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=-0.8 required=3.0 tests=HEADER_FROM_DIFFERENT_DOMAINS, MAILING_LIST_MULTI,SPF_HELO_NONE,SPF_PASS autolearn=unavailable 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 A612BC0650E for ; Tue, 11 Jun 2019 17:07:47 +0000 (UTC) Received: from mail.linuxfoundation.org (mail.linuxfoundation.org [140.211.169.12]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by mail.kernel.org (Postfix) with ESMTPS id 873082086D for ; Tue, 11 Jun 2019 17:07:47 +0000 (UTC) DMARC-Filter: OpenDMARC Filter v1.3.2 mail.kernel.org 873082086D Authentication-Results: mail.kernel.org; dmarc=fail (p=none dis=none) header.from=linux.intel.com Authentication-Results: mail.kernel.org; spf=pass smtp.mailfrom=iommu-bounces@lists.linux-foundation.org Received: from mail.linux-foundation.org (localhost [127.0.0.1]) by mail.linuxfoundation.org (Postfix) with ESMTP id 57AE6DB4; Tue, 11 Jun 2019 17:07:47 +0000 (UTC) Received: from smtp1.linuxfoundation.org (smtp1.linux-foundation.org [172.17.192.35]) by mail.linuxfoundation.org (Postfix) with ESMTPS id 4383AD0A for ; Tue, 11 Jun 2019 17:07:46 +0000 (UTC) X-Greylist: domain auto-whitelisted by SQLgrey-1.7.6 Received: from mga01.intel.com (mga01.intel.com [192.55.52.88]) by smtp1.linuxfoundation.org (Postfix) with ESMTPS id B70B8174 for ; Tue, 11 Jun 2019 17:07:45 +0000 (UTC) X-Amp-Result: SKIPPED(no attachment in message) X-Amp-File-Uploaded: False Received: from orsmga006.jf.intel.com ([10.7.209.51]) by fmsmga101.fm.intel.com with ESMTP/TLS/DHE-RSA-AES256-GCM-SHA384; 11 Jun 2019 10:07:44 -0700 X-ExtLoop1: 1 Received: from jacob-builder.jf.intel.com (HELO jacob-builder) ([10.7.199.155]) by orsmga006.jf.intel.com with ESMTP; 11 Jun 2019 10:07:44 -0700 Date: Tue, 11 Jun 2019 10:10:52 -0700 From: Jacob Pan To: Jean-Philippe Brucker Subject: Re: [PATCH 1/8] iommu: Add I/O ASID allocator Message-ID: <20190611101052.35af46df@jacob-builder> In-Reply-To: <95292b47-4cf4-5fd9-b096-1cb016e2264f@arm.com> References: <20190610184714.6786-1-jean-philippe.brucker@arm.com> <20190610184714.6786-2-jean-philippe.brucker@arm.com> <20190611052626.20bed59a@jacob-builder> <95292b47-4cf4-5fd9-b096-1cb016e2264f@arm.com> Organization: OTC X-Mailer: Claws Mail 3.13.2 (GTK+ 2.24.30; x86_64-pc-linux-gnu) MIME-Version: 1.0 Cc: mark.rutland@arm.com, devicetree@vger.kernel.org, will.deacon@arm.com, linux-kernel@vger.kernel.org, iommu@lists.linux-foundation.org, robh+dt@kernel.org, robin.murphy@arm.com, linux-arm-kernel@lists.infradead.org X-BeenThere: iommu@lists.linux-foundation.org X-Mailman-Version: 2.1.12 Precedence: list List-Id: Development issues for Linux IOMMU support List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Content-Type: text/plain; charset="us-ascii" Content-Transfer-Encoding: 7bit Sender: iommu-bounces@lists.linux-foundation.org Errors-To: iommu-bounces@lists.linux-foundation.org On Tue, 11 Jun 2019 15:37:42 +0100 Jean-Philippe Brucker wrote: > On 11/06/2019 13:26, Jacob Pan wrote: > >> +/** > >> + * ioasid_set_data - Set private data for an allocated ioasid > >> + * @ioasid: the ID to set data > >> + * @data: the private data > >> + * > >> + * For IOASID that is already allocated, private data can be set > >> + * via this API. Future lookup can be done via ioasid_find. > >> + */ > >> +int ioasid_set_data(ioasid_t ioasid, void *data) > >> +{ > >> + struct ioasid_data *ioasid_data; > >> + int ret = 0; > >> + > >> + xa_lock(&ioasid_xa); > > Just wondering if this is necessary, since xa_load is under > > rcu_read_lock and we are not changing anything internal to xa. For > > custom allocator I still need to have the mutex against allocator > > removal. > > I think we do need this because of a possible race with ioasid_free(): > > CPU1 CPU2 > ioasid_free(ioasid) ioasid_set_data(ioasid, foo) > data = xa_load(...) > xa_erase(...) > kfree_rcu(data) (no RCU lock held) > ...free(data) > data->private = foo; > make sense, thanks for explaining. > The issue is theoretical at the moment because no users do this, but > I'd be more comfortable taking the xa_lock, which prevents a > concurrent xa_erase()+free(). (I commented on your v3 but you might > have missed it) > Did you reply to my v3? I did not see it. I only saw your comments about v3 in your commit message. > >> + ioasid_data = xa_load(&ioasid_xa, ioasid); > >> + if (ioasid_data) > >> + rcu_assign_pointer(ioasid_data->private, data); > > it is good to publish and have barrier here. But I just wonder even > > for weakly ordered machine, this pointer update is quite far away > > from its data update. > > I don't know, it could be right before calling ioasid_set_data(): > > mydata = kzalloc(sizeof(*mydata)); > mydata->ops = &my_ops; (1) > ioasid_set_data(ioasid, mydata); > ... /* no write barrier here */ > data->private = mydata; (2) > > And then another thread calls ioasid_find(): > > mydata = ioasid_find(ioasid); > if (mydata) > mydata->ops->do_something(); > > On a weakly ordered machine, this thread could observe the pointer > assignment (2) before the ops assignment (1), and dereference NULL. > Using rcu_assign_pointer() should fix that > I agree it is better to have the barrier. Just thought there is already a rcu_read_lock() in xa_load() in between. rcu_read_lock() may have barrier in some case but better not count on it. No issues here. I will integrate this in the next version. > Thanks, > Jean [Jacob Pan] _______________________________________________ iommu mailing list iommu@lists.linux-foundation.org https://lists.linuxfoundation.org/mailman/listinfo/iommu