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 20B77C05027 for ; Tue, 14 Feb 2023 16:23:32 +0000 (UTC) Received: from localhost ([::1] helo=lists1p.gnu.org) by lists.gnu.org with esmtp (Exim 4.90_1) (envelope-from ) id 1pRy5O-0003No-5V; Tue, 14 Feb 2023 11:23:18 -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 1pRy5M-0003MG-7q for qemu-devel@nongnu.org; Tue, 14 Feb 2023 11:23:16 -0500 Received: from us-smtp-delivery-124.mimecast.com ([170.10.133.124]) by eggs.gnu.org with esmtps (TLS1.2:ECDHE_RSA_AES_256_GCM_SHA384:256) (Exim 4.90_1) (envelope-from ) id 1pRy5K-0007lj-L6 for qemu-devel@nongnu.org; Tue, 14 Feb 2023 11:23:15 -0500 DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=redhat.com; s=mimecast20190719; t=1676391793; 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: content-transfer-encoding:content-transfer-encoding: in-reply-to:in-reply-to:references:references; bh=ifuGtW2bpPcLPQXRxiUJ9EqHDn4hB9NZX7UILXOpe8E=; b=Qo9PfpHzXipCXgHhggtroPl7k1VVnajp7oqVeWB8INDjK+MHfVlO2bwSKYyNYr/1NcZZ6o 5NaJXXHAEefQxtiNTix22GsH2dWzbn1cMR5lIHQW+1dLq83GCliiivk+e4+JBE82ERfTv5 YVEqES3lQcPrE/RE9pDh8Zj2bF8MQgk= Received: from mimecast-mx02.redhat.com (mimecast-mx02.redhat.com [66.187.233.88]) by relay.mimecast.com with ESMTP with STARTTLS (version=TLSv1.2, cipher=TLS_ECDHE_RSA_WITH_AES_256_GCM_SHA384) id us-mta-664-iwk2S2OePZWGm6zk1fOInQ-1; Tue, 14 Feb 2023 11:23:10 -0500 X-MC-Unique: iwk2S2OePZWGm6zk1fOInQ-1 Received: from smtp.corp.redhat.com (int-mx06.intmail.prod.int.rdu2.redhat.com [10.11.54.6]) (using TLSv1.2 with cipher AECDH-AES256-SHA (256/256 bits)) (No client certificate requested) by mimecast-mx02.redhat.com (Postfix) with ESMTPS id 77AE01991C47; Tue, 14 Feb 2023 16:23:09 +0000 (UTC) Received: from blackfin.pond.sub.org (unknown [10.39.192.13]) by smtp.corp.redhat.com (Postfix) with ESMTPS id 395FB2166B26; Tue, 14 Feb 2023 16:23:09 +0000 (UTC) Received: by blackfin.pond.sub.org (Postfix, from userid 1000) id 23BCB21E6A1F; Tue, 14 Feb 2023 17:23:08 +0100 (CET) From: Markus Armbruster To: Daniel P. =?utf-8?Q?Berrang=C3=A9?= Cc: =?utf-8?Q?Marc-Andr=C3=A9?= Lureau , Markus Armbruster , "Dr. David Alan Gilbert" , qemu-devel@nongnu.org, Beraldo Leal , Eric Blake , Stefan Weil , Alex =?utf-8?Q?Benn?= =?utf-8?Q?=C3=A9e?= , Paolo Bonzini , Laurent Vivier , Gerd Hoffmann , Michael Roth , Philippe =?utf-8?Q?Mathieu-Daud=C3=A9?= , Thomas Huth , Wainer dos Santos Moschetta , Peter Xu Subject: Re: [PATCH v3 06/10] monitor: release the lock before calling close() References: <20230207142535.1153722-1-marcandre.lureau@redhat.com> <20230207142535.1153722-7-marcandre.lureau@redhat.com> <873578id74.fsf@pond.sub.org> Date: Tue, 14 Feb 2023 17:23:08 +0100 In-Reply-To: ("Daniel P. =?utf-8?Q?Berrang?= =?utf-8?Q?=C3=A9=22's?