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=ham 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 3C5FEC4321A for ; Tue, 11 Jun 2019 14:38:11 +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 1683F2089E for ; Tue, 11 Jun 2019 14:38:11 +0000 (UTC) DMARC-Filter: OpenDMARC Filter v1.3.2 mail.kernel.org 1683F2089E 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 F0ECDEC9; Tue, 11 Jun 2019 14:38:10 +0000 (UTC) Received: from smtp1.linuxfoundation.org (smtp1.linux-foundation.org [172.17.192.35]) by mail.linuxfoundation.org (Postfix) with ESMTPS id 0D55FD07 for ; Tue, 11 Jun 2019 14:38:10 +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 A9E747C3 for ; Tue, 11 Jun 2019 14:38:09 +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 35FBD337; Tue, 11 Jun 2019 07:38:09 -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 0C1583F557; Tue, 11 Jun 2019 07:38:07 -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> From: Jean-Philippe Brucker Message-ID: <95292b47-4cf4-5fd9-b096-1cb016e2264f@arm.com> Date: Tue, 11 Jun 2019 15:37:42 +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: <20190611052626.20bed59a@jacob-builder> Content-Language: en-US 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 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; 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) >> + 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 Thanks, Jean _______________________________________________ iommu mailing list iommu@lists.linux-foundation.org https://lists.linuxfoundation.org/mailman/listinfo/iommu