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Tue, 3 Mar 2020 17:37:13 GMT Received: from userv0122.oracle.com (userv0122.oracle.com [156.151.31.75]) by userp3030.oracle.com with ESMTP id 2yg1em8ujh-1 (version=TLSv1.2 cipher=ECDHE-RSA-AES256-GCM-SHA384 bits=256 verify=OK); Tue, 03 Mar 2020 17:37:11 +0000 Received: from abhmp0009.oracle.com (abhmp0009.oracle.com [141.146.116.15]) by userv0122.oracle.com (8.14.4/8.14.4) with ESMTP id 023HaX6a026231; Tue, 3 Mar 2020 17:36:33 GMT Received: from dhcp-10-211-47-111.usdhcp.oraclecorp.com (/10.211.47.111) by default (Oracle Beehive Gateway v4.0) with ESMTP ; Tue, 03 Mar 2020 09:36:33 -0800 Subject: Re: [Xen-devel] [PATCH 1/2] xenbus: req->body should be updated before req->state To: Julien Grall , xen-devel@lists.xenproject.org, linux-kernel@vger.kernel.org Cc: jgross@suse.com, boris.ostrovsky@oracle.com, sstabellini@kernel.org, joe.jin@oracle.com References: <20200303015859.18813-1-dongli.zhang@oracle.com> <2f175c30-b6b9-5f21-6cf3-2ee89e0c475e@xen.org> From: dongli.zhang@oracle.com Organization: Oracle Corporation Message-ID: <4d2428a4-01f7-cf23-82e1-6a9bec2c6d19@oracle.com> Date: Tue, 3 Mar 2020 09:36:32 -0800 User-Agent: Mozilla/5.0 (Macintosh; Intel Mac OS X 10.14; rv:68.0) Gecko/20100101 Thunderbird/68.2.2 MIME-Version: 1.0 In-Reply-To: <2f175c30-b6b9-5f21-6cf3-2ee89e0c475e@xen.org> Content-Type: text/plain; charset=utf-8 Content-Language: en-US Content-Transfer-Encoding: 8bit X-Proofpoint-Virus-Version: vendor=nai engine=6000 definitions=9549 signatures=668685 X-Proofpoint-Spam-Details: rule=notspam policy=default score=0 adultscore=0 suspectscore=3 spamscore=0 mlxlogscore=999 malwarescore=0 bulkscore=0 mlxscore=0 phishscore=0 classifier=spam adjust=0 reason=mlx scancount=1 engine=8.12.0-2001150001 definitions=main-2003030118 X-Proofpoint-Virus-Version: vendor=nai engine=6000 definitions=9549 signatures=668685 X-Proofpoint-Spam-Details: rule=notspam policy=default score=0 malwarescore=0 phishscore=0 spamscore=0 impostorscore=0 mlxscore=0 adultscore=0 mlxlogscore=999 lowpriorityscore=0 priorityscore=1501 bulkscore=0 clxscore=1011 suspectscore=3 classifier=spam adjust=0 reason=mlx scancount=1 engine=8.12.0-2001150001 definitions=main-2003030117 Sender: linux-kernel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On 3/3/20 1:40 AM, Julien Grall wrote: > Hi, > > On 03/03/2020 01:58, Dongli Zhang wrote: >> The req->body should be updated before req->state is updated and the >> order should be guaranteed by a barrier. >> >> Otherwise, read_reply() might return req->body = NULL. >> >> Below is sample callstack when the issue is reproduced on purpose by >> reordering the updates of req->body and req->state and adding delay in >> code between updates of req->state and req->body. >> >> [   22.356105] general protection fault: 0000 [#1] SMP PTI >> [   22.361185] CPU: 2 PID: 52 Comm: xenwatch Not tainted 5.5.0xen+ #6 >> [   22.366727] Hardware name: Xen HVM domU, BIOS ... >> [   22.372245] RIP: 0010:_parse_integer_fixup_radix+0x6/0x60 >> ... ... >> [   22.392163] RSP: 0018:ffffb2d64023fdf0 EFLAGS: 00010246 >> [   22.395933] RAX: 0000000000000000 RBX: 75746e7562755f6d RCX: 0000000000000000 >> [   22.400871] RDX: 0000000000000000 RSI: ffffb2d64023fdfc RDI: 75746e7562755f6d >> [   22.405874] RBP: 0000000000000000 R08: 00000000000001e8 R09: 0000000000cdcdcd >> [   22.410945] R10: ffffb2d6402ffe00 R11: ffff9d95395eaeb0 R12: ffff9d9535935000 >> [   22.417613] R13: ffff9d9526d4a000 R14: ffff9d9526f4f340 R15: ffff9d9537654000 >> [   22.423726] FS:  0000000000000000(0000) GS:ffff9d953bc80000(0000) >> knlGS:0000000000000000 >> [   22.429898] CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033 >> [   22.434342] CR2: 000000c4206a9000 CR3: 00000001ea3fc002 CR4: 00000000001606e0 >> [   22.439645] DR0: 0000000000000000 DR1: 0000000000000000 DR2: 0000000000000000 >> [   22.444941] DR3: 0000000000000000 DR6: 00000000fffe0ff0 DR7: 0000000000000400 >> [   22.450342] Call Trace: >> [   22.452509]  simple_strtoull+0x27/0x70 >> [   22.455572]  xenbus_transaction_start+0x31/0x50 >> [   22.459104]  netback_changed+0x76c/0xcc1 [xen_netfront] >> [   22.463279]  ? find_watch+0x40/0x40 >> [   22.466156]  xenwatch_thread+0xb4/0x150 >> [   22.469309]  ? wait_woken+0x80/0x80 >> [   22.472198]  kthread+0x10e/0x130 >> [   22.474925]  ? kthread_park+0x80/0x80 >> [   22.477946]  ret_from_fork+0x35/0x40 >> [   22.480968] Modules linked in: xen_kbdfront xen_fbfront(+) xen_netfront >> xen_blkfront >> [   22.486783] ---[ end trace a9222030a747c3f7 ]--- >> [   22.490424] RIP: 0010:_parse_integer_fixup_radix+0x6/0x60 >> >> The "while" is changed to "do while" so that wait_event() is used as a >> barrier. > > The correct barrier for read_reply() should be virt_rmb(). While on x86, this is > equivalent to barrier(), on Arm this will be a dmb(ish) to prevent the processor > re-ordering memory access. > > Therefore the barrier in test_reply() (called by wait_event()) is not going to > be sufficient for Arm. Sorry that I just erroneously thought wait_event() would be used as read barrier. I would change barrier() to virt_rmb() for read_reply() in v2. Thank you very much! Dongli Zhang