From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from eggs.gnu.org ([2001:4830:134:3::10]:40574) by lists.gnu.org with esmtp (Exim 4.71) (envelope-from ) id 1gTn9J-0005GI-Ty for qemu-devel@nongnu.org; Mon, 03 Dec 2018 07:16:30 -0500 Received: from Debian-exim by eggs.gnu.org with spam-scanned (Exim 4.71) (envelope-from ) id 1gTn9F-0008Ry-Ew for qemu-devel@nongnu.org; Mon, 03 Dec 2018 07:16:29 -0500 Received: from mx1.redhat.com ([209.132.183.28]:29982) by eggs.gnu.org with esmtps (TLS1.0:DHE_RSA_AES_256_CBC_SHA1:32) (Exim 4.71) (envelope-from ) id 1gTn9F-0008RB-84 for qemu-devel@nongnu.org; Mon, 03 Dec 2018 07:16:25 -0500 Received: from smtp.corp.redhat.com (int-mx03.intmail.prod.int.phx2.redhat.com [10.5.11.13]) (using TLSv1.2 with cipher AECDH-AES256-SHA (256/256 bits)) (No client certificate requested) by mx1.redhat.com (Postfix) with ESMTPS id 465403001FE5 for ; Mon, 3 Dec 2018 12:16:24 +0000 (UTC) From: Markus Armbruster References: <20181029125733.14597-1-marcandre.lureau@redhat.com> <20181029125733.14597-6-marcandre.lureau@redhat.com> <87mupn6l5v.fsf@dusky.pond.sub.org> Date: Mon, 03 Dec 2018 13:16:09 +0100 In-Reply-To: (=?utf-8?Q?=22Marc-Andr=C3=A9?= Lureau"'s message of "Mon, 3 Dec 2018 13:55:43 +0400") Message-ID: <87o9a24xh2.fsf@dusky.pond.sub.org> MIME-Version: 1.0 Content-Type: text/plain; charset=utf-8 Content-Transfer-Encoding: quoted-printable Subject: Re: [Qemu-devel] [PATCH v2 5/6] monitor: prevent inserting new monitors after cleanup List-Id: List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , To: =?utf-8?Q?Marc-Andr=C3=A9?= Lureau Cc: "Bonzini, Paolo" , qemu-devel , Peter Xu , "Dr. David Alan Gilbert" Marc-Andr=C3=A9 Lureau writes: > Hi > On Mon, Dec 3, 2018 at 12:59 PM Markus Armbruster wro= te: >> >> Marc-Andr=C3=A9 Lureau writes: >> >> > Add a monitor_destroyed global to check if monitor_cleanup() has been >> > already called. In this case, don't insert the new monitor in the >> > list, but free it instead. >> > >> > Signed-off-by: Marc-Andr=C3=A9 Lureau >> >> The commit message explains what the patch does, but not why we want to >> do it. >> >> > --- >> > monitor.c | 14 ++++++++++++-- >> > 1 file changed, 12 insertions(+), 2 deletions(-) >> > >> > diff --git a/monitor.c b/monitor.c >> > index fffeb27ef9..7fe89daa87 100644 >> > --- a/monitor.c >> > +++ b/monitor.c >> > @@ -263,10 +263,11 @@ typedef struct QMPRequest QMPRequest; >> > /* QMP checker flags */ >> > #define QMP_ACCEPT_UNKNOWNS 1 >> > >> > -/* Protects mon_list, monitor_qapi_event_state. */ >> > +/* Protects mon_list, monitor_qapi_event_state, monitor_destroyed. */ >> > static QemuMutex monitor_lock; >> > static GHashTable *monitor_qapi_event_state; >> > static QTAILQ_HEAD(mon_list, Monitor) mon_list; >> > +static bool monitor_destroyed; >> > >> > /* Protects mon_fdsets */ >> > static QemuMutex mon_fdsets_lock; >> > @@ -4536,8 +4537,16 @@ void error_vprintf_unless_qmp(const char *fmt, = va_list ap) >> > static void monitor_list_append(Monitor *mon) >> > { >> > qemu_mutex_lock(&monitor_lock); >> > - QTAILQ_INSERT_HEAD(&mon_list, mon, entry); >> > + if (!monitor_destroyed) { >> > + QTAILQ_INSERT_HEAD(&mon_list, mon, entry); >> > + mon =3D NULL; >> > + } >> > qemu_mutex_unlock(&monitor_lock); >> > + >> > + if (mon) { >> > + monitor_data_destroy(mon); >> > + g_free(mon); >> > + } >> > } >> > >> > static void monitor_qmp_setup_handlers_bh(void *opaque) >> > @@ -4631,6 +4640,7 @@ void monitor_cleanup(void) >> > >> > /* Flush output buffers and destroy monitors */ >> > qemu_mutex_lock(&monitor_lock); >> > + monitor_destroyed =3D true; >> > QTAILQ_FOREACH_SAFE(mon, &mon_list, entry, next) { >> > QTAILQ_REMOVE(&mon_list, mon, entry); >> > monitor_flush(mon); >> >> monitor_cleanup() is one of the last things main() calls before it >> returns. If another thread creates a monitor afterwards, it won't be >> cleaned up. I figure that's the reason we want this patch. There might >> be more serious issues than failure to clean up. Please explain in your >> commit message. > > Is this clearer? > > monitor_cleanup() is one of the last things main() calls before it > returns. In the following patch, monitor_cleanup() will release the > monitor_lock during flushing. There may be pending commands to insert > new monitors, which would modify the mon_list during iteration, and > the clean-up could thus miss those new insertions. > > Add a monitor_destroyed global to check if monitor_cleanup() has been > already called. In this case, don't insert the new monitor in the > list, but free it instead. Yes. The solution feels a bit clunky: it duplicates part of the work monitor_cleanup() does into monitor_list_append(). Works because the part it doesn't duplicate needn't be done in this case: removing from @mon_list, and monitor_flush(). I suspect a cleaner solution would involve the main thread telling other threads to terminate, waiting for their termination with pthread_join(), and only then do final cleanup. I'm not asking for that solution now. A clunky fix we have is better than a refactoring we don't have. A comment admitting the clunkiness would be nice. >> monitor_list_append() is called by monitor_init(), either directly right >> before it returns, or via a bottom half if mon->use_io_thread. >> Therefore, @monitor_destroyed can only be true if monitor_init() >> commonly runs in a thread other than the main thread. Please give an >> example where it does, in your commit message.