From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from eggs.gnu.org ([2001:4830:134:3::10]:57695) by lists.gnu.org with esmtp (Exim 4.71) (envelope-from ) id 1cqKXT-00086J-Ou for qemu-devel@nongnu.org; Tue, 21 Mar 2017 10:13:33 -0400 Received: from Debian-exim by eggs.gnu.org with spam-scanned (Exim 4.71) (envelope-from ) id 1cqKXP-0002sZ-Mk for qemu-devel@nongnu.org; Tue, 21 Mar 2017 10:13:31 -0400 Received: from mx0b-001b2d01.pphosted.com ([148.163.158.5]:44453 helo=mx0a-001b2d01.pphosted.com) by eggs.gnu.org with esmtps (TLS1.0:RSA_AES_256_CBC_SHA1:32) (Exim 4.71) (envelope-from ) id 1cqKXP-0002sL-GH for qemu-devel@nongnu.org; Tue, 21 Mar 2017 10:13:27 -0400 Received: from pps.filterd (m0098417.ppops.net [127.0.0.1]) by mx0a-001b2d01.pphosted.com (8.16.0.20/8.16.0.20) with SMTP id v2LE93hp147417 for ; Tue, 21 Mar 2017 10:13:26 -0400 Received: from e06smtp10.uk.ibm.com (e06smtp10.uk.ibm.com [195.75.94.106]) by mx0a-001b2d01.pphosted.com with ESMTP id 29b50cajqw-1 (version=TLSv1.2 cipher=AES256-SHA bits=256 verify=NOT) for ; Tue, 21 Mar 2017 10:13:26 -0400 Received: from localhost by e06smtp10.uk.ibm.com with IBM ESMTP SMTP Gateway: Authorized Use Only! Violators will be prosecuted for from ; Tue, 21 Mar 2017 14:13:24 -0000 From: Greg Kurz Date: Tue, 21 Mar 2017 15:12:59 +0100 In-Reply-To: <1490105580-5008-1-git-send-email-groug@kaod.org> References: <1490105580-5008-1-git-send-email-groug@kaod.org> Message-Id: <1490105580-5008-2-git-send-email-groug@kaod.org> Subject: [Qemu-devel] [PULL 1/2] 9pfs: don't try to flush self and avoid QEMU hang on reset List-Id: List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , To: qemu-devel@nongnu.org Cc: Peter Maydell , Greg Kurz According to the 9P spec [*], when a client wants to cancel a pending I/O request identified by a given tag (uint16), it must send a Tflush message and wait for the server to respond with a Rflush message before reusing this tag for another I/O. The server may still send a completion message for the I/O if it wasn't actually cancelled but the Rflush message must arrive after that. QEMU hence waits for the flushed PDU to complete before sending the Rflush message back to the client. If a client sends 'Tflush tag oldtag' and tag == oldtag, QEMU will then allocate a PDU identified by tag, find it in the PDU list and wait for this same PDU to complete... i.e. wait for a completion that will never happen. This causes a tag and ring slot leak in the guest, and a PDU leak in QEMU, all of them limited by the maximal number of PDUs (128). But, worse, this causes QEMU to hang on device reset since v9fs_reset() wants to drain all pending I/O. This insane behavior is likely to denote a bug in the client, and it would deserve an Rerror message to be sent back. Unfortunately, the protocol allows it and requires all flush requests to suceed (only a Tflush response is expected). The only option is to detect when we have to handle a self-referencing flush request and report success to the client right away. [*] http://man.cat-v.org/plan_9/5/flush Reported-by: Al Viro Signed-off-by: Greg Kurz --- hw/9pfs/9p.c | 12 ++++++++---- 1 file changed, 8 insertions(+), 4 deletions(-) diff --git a/hw/9pfs/9p.c b/hw/9pfs/9p.c index 76c9247c777d..b8c0b993580c 100644 --- a/hw/9pfs/9p.c +++ b/hw/9pfs/9p.c @@ -2353,7 +2353,7 @@ static void coroutine_fn v9fs_flush(void *opaque) ssize_t err; int16_t tag; size_t offset = 7; - V9fsPDU *cancel_pdu; + V9fsPDU *cancel_pdu = NULL; V9fsPDU *pdu = opaque; V9fsState *s = pdu->s; @@ -2364,9 +2364,13 @@ static void coroutine_fn v9fs_flush(void *opaque) } trace_v9fs_flush(pdu->tag, pdu->id, tag); - QLIST_FOREACH(cancel_pdu, &s->active_list, next) { - if (cancel_pdu->tag == tag) { - break; + if (pdu->tag == tag) { + error_report("Warning: the guest sent a self-referencing 9P flush request"); + } else { + QLIST_FOREACH(cancel_pdu, &s->active_list, next) { + if (cancel_pdu->tag == tag) { + break; + } } } if (cancel_pdu) { -- 2.7.4