From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from eggs.gnu.org ([2001:4830:134:3::10]:54178) by lists.gnu.org with esmtp (Exim 4.71) (envelope-from ) id 1a6HPK-0007F7-A2 for qemu-devel@nongnu.org; Tue, 08 Dec 2015 07:30:20 -0500 Received: from Debian-exim by eggs.gnu.org with spam-scanned (Exim 4.71) (envelope-from ) id 1a6HPH-0008Ao-3a for qemu-devel@nongnu.org; Tue, 08 Dec 2015 07:30:14 -0500 Received: from e06smtp17.uk.ibm.com ([195.75.94.113]:45288) by eggs.gnu.org with esmtp (Exim 4.71) (envelope-from ) id 1a6HPG-0008Ac-NY for qemu-devel@nongnu.org; Tue, 08 Dec 2015 07:30:11 -0500 Received: from localhost by e06smtp17.uk.ibm.com with IBM ESMTP SMTP Gateway: Authorized Use Only! Violators will be prosecuted for from ; Tue, 8 Dec 2015 12:30:07 -0000 Received: from b06cxnps3075.portsmouth.uk.ibm.com (d06relay10.portsmouth.uk.ibm.com [9.149.109.195]) by d06dlp02.portsmouth.uk.ibm.com (Postfix) with ESMTP id D4E1B2190067 for ; Tue, 8 Dec 2015 12:29:55 +0000 (GMT) Received: from d06av04.portsmouth.uk.ibm.com (d06av04.portsmouth.uk.ibm.com [9.149.37.216]) by b06cxnps3075.portsmouth.uk.ibm.com (8.14.9/8.14.9/NCO v10.0) with ESMTP id tB8CU3mZ32964766 for ; Tue, 8 Dec 2015 12:30:03 GMT Received: from d06av04.portsmouth.uk.ibm.com (localhost [127.0.0.1]) by d06av04.portsmouth.uk.ibm.com (8.14.4/8.14.4/NCO v10.0 AVout) with ESMTP id tB8CU1rM001074 for ; Tue, 8 Dec 2015 05:30:02 -0700 References: <1449118802-12047-1-git-send-email-stefanha@redhat.com> <1449118802-12047-3-git-send-email-stefanha@redhat.com> <20151207110251.6391b306.cornelia.huck@de.ibm.com> <20151207174229.4edc6004.cornelia.huck@de.ibm.com> <20151208095954.GD5071@noname.str.redhat.com> <20151208130008.5d0fc318.cornelia.huck@de.ibm.com> From: Christian Borntraeger Message-ID: <5666CD48.4050600@de.ibm.com> Date: Tue, 8 Dec 2015 13:30:00 +0100 MIME-Version: 1.0 In-Reply-To: <20151208130008.5d0fc318.cornelia.huck@de.ibm.com> Content-Type: text/plain; charset=windows-1252 Content-Transfer-Encoding: 7bit Subject: Re: [Qemu-devel] [PULL for-2.5 2/4] block: Don't wait serialising for non-COR read requests List-Id: List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , To: Cornelia Huck , Kevin Wolf Cc: Peter Maydell , Fam Zheng , qemu-devel@nongnu.org, Stefan Hajnoczi On 12/08/2015 01:00 PM, Cornelia Huck wrote: > On Tue, 8 Dec 2015 10:59:54 +0100 > Kevin Wolf wrote: > >> Am 07.12.2015 um 17:42 hat Cornelia Huck geschrieben: >>> On Mon, 7 Dec 2015 11:02:51 +0100 >>> Cornelia Huck wrote: >>> >>>> On Thu, 3 Dec 2015 13:00:00 +0800 >>>> Stefan Hajnoczi wrote: >>>> >>>>> From: Fam Zheng >>>>> >>>>> The assertion problem was noticed in 06c3916b35a, but it wasn't >>>>> completely fixed, because even though the req is not marked as >>>>> serialising, it still gets serialised by wait_serialising_requests >>>>> against other serialising requests, which could lead to the same >>>>> assertion failure. >>>>> >>>>> Fix it by even more explicitly skipping the serialising for this >>>>> specific case. >>>>> >>>>> Signed-off-by: Fam Zheng >>>>> Message-id: 1448962590-2842-2-git-send-email-famz@redhat.com >>>>> Signed-off-by: Stefan Hajnoczi >>>>> --- >>>>> block/backup.c | 2 +- >>>>> block/io.c | 12 +++++++----- >>>>> include/block/block.h | 4 ++-- >>>>> trace-events | 2 +- >>>>> 4 files changed, 11 insertions(+), 9 deletions(-) >>>> >>>> This one causes segfaults for me: >>>> >>>> Program received signal SIGSEGV, Segmentation fault. >>>> bdrv_is_inserted (bs=0x800000000000) at /data/git/yyy/qemu/block.c:3071 >>>> 3071 if (!drv) { >>>> >>>> (gdb) bt >>>> #0 bdrv_is_inserted (bs=0x800000000000) at /data/git/yyy/qemu/block.c:3071 >> >> This looks like some kind of memory corruption that hit blk->bs. It's >> most definitely not a valid pointer anyway. >> >>>> #1 0x0000000080216974 in blk_is_inserted (blk=) >>>> at /data/git/yyy/qemu/block/block-backend.c:986 >>>> #2 0x00000000802169c6 in blk_is_available (blk=blk@entry=0x3ffb17e7960) >>>> at /data/git/yyy/qemu/block/block-backend.c:991 >>>> #3 0x0000000080216d12 in blk_check_byte_request (blk=blk@entry=0x3ffb17e7960, >>>> offset=offset@entry=4928966656, size=16384) >>>> at /data/git/yyy/qemu/block/block-backend.c:558 >>>> #4 0x0000000080216df2 in blk_check_request (blk=blk@entry=0x3ffb17e7960, >>>> sector_num=sector_num@entry=9626888, nb_sectors=nb_sectors@entry=32) >>>> at /data/git/yyy/qemu/block/block-backend.c:589 >>>> #5 0x0000000080217ee8 in blk_aio_readv (blk=0x3ffb17e7960, sector_num= >>>> 9626888, iov=0x8098c658, nb_sectors=, cb= >>>> 0x80081150 , opaque=0x80980620) >>>> at /data/git/yyy/qemu/block/block-backend.c:727 >>>> #6 0x000000008008186e in submit_requests (niov=, >>>> num_reqs=, start=, mrb=, >>>> blk=) at /data/git/yyy/qemu/hw/block/virtio-blk.c:366 >>>> #7 virtio_blk_submit_multireq (mrb=, blk=) >>>> at /data/git/yyy/qemu/hw/block/virtio-blk.c:444 >>>> #8 virtio_blk_submit_multireq (blk=0x3ffb17e7960, mrb=0x3ffffffeb58) >>>> at /data/git/yyy/qemu/hw/block/virtio-blk.c:389 >>>> #9 0x00000000800823ee in virtio_blk_handle_output (vdev=, >>>> vq=) at /data/git/yyy/qemu/hw/block/virtio-blk.c:615 >>>> #10 0x00000000801e367e in aio_dispatch (ctx=0x80918520) >>>> at /data/git/yyy/qemu/aio-posix.c:326 >>>> #11 0x00000000801d28b0 in aio_ctx_dispatch (source=, >>>> callback=, user_data=) >>>> at /data/git/yyy/qemu/async.c:231 >>>> #12 0x000003fffd36a05a in g_main_context_dispatch () >>>> from /lib64/libglib-2.0.so.0 >>>> #13 0x00000000801e0ffa in glib_pollfds_poll () >>>> at /data/git/yyy/qemu/main-loop.c:211 >>>> #14 os_host_main_loop_wait (timeout=) >>>> at /data/git/yyy/qemu/main-loop.c:256 >>>> #15 main_loop_wait (nonblocking=) >>>> at /data/git/yyy/qemu/main-loop.c:504 >>>> #16 0x00000000800148a6 in main_loop () at /data/git/yyy/qemu/vl.c:1923 >>>> #17 main (argc=, argv=, envp=) >>>> at /data/git/yyy/qemu/vl.c:4684 >>>> >>>> Relevant part of command line: >>>> >>>> -drive file=/dev/sda,if=none,id=drive-virtio-disk0,format=raw,serial=ccwzfcp1,cache=none -device virtio-blk-ccw,devno=fe.0.0001,drive=drive-virtio-disk0,id=virtio-disk0,bootindex=1,scsi=off >>> >>> I played around a bit. The main part of this change seems to be calling >>> wait_serialising_requests() conditionally; reverting this makes the >>> guest boot again. >>> >>> I then tried to find out when wait_serialising_requests() was NOT >>> called and added fprintfs: well, it was _always_ called. I then added a >>> fprintf for flags at the beginning of the function: this produced a >>> segfault no matter whether wait_serialising_requests() was called >>> conditionally or unconditionally. Weird race? >>> >>> Anything further I can do? I guess this patch fixes a bug for someone, >>> but it means insta-death for my setup... >> >> If it happens immediately, perhaps running under valgrind is possible >> and could give some hints about what happened with blk->bs? > > Just a quick update: This triggers on a qemu built with a not-so-fresh > gcc 4.7.2 (and it seems to depend on compiler optimizations as well). > We can't trigger it with newer gccs. No hints from valgrind, sadly. > Investigation ongoing. > > Some updates after looking at Cornelias system. It seem to be related to the F18 gcc that is still on that test system. Problem happens when hw/block/virtio-blk.c is compiled with -O2 and goes away with -O1 and -O0 (I trimmed that down to -fexpensive-optimizations) The system calls virtio_blk_submit_multireq 26 times and then it messes up the blk pointer: (gdb) display blk->name 1: blk->name = 0x80894300 "drive-virtio-disk0" (gdb) next 419 if (niov + req->qiov.niov > IOV_MAX) { 1: blk->name = 0x80894300 "drive-virtio-disk0" (gdb) 424 if (req->qiov.size / BDRV_SECTOR_SIZE + nb_sectors > max_xfer_len) { 1: blk->name = 0x80894300 "drive-virtio-disk0" (gdb) 419 if (niov + req->qiov.niov > IOV_MAX) { 1: blk->name = 0x80894300 "drive-virtio-disk0" (gdb) 429 if (sector_num + nb_sectors != req->sector_num) { 1: blk->name = 0x80894300 "drive-virtio-disk0" (gdb) 434 submit_requests(blk, mrb, start, num_reqs, niov); 1: blk->name = 0x80894300 "drive-virtio-disk0" (gdb) 413 for (i = 0; i < mrb->num_reqs; i++) { 1: blk->name = 0x8089ae40 "" uninlining submit_request makes the problem go away, so might be a gcc bug in Fedora18. I am now going to look at the disassembly to be sure. Christian