From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from eggs.gnu.org ([2001:4830:134:3::10]:38220) by lists.gnu.org with esmtp (Exim 4.71) (envelope-from ) id 1fNmN4-00030k-Su for qemu-devel@nongnu.org; Tue, 29 May 2018 17:41:35 -0400 Received: from Debian-exim by eggs.gnu.org with spam-scanned (Exim 4.71) (envelope-from ) id 1fNmN2-0007iX-AG for qemu-devel@nongnu.org; Tue, 29 May 2018 17:41:34 -0400 Received: from 4.mo178.mail-out.ovh.net ([46.105.49.171]:52282) by eggs.gnu.org with esmtps (TLS1.0:DHE_RSA_AES_256_CBC_SHA1:32) (Exim 4.71) (envelope-from ) id 1fNmN2-0007em-3G for qemu-devel@nongnu.org; Tue, 29 May 2018 17:41:32 -0400 Received: from player735.ha.ovh.net (unknown [10.109.108.46]) by mo178.mail-out.ovh.net (Postfix) with ESMTP id 4F68E184FB for ; Tue, 29 May 2018 23:41:30 +0200 (CEST) Date: Tue, 29 May 2018 23:41:22 +0200 From: Greg Kurz Message-ID: <20180529234122.7cbebcc8@bahia.lan> In-Reply-To: <20180529201917.GL4756@localhost.localdomain> References: <152750903916.663961.9369851345277129751.stgit@bahia.lan> <20180529201917.GL4756@localhost.localdomain> MIME-Version: 1.0 Content-Type: text/plain; charset=US-ASCII Content-Transfer-Encoding: 7bit Subject: Re: [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: Kevin Wolf Cc: qemu-devel@nongnu.org, Max Reitz , Stefan Hajnoczi , Paolo Bonzini , qemu-stable@nongnu.org, qemu-block@nongnu.org On Tue, 29 May 2018 22:19:17 +0200 Kevin Wolf wrote: > Am 28.05.2018 um 14:03 hat Greg Kurz geschrieben: > > 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 > > Hmm... It sounded convincing, but 'make check-tests/test-replication' > fails now. The good news is that with the drain fixes, for which I sent > v2 today, it passes, so instead of staging it in my block branch, I'll > put it at the end of my branch for the drain fixes. > > Might take a bit longer than planned until it's in master, sorry. > > Kevin Works for me :) Thanks ! -- Greg