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 5F3A6C31E46 for ; Wed, 12 Jun 2019 11:33:11 +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 3C1A6208CA for ; Wed, 12 Jun 2019 11:33:11 +0000 (UTC) DMARC-Filter: OpenDMARC Filter v1.3.2 mail.kernel.org 3C1A6208CA Authentication-Results: mail.kernel.org; dmarc=none (p=none dis=none) header.from=arm.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 ECDCB1B05; Wed, 12 Jun 2019 11:33:10 +0000 (UTC) Received: from smtp1.linuxfoundation.org (smtp1.linux-foundation.org [172.17.192.35]) by mail.linuxfoundation.org (Postfix) with ESMTPS id 2B1001AFB for ; Wed, 12 Jun 2019 11:30:34 +0000 (UTC) X-Greylist: domain auto-whitelisted by SQLgrey-1.7.6 Received: from foss.arm.com (foss.arm.com [217.140.110.172]) by smtp1.linuxfoundation.org (Postfix) with ESMTP id BE3DD79 for ; Wed, 12 Jun 2019 11:30:33 +0000 (UTC) Received: from usa-sjc-imap-foss1.foss.arm.com (unknown [10.121.207.14]) by usa-sjc-mx-foss1.foss.arm.com (Postfix) with ESMTP id 4F9C428; Wed, 12 Jun 2019 04:30:33 -0700 (PDT) Received: from [10.1.196.129] (ostrya.cambridge.arm.com [10.1.196.129]) by usa-sjc-imap-foss1.foss.arm.com (Postfix) with ESMTPSA id C93383F246; Wed, 12 Jun 2019 04:32:14 -0700 (PDT) Subject: Re: [PATCH 1/8] iommu: Add I/O ASID allocator To: Jacob Pan 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> <20190611101052.35af46df@jacob-builder> From: Jean-Philippe Brucker Message-ID: Date: Wed, 12 Jun 2019 12:30:05 +0100 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:60.0) Gecko/20100101 Thunderbird/60.7.0 MIME-Version: 1.0 In-Reply-To: <20190611101052.35af46df@jacob-builder> Content-Language: en-US Cc: Mark Rutland , "devicetree@vger.kernel.org" , Will Deacon , "linux-kernel@vger.kernel.org" , "iommu@lists.linux-foundation.org" , "robh+dt@kernel.org" , Robin Murphy , "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 11/06/2019 18:10, Jacob Pan wrote: >> 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. My fault, I sneaked the comments in a random reply three levels down the thread: https://lore.kernel.org/linux-iommu/836caf0d-699e-33ba-5303-b1c9c949c9ca@arm.com/ (Great, linux-iommu is indexed by lore! I won't have to Cc lkml anymore) >>>> + 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. Yes, and even if rcu_read_lock() provided a barrier I don't think it would be sufficient, because acquire semantics don't guarantee that prior writes appear to happen before the barrier, only the other way round. A lock operation with release semantics, for example spin_unlock(), should work. Thanks, Jean > 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