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=DKIM_SIGNED,DKIM_VALID, HEADER_FROM_DIFFERENT_DOMAINS,MAILING_LIST_MULTI,SPF_HELO_NONE,SPF_PASS, T_DKIMWL_WL_HIGH 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 71B99C43218 for ; Tue, 11 Jun 2019 14:38:20 +0000 (UTC) Received: from bombadil.infradead.org (bombadil.infradead.org [198.137.202.133]) (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 42AA520896 for ; Tue, 11 Jun 2019 14:38:20 +0000 (UTC) Authentication-Results: mail.kernel.org; dkim=pass (2048-bit key) header.d=lists.infradead.org header.i=@lists.infradead.org header.b="bg/6YP3S" DMARC-Filter: OpenDMARC Filter v1.3.2 mail.kernel.org 42AA520896 Authentication-Results: mail.kernel.org; dmarc=none (p=none dis=none) header.from=arm.com Authentication-Results: mail.kernel.org; spf=none smtp.mailfrom=linux-arm-kernel-bounces+infradead-linux-arm-kernel=archiver.kernel.org@lists.infradead.org DKIM-Signature: v=1; a=rsa-sha256; q=dns/txt; c=relaxed/relaxed; d=lists.infradead.org; s=bombadil.20170209; h=Sender: Content-Transfer-Encoding:Content-Type:Cc:List-Subscribe:List-Help:List-Post: List-Archive:List-Unsubscribe:List-Id:In-Reply-To:MIME-Version:Date: Message-ID:From:References:To:Subject:Reply-To:Content-ID:Content-Description :Resent-Date:Resent-From:Resent-Sender:Resent-To:Resent-Cc:Resent-Message-ID: List-Owner; bh=VmYXVoW96scbLOasNELWri83oWWumn1wox9vNACknxQ=; b=bg/6YP3Sc18i58 Po1X7dF5Bf/xHdGdgkws/bvLeRGVGvCtKDkE6lEXzYm7x8nq4JzttuX8rTDgHYJ5HZi9PaR9nhFHG TiRkdfy+K+arl7fwSOitrzpllDj9RhQfgjUTqroGV2G8dHfSUHl037DW5w8f24hBgK5C7kAASAkTn KIXDUXYDMo8XzXF7WA2KmifcScPBNiLk3rlrhgGzJ+cT8gFM0GZooz3Pjl7xKl8U6dNwWo4nhHZYH XsomNr8SOXi6JMlxFe83nolA6ENSkxrIpJOI47mhLpZ1sMw5XNf+omOfk/RU8UVOdRfI2YFiGezj2 c+jA96u1AdCrI8Ono+zw==; Received: from localhost ([127.0.0.1] helo=bombadil.infradead.org) by bombadil.infradead.org with esmtp (Exim 4.92 #3 (Red Hat Linux)) id 1hahuf-0002xF-Ie; Tue, 11 Jun 2019 14:38:13 +0000 Received: from foss.arm.com ([217.140.110.172]) by bombadil.infradead.org with esmtp (Exim 4.92 #3 (Red Hat Linux)) id 1hahuc-0002w7-9H for linux-arm-kernel@lists.infradead.org; Tue, 11 Jun 2019 14:38:11 +0000 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 X-CRM114-Version: 20100106-BlameMichelson ( TRE 0.8.0 (BSD) ) MR-646709E3 X-CRM114-CacheID: sfid-20190611_073810_371870_E99D4D1F X-CRM114-Status: GOOD ( 13.93 ) X-BeenThere: linux-arm-kernel@lists.infradead.org X-Mailman-Version: 2.1.29 Precedence: list List-Id: List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , 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 Content-Type: text/plain; charset="us-ascii" Content-Transfer-Encoding: 7bit Sender: "linux-arm-kernel" Errors-To: linux-arm-kernel-bounces+infradead-linux-arm-kernel=archiver.kernel.org@lists.infradead.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 _______________________________________________ linux-arm-kernel mailing list linux-arm-kernel@lists.infradead.org http://lists.infradead.org/mailman/listinfo/linux-arm-kernel