From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from eggs.gnu.org ([2001:4830:134:3::10]:33566) by lists.gnu.org with esmtp (Exim 4.71) (envelope-from ) id 1d7n43-0001nx-Di for qemu-devel@nongnu.org; Mon, 08 May 2017 14:07:20 -0400 Received: from Debian-exim by eggs.gnu.org with spam-scanned (Exim 4.71) (envelope-from ) id 1d7n42-0005WL-Gz for qemu-devel@nongnu.org; Mon, 08 May 2017 14:07:19 -0400 From: Stefan Hajnoczi Date: Mon, 8 May 2017 14:07:05 -0400 Message-Id: <20170508180705.20609-1-stefanha@redhat.com> Subject: [Qemu-devel] [PATCH v3] aio: add missing aio_notify() to aio_enable_external() List-Id: List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , To: qemu-devel@nongnu.org Cc: qemu-block@nongnu.org, Paolo Bonzini , Fam Zheng , Stefan Hajnoczi The main loop uses aio_disable_external()/aio_enable_external() to temporarily disable processing of external AioContext clients like device emulation. This allows monitor commands to quiesce I/O and prevent the guest from submitting new requests while a monitor command is in progress. The aio_enable_external() API is currently broken when an IOThread is in aio_poll() waiting for fd activity when the main loop re-enables external clients. Incrementing ctx->external_disable_cnt does not wake the IOThread from ppoll(2) so fd processing remains suspended and leads to unresponsive emulated devices. This patch adds an aio_notify() call to aio_enable_external() so the IOThread is kicked out of ppoll(2) and will re-arm the file descriptors. The bug can be reproduced as follows: $ qemu -M accel=kvm -m 1024 \ -object iothread,id=iothread0 \ -device virtio-scsi-pci,iothread=iothread0,id=virtio-scsi-pci0 \ -drive if=none,id=drive0,aio=native,cache=none,format=raw,file=test.img \ -device scsi-hd,id=scsi-hd0,drive=drive0 \ -qmp tcp::5555,server,nowait $ scripts/qmp/qmp-shell localhost:5555 (qemu) blockdev-snapshot-sync device=drive0 snapshot-file=sn1.qcow2 mode=absolute-paths format=qcow2 After blockdev-snapshot-sync completes the SCSI disk will be unresponsive. This leads to request timeouts inside the guest. Reported-by: Qianqian Zhu Suggested-by: Fam Zheng Signed-off-by: Stefan Hajnoczi --- v3: * s/dec_fetch/fetch_dec/ [Fam] --- include/block/aio.h | 10 ++++++++-- 1 file changed, 8 insertions(+), 2 deletions(-) diff --git a/include/block/aio.h b/include/block/aio.h index 406e323..e9aeeae 100644 --- a/include/block/aio.h +++ b/include/block/aio.h @@ -454,8 +454,14 @@ static inline void aio_disable_external(AioContext *ctx) */ static inline void aio_enable_external(AioContext *ctx) { - assert(ctx->external_disable_cnt > 0); - atomic_dec(&ctx->external_disable_cnt); + int old; + + old = atomic_fetch_dec(&ctx->external_disable_cnt); + assert(old > 0); + if (old == 1) { + /* Kick event loop so it re-arms file descriptors */ + aio_notify(ctx); + } } /** -- 2.9.3