From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: X-Spam-Checker-Version: SpamAssassin 3.4.0 (2014-02-07) on aws-us-west-2-korg-lkml-1.web.codeaurora.org Received: from lists.gnu.org (lists.gnu.org [209.51.188.17]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by smtp.lore.kernel.org (Postfix) with ESMTPS id 99955C05027 for ; Tue, 14 Feb 2023 14:07:00 +0000 (UTC) Received: from localhost ([::1] helo=lists1p.gnu.org) by lists.gnu.org with esmtp (Exim 4.90_1) (envelope-from ) id 1pRvxH-0001ci-C4; Tue, 14 Feb 2023 09:06:47 -0500 Received: from eggs.gnu.org ([2001:470:142:3::10]) by lists.gnu.org with esmtps (TLS1.2:ECDHE_RSA_AES_256_GCM_SHA384:256) (Exim 4.90_1) (envelope-from ) id 1pRvxF-0001cI-Hq for qemu-devel@nongnu.org; Tue, 14 Feb 2023 09:06:45 -0500 Received: from us-smtp-delivery-124.mimecast.com ([170.10.129.124]) by eggs.gnu.org with esmtps (TLS1.2:ECDHE_RSA_AES_256_GCM_SHA384:256) (Exim 4.90_1) (envelope-from ) id 1pRvxB-0001sL-Sq for qemu-devel@nongnu.org; Tue, 14 Feb 2023 09:06:44 -0500 DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=redhat.com; s=mimecast20190719; t=1676383600; h=from:from:reply-to:subject:subject:date:date:message-id:message-id: to:to:cc:cc:mime-version:mime-version:content-type:content-type: in-reply-to:in-reply-to:references:references; bh=ZfP8To/cT7L0MeoRKjpw9fRhkpf8gYjQ1xolTItFTPY=; b=LePL9FA+3x3x754bDY6Dz6JknvO+YnOoGqtlJxjPZkQ5zCC91hU8swluYIy6g73+5MUjpl AX2Jxayd1dkC+dnHfGCHVsLrf3unFAjjKvRjpLzFUOcfXCS04omAf1xvlz80NyzL5Ng2h6 Tk8Y6A8F+vhln55tzrXIjhsTHQvNvnU= Received: from mimecast-mx02.redhat.com (mx3-rdu2.redhat.com [66.187.233.73]) by relay.mimecast.com with ESMTP with STARTTLS (version=TLSv1.2, cipher=TLS_ECDHE_RSA_WITH_AES_256_GCM_SHA384) id us-mta-497-i2Nte2s3PSWChhfvJw5WmQ-1; Tue, 14 Feb 2023 09:06:38 -0500 X-MC-Unique: i2Nte2s3PSWChhfvJw5WmQ-1 Received: from smtp.corp.redhat.com (int-mx09.intmail.prod.int.rdu2.redhat.com [10.11.54.9]) (using TLSv1.2 with cipher AECDH-AES256-SHA (256/256 bits)) (No client certificate requested) by mimecast-mx02.redhat.com (Postfix) with ESMTPS id 69EC81C041A8; Tue, 14 Feb 2023 14:06:38 +0000 (UTC) Received: from redhat.com (unknown [10.39.192.80]) by smtp.corp.redhat.com (Postfix) with ESMTPS id 79573492B03; Tue, 14 Feb 2023 14:06:37 +0000 (UTC) Date: Tue, 14 Feb 2023 15:06:36 +0100 From: Kevin Wolf To: Stefano Garzarella Cc: qemu-devel@nongnu.org, qemu-block@nongnu.org, Hanna Reitz , Aihua Liang Subject: Re: [PATCH] block: temporarily hold the new AioContext of bs_top in bdrv_append() Message-ID: References: <20230214105156.316586-1-sgarzare@redhat.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: X-Scanned-By: MIMEDefang 3.1 on 10.11.54.9 Received-SPF: pass client-ip=170.10.129.124; envelope-from=kwolf@redhat.com; helo=us-smtp-delivery-124.mimecast.com X-Spam_score_int: -20 X-Spam_score: -2.1 X-Spam_bar: -- X-Spam_report: (-2.1 / 5.0 requ) BAYES_00=-1.9, DKIMWL_WL_HIGH=-0.001, DKIM_SIGNED=0.1, DKIM_VALID=-0.1, DKIM_VALID_AU=-0.1, DKIM_VALID_EF=-0.1, RCVD_IN_DNSWL_NONE=-0.0001, RCVD_IN_MSPIKE_H2=-0.001, SPF_HELO_NONE=0.001, SPF_PASS=-0.001 autolearn=ham autolearn_force=no X-Spam_action: no action X-BeenThere: qemu-devel@nongnu.org X-Mailman-Version: 2.1.29 Precedence: list List-Id: List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Errors-To: qemu-devel-bounces+qemu-devel=archiver.kernel.org@nongnu.org Sender: qemu-devel-bounces+qemu-devel=archiver.kernel.org@nongnu.org Am 14.02.2023 um 13:22 hat Stefano Garzarella geschrieben: > On Tue, Feb 14, 2023 at 12:56 PM Kevin Wolf wrote: > > > > Am 14.02.2023 um 11:51 hat Stefano Garzarella geschrieben: > > > bdrv_append() is called with bs_top AioContext held, but > > > bdrv_attach_child_noperm() could change the AioContext of bs_top. > > > > > > bdrv_replace_node_noperm() calls bdrv_drained_begin() starting from > > > commit 2398747128 ("block: Don't poll