From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from eggs.gnu.org ([2001:4830:134:3::10]:49568) by lists.gnu.org with esmtp (Exim 4.71) (envelope-from ) id 1YMp8q-00014C-IA for qemu-devel@nongnu.org; Sat, 14 Feb 2015 21:41:05 -0500 Received: from Debian-exim by eggs.gnu.org with spam-scanned (Exim 4.71) (envelope-from ) id 1YMp8o-0001la-Hy for qemu-devel@nongnu.org; Sat, 14 Feb 2015 21:41:04 -0500 Received: from mx1.redhat.com ([209.132.183.28]:39046) by eggs.gnu.org with esmtp (Exim 4.71) (envelope-from ) id 1YMp8o-0001lV-A6 for qemu-devel@nongnu.org; Sat, 14 Feb 2015 21:41:02 -0500 Received: from int-mx10.intmail.prod.int.phx2.redhat.com (int-mx10.intmail.prod.int.phx2.redhat.com [10.5.11.23]) by mx1.redhat.com (8.14.4/8.14.4) with ESMTP id t1F2f13K027569 (version=TLSv1/SSLv3 cipher=DHE-RSA-AES256-GCM-SHA384 bits=256 verify=FAIL) for ; Sat, 14 Feb 2015 21:41:01 -0500 From: Fam Zheng Date: Sun, 15 Feb 2015 10:40:51 +0800 Message-Id: <1423968052-17461-2-git-send-email-famz@redhat.com> In-Reply-To: <1423968052-17461-1-git-send-email-famz@redhat.com> References: <1423968052-17461-1-git-send-email-famz@redhat.com> Subject: [Qemu-devel] [PATCH v2 1/2] block: Forbid bdrv_set_aio_context outside BQL List-Id: List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , To: qemu-devel@nongnu.org Cc: kwolf@redhat.com, Paolo Bonzini , stefanha@redhat.com Even if the caller has the old #AioContext, there can be a deadlock, due to the leading bdrv_drain_all: Suppose there are three io threads (a, b, c) with each owning a BDS (bds_a, bds_b, bds_c), and a and b want to move their own BDS to c at the same time: iothread a iothread b -------------------------------------------------------------------------- bdrv_set_aio_context(bds_a, c) bdrv_set_aio_context(bds_b, c) -> bdrv_drain_all() -> bdrv_drain_all() -> acquire a (OK, already has) -> acquire a (blocked) -> acquire b (blocked) -> acquire b -> acquire c -> acquire c Current caller of bdrv_set_aio_context outside BQL is virtio-scsi-dataplane, which will be fixed in the next patches. Signed-off-by: Fam Zheng Reviewed-by: Paolo Bonzini --- include/block/block.h | 3 +-- 1 file changed, 1 insertion(+), 2 deletions(-) diff --git a/include/block/block.h b/include/block/block.h index 321295e..4fce25d 100644 --- a/include/block/block.h +++ b/include/block/block.h @@ -546,8 +546,7 @@ AioContext *bdrv_get_aio_context(BlockDriverState *bs); * Changes the #AioContext used for fd handlers, timers, and BHs by this * BlockDriverState and all its children. * - * This function must be called from the old #AioContext or with a lock held so - * the old #AioContext is not executing. + * This function must be called with iothread lock held. */ void bdrv_set_aio_context(BlockDriverState *bs, AioContext *new_context); -- 2.1.0