From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from eggs.gnu.org ([2001:4830:134:3::10]:33471) by lists.gnu.org with esmtp (Exim 4.71) (envelope-from ) id 1fNGsr-0002Xg-Fe for qemu-devel@nongnu.org; Mon, 28 May 2018 08:04:18 -0400 Received: from Debian-exim by eggs.gnu.org with spam-scanned (Exim 4.71) (envelope-from ) id 1fNGso-000370-D4 for qemu-devel@nongnu.org; Mon, 28 May 2018 08:04:17 -0400 Received: from 6.mo68.mail-out.ovh.net ([46.105.63.100]:41796) by eggs.gnu.org with esmtps (TLS1.0:DHE_RSA_AES_256_CBC_SHA1:32) (Exim 4.71) (envelope-from ) id 1fNGso-000369-7A for qemu-devel@nongnu.org; Mon, 28 May 2018 08:04:14 -0400 Received: from player755.ha.ovh.net (unknown [10.109.122.51]) by mo68.mail-out.ovh.net (Postfix) with ESMTP id 27E3AE3F65 for ; Mon, 28 May 2018 14:04:11 +0200 (CEST) From: Greg Kurz Date: Mon, 28 May 2018 14:03:59 +0200 Message-ID: <152750903916.663961.9369851345277129751.stgit@bahia.lan> MIME-Version: 1.0 Content-Type: text/plain; charset="utf-8" Content-Transfer-Encoding: 7bit Subject: [Qemu-devel] [PATCH v4] block: fix QEMU crash with scsi-hd and drive_del List-Id: List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , To: qemu-devel@nongnu.org Cc: Kevin Wolf , Max Reitz , Stefan Hajnoczi , Paolo Bonzini , qemu-stable@nongnu.org Removing a drive with drive_del while it is being used to run an I/O intensive workload can cause QEMU to crash. An AIO flush can yield at some point: blk_aio_flush_entry() blk_co_flush(blk) bdrv_co_flush(blk->root->bs) ... qemu_coroutine_yield() and let the HMP command to run, free blk->root and give control back to the AIO flush: hmp_drive_del() blk_remove_bs() bdrv_root_unref_child(blk->root) child_bs = blk->root->bs bdrv_detach_child(blk->root) bdrv_replace_child(blk->root, NULL) blk->root->bs = NULL g_free(blk->root) <============== blk->root becomes stale bdrv_unref(child_bs) bdrv_delete(child_bs) bdrv_close() bdrv_drained_begin() bdrv_do_drained_begin() bdrv_drain_recurse() aio_poll() ... qemu_coroutine_switch() and the AIO flush completion ends up dereferencing blk->root: blk_aio_complete() scsi_aio_complete() blk_get_aio_context(blk) bs = blk_bs(blk) ie, bs = blk->root ? blk->root->bs : NULL ^^^^^ stale The problem is that we should avoid making block driver graph changes while we have in-flight requests. Let's drain all I/O for this BB before calling bdrv_root_unref_child(). Signed-off-by: Greg Kurz --- v4: - call blk_drain() in blk_remove_bs() (Kevin) v3: - start drained section before modifying the graph (Stefan) v2: - drain I/O requests when detaching the BDS (Stefan, Paolo) --- block/block-backend.c | 5 +++++ 1 file changed, 5 insertions(+) diff --git a/block/block-backend.c b/block/block-backend.c index 89f47b00ea24..bee1f0e41461 100644 --- a/block/block-backend.c +++ b/block/block-backend.c @@ -768,6 +768,11 @@ void blk_remove_bs(BlockBackend *blk) blk_update_root_state(blk); + /* bdrv_root_unref_child() will cause blk->root to become stale and may + * switch to a completion coroutine later on. Let's drain all I/O here + * to avoid that and a potential QEMU crash. + */ + blk_drain(blk); bdrv_root_unref_child(blk->root); blk->root = NULL; }