in bdrv_replace_child_noperm()"). > > > bdrv_drained_begin() can call BDRV_POLL_WHILE that assumes the new lock > > > is taken, so let's temporarily hold the new AioContext to prevent QEMU > > > from failing in BDRV_POLL_WHILE when it tries to release the wrong > > > AioContext. > > > > > > Buglink: https://bugzilla.redhat.com/show_bug.cgi?id=2168209 > > > Reported-by: Aihua Liang > > > Signed-off-by: Stefano Garzarella > > > --- > > > I'm not sure whether to use the following Fixes tag. That commit added the > > > calls to bdrv_drained_begin() in bdrv_replace_node_noperm(), but maybe the > > > problem was pre-existing. > > > > > > Fixes: 2398747128 ("block: Don't poll in bdrv_replace_child_noperm()") > > > > > > Note: a local reproducer is attached in the BZ, it is based on the Aihua Liang > > > report and it hits the issue with a 20% ratio. > > > --- > > > block.c | 23 +++++++++++++++++++++++ > > > 1 file changed, 23 insertions(+) > > > > > > diff --git a/block.c b/block.c > > > index aa9062f2c1..0e2bc11e0b 100644 > > > --- a/block.c > > > +++ b/block.c > > > @@ -5266,6 +5266,8 @@ int bdrv_drop_filter(BlockDriverState *bs, Error **errp) > > > * child. > > > * > > > * This function does not create any image files. > > > + * > > > + * The caller must hold the AioContext lock for @bs_top. > > > */ > > > int bdrv_append(BlockDriverState *bs_new, BlockDriverState *bs_top, > > > Error **errp) > > > @@ -5273,11 +5275,14 @@ int bdrv_append(BlockDriverState *bs_new, BlockDriverState *bs_top, > > > int ret; > > > BdrvChild *child; > > > Transaction *tran = tran_new(); > > > + AioContext *old_context, *new_context; > > > > > > GLOBAL_STATE_CODE(); > > > > > > assert(!bs_new->backing); > > > > > > + old_context = bdrv_get_aio_context(bs_top); > > > + > > > child = bdrv_attach_child_noperm(bs_new, bs_top, "backing", > > > &child_of_bds, bdrv_backing_role(bs_new), > > > tran, errp); > > > @@ -5286,11 +5291,29 @@ int bdrv_append(BlockDriverState *bs_new, BlockDriverState *bs_top, > > > goto out; > > > } > > > > > > + /* > > > + * bdrv_attach_child_noperm could change the AioContext of bs_top. > > > + * bdrv_replace_node_noperm calls bdrv_drained_begin, so let's temporarily > > > + * hold the new AioContext, since bdrv_drained_begin calls BDRV_POLL_WHILE > > > + * that assumes the new lock is taken. > > > + */ > > > + new_context = bdrv_get_aio_context(bs_top); > > > + > > > + if (old_context != new_context) { > > > + aio_context_release(old_context); > > > + aio_context_acquire(new_context); > > > + } > > > + > > > ret = bdrv_replace_node_noperm(bs_top, bs_new, true, tran, errp); > > > if (ret < 0) { > > > goto out; > > > > If we take the error path, we return with new_context locked instead of > > old_context now. > > Grr, I'm blind... > > > > > > } > > > > > > + if (old_context != new_context) { > > > + aio_context_release(new_context); > > > + aio_context_acquire(old_context); > > > + } > > > + > > > ret = bdrv_refresh_perms(bs_new, tran, errp); > > > out: > > > tran_finalize(tran, ret); > > > > Strictly speaking, don't we need to hold the lock across > > tran_finalize(), too? It completes the bdrv_replace_node_noperm() call > > you covered above. > > Right! > > > > > Maybe bdrv_refresh_perms() and bdrv_refresh_limits(), too, in fact. We > > never clearly defined which functions need the lock and which don't, so > > hard to tell. > > Okay, so to be on the safe side, I'll switch them back just before return. > > > It's really time to get rid of it. > > How could one disagree? :-) > > What about the Fixes tag? Should I include it? I'm not sure. Before the patch, bdrv_replace_child_noperm() had a drain which could have caused the same kind of problems. But we're now draining two BDSes, maybe that is the relevant difference. I guess we've always had a bug, but that commit made it more likely to actually trigger? Kevin