From nobody Sun May 5 10:32:01 2024 Delivered-To: importer@patchew.org Received-SPF: pass (zoho.com: domain of gnu.org designates 209.51.188.17 as permitted sender) client-ip=209.51.188.17; envelope-from=qemu-devel-bounces+importer=patchew.org@nongnu.org; helo=lists.gnu.org; Authentication-Results: mx.zohomail.com; spf=pass (zoho.com: domain of gnu.org designates 209.51.188.17 as permitted sender) smtp.mailfrom=qemu-devel-bounces+importer=patchew.org@nongnu.org; dmarc=fail(p=none dis=none) header.from=redhat.com Return-Path: Received: from lists.gnu.org (lists.gnu.org [209.51.188.17]) by mx.zohomail.com with SMTPS id 1547031840113201.8166757252136; Wed, 9 Jan 2019 03:04:00 -0800 (PST) Received: from localhost ([127.0.0.1]:55764 helo=lists.gnu.org) by lists.gnu.org with esmtp (Exim 4.71) (envelope-from ) id 1ghBeR-0002LR-5W for importer@patchew.org; Wed, 09 Jan 2019 06:03:59 -0500 Received: from eggs.gnu.org ([209.51.188.92]:44762) by lists.gnu.org with esmtp (Exim 4.71) (envelope-from ) id 1ghBcV-0001EF-Ru for qemu-devel@nongnu.org; Wed, 09 Jan 2019 06:02:04 -0500 Received: from Debian-exim by eggs.gnu.org with spam-scanned (Exim 4.71) (envelope-from ) id 1ghBcT-0003TY-TT for qemu-devel@nongnu.org; Wed, 09 Jan 2019 06:01:59 -0500 Received: from mx1.redhat.com ([209.132.183.28]:27765) by eggs.gnu.org with esmtps (TLS1.0:DHE_RSA_AES_256_CBC_SHA1:32) (Exim 4.71) (envelope-from ) id 1ghBcJ-0003IJ-AG; Wed, 09 Jan 2019 06:01:47 -0500 Received: from smtp.corp.redhat.com (int-mx08.intmail.prod.int.phx2.redhat.com [10.5.11.23]) (using TLSv1.2 with cipher AECDH-AES256-SHA (256/256 bits)) (No client certificate requested) by mx1.redhat.com (Postfix) with ESMTPS id 6FEDC88E63; Wed, 9 Jan 2019 11:01:46 +0000 (UTC) Received: from localhost (ovpn-116-145.ams2.redhat.com [10.36.116.145]) by smtp.corp.redhat.com (Postfix) with ESMTP id F3D952619E; Wed, 9 Jan 2019 11:01:45 +0000 (UTC) From: Stefan Hajnoczi To: qemu-devel@nongnu.org Date: Wed, 9 Jan 2019 11:01:44 +0000 Message-Id: <20190109110144.18633-1-stefanha@redhat.com> MIME-Version: 1.0 X-Scanned-By: MIMEDefang 2.84 on 10.5.11.23 X-Greylist: Sender IP whitelisted, not delayed by milter-greylist-4.5.16 (mx1.redhat.com [10.5.110.25]); Wed, 09 Jan 2019 11:01:46 +0000 (UTC) Content-Transfer-Encoding: quoted-printable X-detected-operating-system: by eggs.gnu.org: GNU/Linux 2.2.x-3.x [generic] X-Received-From: 209.132.183.28 Subject: [Qemu-devel] [PATCH] throttle-groups: fix restart coroutine iothread race 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 , Alberto Garcia , qemu-block@nongnu.org, Max Reitz , Stefan Hajnoczi , Paolo Bonzini Errors-To: qemu-devel-bounces+importer=patchew.org@nongnu.org Sender: "Qemu-devel" Content-Type: text/plain; charset="utf-8" The following QMP command leads to a crash when iothreads are used: { 'execute': 'device_del', 'arguments': {'id': 'data'} } The backtrace involves the queue restart coroutine where tgm->throttle_state is a NULL pointer because throttle_group_unregister_tgm() has already been called: (gdb) bt full #0 0x00005585a7a3b378 in qemu_mutex_lock_impl (mutex=3D0xffffffffffffffd= 0, file=3D0x5585a7bb3d54 "block/throttle-groups.c", line=3D412) at util/qem= u-thread-posix.c:64 err =3D __PRETTY_FUNCTION__ =3D "qemu_mutex_lock_impl" __func__ =3D "qemu_mutex_lock_impl" #1 0x00005585a79be074 in throttle_group_restart_queue_entry (opaque=3D0x= 5585a9de4eb0) at block/throttle-groups.c:412 _f =3D data =3D 0x5585a9de4eb0 tgm =3D 0x5585a9079440 ts =3D 0x0 tg =3D 0xffffffffffffff98 is_write =3D false empty_queue =3D 255 This coroutine should not execute in the iothread after the throttle group member has been unregistered! The root cause is that the device_del code path schedules the restart coroutine in the iothread while holding the AioContext lock. Therefore the iothread cannot execute the coroutine until after device_del releases the lock - by this time it's too late. This patch adds a reference count to ThrottleGroupMember so we can synchronously wait for restart coroutines to complete. Once they are done it is safe to unregister the ThrottleGroupMember. Signed-off-by: Stefan Hajnoczi Reviewed-by: Alberto Garcia --- I'm unhappy about adding another synchronous point but haven't found a nicer solution. Only the main loop calls throttle_group_unregister_tgm() and the AioContext lock is held, so using AIO_WAIT_WHILE() is safe. Both iothread and non-iothread mode have been tested. I also tested 2 scsi-hd instances in the same throttling group to make sure that hot unplugging one drive doesn't interfere with the remaining drive in the throttling group. include/block/throttle-groups.h | 5 +++++ block/throttle-groups.c | 9 +++++++++ 2 files changed, 14 insertions(+) diff --git a/include/block/throttle-groups.h b/include/block/throttle-group= s.h index e2fd0513c4..7492bbc8a8 100644 --- a/include/block/throttle-groups.h +++ b/include/block/throttle-groups.h @@ -43,6 +43,11 @@ typedef struct ThrottleGroupMember { */ unsigned int io_limits_disabled; =20 + /* Number of pending throttle_group_restart_queue_entry() coroutines. + * Accessed under aio_context lock. + */ + unsigned int restart_pending; + /* The following fields are protected by the ThrottleGroup lock. * See the ThrottleGroup documentation for details. * throttle_state tells us if I/O limits are configured. */ diff --git a/block/throttle-groups.c b/block/throttle-groups.c index 5d8213a443..a4b15ea428 100644 --- a/block/throttle-groups.c +++ b/block/throttle-groups.c @@ -415,6 +415,9 @@ static void coroutine_fn throttle_group_restart_queue_e= ntry(void *opaque) } =20 g_free(data); + + tgm->restart_pending--; + aio_wait_kick(); } =20 static void throttle_group_restart_queue(ThrottleGroupMember *tgm, bool is= _write) @@ -430,6 +433,8 @@ static void throttle_group_restart_queue(ThrottleGroupM= ember *tgm, bool is_write * be no timer pending on this tgm at this point */ assert(!timer_pending(tgm->throttle_timers.timers[is_write])); =20 + tgm->restart_pending++; + co =3D qemu_coroutine_create(throttle_group_restart_queue_entry, rd); aio_co_enter(tgm->aio_context, co); } @@ -538,6 +543,7 @@ void throttle_group_register_tgm(ThrottleGroupMember *t= gm, =20 tgm->throttle_state =3D ts; tgm->aio_context =3D ctx; + tgm->restart_pending =3D 0; =20 qemu_mutex_lock(&tg->lock); /* If the ThrottleGroup is new set this ThrottleGroupMember as the tok= en */ @@ -584,6 +590,9 @@ void throttle_group_unregister_tgm(ThrottleGroupMember = *tgm) return; } =20 + /* Wait for throttle_group_restart_queue_entry() coroutines to finish = */ + AIO_WAIT_WHILE(tgm->aio_context, tgm->restart_pending > 0); + qemu_mutex_lock(&tg->lock); for (i =3D 0; i < 2; i++) { assert(tgm->pending_reqs[i] =3D=3D 0); --=20 2.20.1