From nobody Mon Feb 9 10:58:20 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 1495041112704824.1241020423954; Wed, 17 May 2017 10:11:52 -0700 (PDT) Received: from localhost ([::1]:50050 helo=lists.gnu.org) by lists.gnu.org with esmtp (Exim 4.71) (envelope-from ) id 1dB2UH-0007IK-CG for importer@patchew.org; Wed, 17 May 2017 13:11:49 -0400 Received: from eggs.gnu.org ([2001:4830:134:3::10]:38803) by lists.gnu.org with esmtp (Exim 4.71) (envelope-from ) id 1dB2Sd-00068i-GY for qemu-devel@nongnu.org; Wed, 17 May 2017 13:10:08 -0400 Received: from Debian-exim by eggs.gnu.org with spam-scanned (Exim 4.71) (envelope-from ) id 1dB2Sc-00055u-Hl for qemu-devel@nongnu.org; Wed, 17 May 2017 13:10:07 -0400 Received: from mx1.redhat.com ([209.132.183.28]:54738) by eggs.gnu.org with esmtps (TLS1.0:DHE_RSA_AES_256_CBC_SHA1:32) (Exim 4.71) (envelope-from ) id 1dB2SW-000539-S7; Wed, 17 May 2017 13:10:00 -0400 Received: from smtp.corp.redhat.com (int-mx04.intmail.prod.int.phx2.redhat.com [10.5.11.14]) (using TLSv1.2 with cipher AECDH-AES256-SHA (256/256 bits)) (No client certificate requested) by mx1.redhat.com (Postfix) with ESMTPS id E296DBDD0; Wed, 17 May 2017 17:09:59 +0000 (UTC) Received: from localhost (unknown [10.36.118.35]) by smtp.corp.redhat.com (Postfix) with ESMTP id 42FB31816F; Wed, 17 May 2017 17:09:59 +0000 (UTC) DMARC-Filter: OpenDMARC Filter v1.3.2 mx1.redhat.com E296DBDD0 Authentication-Results: ext-mx05.extmail.prod.ext.phx2.redhat.com; dmarc=none (p=none dis=none) header.from=redhat.com Authentication-Results: ext-mx05.extmail.prod.ext.phx2.redhat.com; spf=pass smtp.mailfrom=stefanha@redhat.com DKIM-Filter: OpenDKIM Filter v2.11.0 mx1.redhat.com E296DBDD0 From: Stefan Hajnoczi To: qemu-devel@nongnu.org Date: Wed, 17 May 2017 18:09:41 +0100 Message-Id: <20170517170941.25850-4-stefanha@redhat.com> In-Reply-To: <20170517170941.25850-1-stefanha@redhat.com> References: <20170517170941.25850-1-stefanha@redhat.com> X-Scanned-By: MIMEDefang 2.79 on 10.5.11.14 X-Greylist: Sender IP whitelisted, not delayed by milter-greylist-4.5.16 (mx1.redhat.com [10.5.110.29]); Wed, 17 May 2017 17:10:00 +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 3/3] migration: avoid recursive AioContext locking in save_vmstate() 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 , Paolo Bonzini , Fam Zheng , Stefan Hajnoczi , qemu-block@nongnu.org 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" AioContext was designed to allow nested acquire/release calls. It uses a recursive mutex so callers don't need to worry about nesting...or so we thought. BDRV_POLL_WHILE() is used to wait for block I/O requests. It releases the AioContext temporarily around aio_poll(). This gives IOThreads a chance to acquire the AioContext to process I/O completions. It turns out that recursive locking and BDRV_POLL_WHILE() don't mix. BDRV_POLL_WHILE() only releases the AioContext once, so the IOThread will not be able to acquire the AioContext if it was acquired multiple times. Instead of trying to release AioContext n times in BDRV_POLL_WHILE(), this patch simply avoids nested locking in save_vmstate(). It's the simplest fix and we should step back to consider the big picture with all the recent changes to block layer threading. This patch is the final fix to solve 'savevm' hanging with -object iothread. Signed-off-by: Stefan Hajnoczi Reviewed-by: Eric Blake --- migration/savevm.c | 12 +++++++++++- 1 file changed, 11 insertions(+), 1 deletion(-) diff --git a/migration/savevm.c b/migration/savevm.c index 7f66d58..a70ba20 100644 --- a/migration/savevm.c +++ b/migration/savevm.c @@ -2153,6 +2153,14 @@ int save_vmstate(const char *name) goto the_end; } =20 + /* The bdrv_all_create_snapshot() call that follows acquires the AioCo= ntext + * for itself. BDRV_POLL_WHILE() does not support nested locking beca= use + * it only releases the lock once. Therefore synchronous I/O will dea= dlock + * unless we release the AioContext before bdrv_all_create_snapshot(). + */ + aio_context_release(aio_context); + aio_context =3D NULL; + ret =3D bdrv_all_create_snapshot(sn, bs, vm_state_size, &bs); if (ret < 0) { error_report("Error while creating snapshot on '%s'", @@ -2163,7 +2171,9 @@ int save_vmstate(const char *name) ret =3D 0; =20 the_end: - aio_context_release(aio_context); + if (aio_context) { + aio_context_release(aio_context); + } if (saved_vm_running) { vm_start(); } --=20 2.9.3