one you want to free unconditionally. > > Well, passing pointer is fine. It's not that it doesn't work, but it feels fragile due to its non-obviousness. > mdev_register_device() has similar little tricky pattern that makes parent = NULL on __find_parent_device() finds duplicate one. I don't think that the two are comparable. > > Ownership transfer is more straight forward code. I have to disagree here. > > It is similar to device_initialize(), device init sequence code, where once device_initialize is done, freeing the device memory will be left to the put_device(), we don't call kfree() on mdev device. This does not really look similar to me: devices are refcounted structures, while strings aren't; you transfer a local pointer to a refcounted structure and then discard the local reference.