= message of "Tue, 14 Feb 2023 13:49:07 +0000") Message-ID: <871qmsgqs3.fsf@pond.sub.org> User-Agent: Gnus/5.13 (Gnus v5.13) Emacs/28.2 (gnu/linux) MIME-Version: 1.0 Content-Type: text/plain; charset=utf-8 Content-Transfer-Encoding: quoted-printable X-Scanned-By: MIMEDefang 3.1 on 10.11.54.6 Received-SPF: pass client-ip=170.10.133.124; envelope-from=armbru@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 Daniel P. Berrang=C3=A9 writes: > On Tue, Feb 14, 2023 at 05:36:32PM +0400, Marc-Andr=C3=A9 Lureau wrote: >> Hi >>=20 >> On Tue, Feb 14, 2023 at 5:34 PM Markus Armbruster wr= ote: >> > >> > marcandre.lureau@redhat.com writes: >> > >> > > From: Marc-Andr=C3=A9 Lureau >> > > >> > > As per comment, presumably to avoid syscall in critical section. >> > > >> > > Fixes: 0210c3b39bef08 ("monitor: Use LOCK_GUARD macros") >> > > Signed-off-by: Marc-Andr=C3=A9 Lureau >> > > --- >> > > monitor/fds.c | 4 +++- >> > > 1 file changed, 3 insertions(+), 1 deletion(-) >> > > >> > > diff --git a/monitor/fds.c b/monitor/fds.c >> > > index 26b39a0ce6..03c5e97c35 100644 >> > > --- a/monitor/fds.c >> > > +++ b/monitor/fds.c >> > > @@ -80,7 +80,7 @@ void qmp_getfd(const char *fdname, Error **errp) >> > > return; >> > > } >> > > >> > > - QEMU_LOCK_GUARD(&cur_mon->mon_lock); >> > > + qemu_mutex_lock(&cur_mon->mon_lock); >> > > QLIST_FOREACH(monfd, &cur_mon->fds, next) { >> > > if (strcmp(monfd->name, fdname) !=3D 0) { >> > > continue; >> > > @@ -88,6 +88,7 @@ void qmp_getfd(const char *fdname, Error **errp) >> > > >> > > tmp_fd =3D monfd->fd; >> > > monfd->fd =3D fd; >> > > + qemu_mutex_unlock(&cur_mon->mon_lock); >> > > /* Make sure close() is outside critical section */ >> > > close(tmp_fd); >> > > return; >> > > @@ -98,6 +99,7 @@ void qmp_getfd(const char *fdname, Error **errp) >> > > monfd->fd =3D fd; >> > > >> > > QLIST_INSERT_HEAD(&cur_mon->fds, monfd, next); >> > > + qemu_mutex_unlock(&cur_mon->mon_lock); >> > > } >> > > >> > > void qmp_closefd(const char *fdname, Error **errp) >> > >> > This confused me. I think I understand now, but let's double-check. >> > >> > You're reverting commit 0210c3b39bef08 for qmp_getfd() because it >> > extended the criticial section beyond the close(), invalidating the >> > comment. Correct? >>=20 >> Correct >>=20 >> > Did it actually break anything? >>=20 >> Not that I know of (David admitted over IRC that this was not intended) > > Conceptually the only risk here is that 'close()' blocks for a > prolonged period of time, which prevents another thread from > acquiring the mutex. > > First, the chances of close() blocking are incredibly low for > socket FDs which have not yet been used to transmit data. It > would require a malicious mgmt app to pass an unexpected FD > type that could block but that's quite hard, and we consider > the QMP client be a trusted entity anyway. > > As for another thread blocking on the mutex I'm not convinced > that'll happen either. The FD set is scoped to the current > monitor. Almost certainly the FD is going to be consumed by > a later QMP device-add/object-add command, in the same thread. > Processing of that later QMP command will be delayed regardless > of whether the close is inside or outside the critical section. > > AFAICT keeping close() oujtside the critical section serves > no purpose and we could just stick with the lock guard and > delete the comment. Makes sense to me. There's another one in monitor_add_fd(). Both are from Peter's commit 9409fc05fe2 "monitor: protect mon->fds with mon_lock". Peter, do you remember why you took the trouble to keep close() outside the critical section? I know it's been a while...