From nobody Tue Feb 10 01:30:45 2026 Delivered-To: importer@patchew.org Received-SPF: pass (zoho.com: domain of gnu.org designates 208.118.235.17 as permitted sender) client-ip=208.118.235.17; envelope-from=qemu-devel-bounces+importer=patchew.org@nongnu.org; helo=lists.gnu.org; Authentication-Results: mx.zoho.com; spf=pass (zoho.com: domain of gnu.org designates 208.118.235.17 as permitted sender) smtp.mailfrom=qemu-devel-bounces+importer=patchew.org@nongnu.org; Return-Path: Received: from lists.gnu.org (lists.gnu.org [208.118.235.17]) by mx.zohomail.com with SMTPS id 1492526100122229.74517780109102; Tue, 18 Apr 2017 07:35:00 -0700 (PDT) Received: from localhost ([::1]:42246 helo=lists.gnu.org) by lists.gnu.org with esmtp (Exim 4.71) (envelope-from ) id 1d0UDa-0004XM-N3 for importer@patchew.org; Tue, 18 Apr 2017 10:34:58 -0400 Received: from eggs.gnu.org ([2001:4830:134:3::10]:37495) by lists.gnu.org with esmtp (Exim 4.71) (envelope-from ) id 1d0UAs-0002mD-N3 for qemu-devel@nongnu.org; Tue, 18 Apr 2017 10:32:11 -0400 Received: from Debian-exim by eggs.gnu.org with spam-scanned (Exim 4.71) (envelope-from ) id 1d0UAo-0006NL-Gl for qemu-devel@nongnu.org; Tue, 18 Apr 2017 10:32:10 -0400 Received: from mx1.redhat.com ([209.132.183.28]:53178) by eggs.gnu.org with esmtps (TLS1.0:DHE_RSA_AES_256_CBC_SHA1:32) (Exim 4.71) (envelope-from ) id 1d0UAi-0006JA-WB; Tue, 18 Apr 2017 10:32:01 -0400 Received: from smtp.corp.redhat.com (int-mx01.intmail.prod.int.phx2.redhat.com [10.5.11.11]) (using TLSv1.2 with cipher AECDH-AES256-SHA (256/256 bits)) (No client certificate requested) by mx1.redhat.com (Postfix) with ESMTPS id ED4E0683F9; Tue, 18 Apr 2017 14:31:59 +0000 (UTC) Received: from lemon.redhat.com (ovpn-8-82.pek2.redhat.com [10.72.8.82]) by smtp.corp.redhat.com (Postfix) with ESMTP id 9FA857E200; Tue, 18 Apr 2017 14:31:15 +0000 (UTC) DMARC-Filter: OpenDMARC Filter v1.3.2 mx1.redhat.com ED4E0683F9 Authentication-Results: ext-mx09.extmail.prod.ext.phx2.redhat.com; dmarc=none (p=none dis=none) header.from=redhat.com Authentication-Results: ext-mx09.extmail.prod.ext.phx2.redhat.com; spf=pass smtp.mailfrom=famz@redhat.com DKIM-Filter: OpenDKIM Filter v2.11.0 mx1.redhat.com ED4E0683F9 From: Fam Zheng To: qemu-devel@nongnu.org Date: Tue, 18 Apr 2017 22:30:43 +0800 Message-Id: <20170418143044.12187-2-famz@redhat.com> In-Reply-To: <20170418143044.12187-1-famz@redhat.com> References: <20170418143044.12187-1-famz@redhat.com> X-Scanned-By: MIMEDefang 2.79 on 10.5.11.11 X-Greylist: Sender IP whitelisted, not delayed by milter-greylist-4.5.16 (mx1.redhat.com [10.5.110.38]); Tue, 18 Apr 2017 14:32:00 +0000 (UTC) X-detected-operating-system: by eggs.gnu.org: GNU/Linux 2.2.x-3.x [generic] [fuzzy] X-Received-From: 209.132.183.28 Subject: [Qemu-devel] [PATCH for-2.9-rc5 v4 1/2] block: Walk bs->children carefully in bdrv_drain_recurse X-BeenThere: qemu-devel@nongnu.org X-Mailman-Version: 2.1.21 Precedence: list List-Id: List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Cc: Kevin Wolf , Fam Zheng , qemu-block@nongnu.org, jcody@redhat.com, Max Reitz , Stefan Hajnoczi , pbonzini@redhat.com Errors-To: qemu-devel-bounces+importer=patchew.org@nongnu.org Sender: "Qemu-devel" X-ZohoMail: RSF_0 Z_629925259 SPT_0 Content-Transfer-Encoding: quoted-printable MIME-Version: 1.0 Content-Type: text/plain; charset="utf-8" The recursive bdrv_drain_recurse may run a block job completion BH that drops nodes. The coming changes will make that more likely and use-after-fr= ee would happen without this patch Stash the bs pointer and use bdrv_ref/bdrv_unref in addition to QLIST_FOREACH_SAFE to prevent such a case from happening. Since bdrv_unref accesses global state that is not protected by the AioCont= ext lock, we cannot use bdrv_ref/bdrv_unref unconditionally. Fortunately the protection is not needed in IOThread because only main loop can modify a gr= aph with the AioContext lock held. Signed-off-by: Fam Zheng --- block/io.c | 23 ++++++++++++++++++++--- 1 file changed, 20 insertions(+), 3 deletions(-) diff --git a/block/io.c b/block/io.c index 8706bfa..a0df8c4 100644 --- a/block/io.c +++ b/block/io.c @@ -158,7 +158,7 @@ bool bdrv_requests_pending(BlockDriverState *bs) =20 static bool bdrv_drain_recurse(BlockDriverState *bs) { - BdrvChild *child; + BdrvChild *child, *tmp; bool waited; =20 waited =3D BDRV_POLL_WHILE(bs, atomic_read(&bs->in_flight) > 0); @@ -167,8 +167,25 @@ static bool bdrv_drain_recurse(BlockDriverState *bs) bs->drv->bdrv_drain(bs); } =20 - QLIST_FOREACH(child, &bs->children, next) { - waited |=3D bdrv_drain_recurse(child->bs); + QLIST_FOREACH_SAFE(child, &bs->children, next, tmp) { + BlockDriverState *bs =3D child->bs; + bool in_main_loop =3D + qemu_get_current_aio_context() =3D=3D qemu_get_aio_context(); + assert(bs->refcnt > 0); + if (in_main_loop) { + /* In case the resursive bdrv_drain_recurse processes a + * block_job_defer_to_main_loop BH and modifies the graph, + * let's hold a reference to bs until we are done. + * + * IOThread doesn't have such a BH, and it is not safe to call + * bdrv_unref without BQL, so skip doing it there. + **/ + bdrv_ref(bs); + } + waited |=3D bdrv_drain_recurse(bs); + if (in_main_loop) { + bdrv_unref(bs); + } } =20 return waited; --=20 2.9.3