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 X-Spam-Level: X-Spam-Status: No, score=-13.6 required=3.0 tests=BAYES_00,DKIM_INVALID, DKIM_SIGNED,HEADER_FROM_DIFFERENT_DOMAINS,INCLUDES_CR_TRAILER,INCLUDES_PATCH, MAILING_LIST_MULTI,SPF_HELO_NONE,SPF_PASS autolearn=ham autolearn_force=no version=3.4.0 Received: from mail.kernel.org (mail.kernel.org [198.145.29.99]) by smtp.lore.kernel.org (Postfix) with ESMTP id 33069C433E0 for ; Tue, 26 Jan 2021 18:35:03 +0000 (UTC) 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 mail.kernel.org (Postfix) with ESMTPS id AD52E2223D for ; Tue, 26 Jan 2021 18:35:02 +0000 (UTC) DMARC-Filter: OpenDMARC Filter v1.3.2 mail.kernel.org AD52E2223D Authentication-Results: mail.kernel.org; dmarc=fail (p=none dis=none) header.from=redhat.com Authentication-Results: mail.kernel.org; spf=pass smtp.mailfrom=qemu-devel-bounces+qemu-devel=archiver.kernel.org@nongnu.org Received: from localhost ([::1]:35864 helo=lists1p.gnu.org) by lists.gnu.org with esmtp (Exim 4.90_1) (envelope-from ) id 1l4TB7-00056v-Rr for qemu-devel@archiver.kernel.org; Tue, 26 Jan 2021 13:35:01 -0500 Received: from eggs.gnu.org ([2001:470:142:3::10]:56242) by lists.gnu.org with esmtps (TLS1.2:ECDHE_RSA_AES_256_GCM_SHA384:256) (Exim 4.90_1) (envelope-from ) id 1l4TA7-00048W-V4 for qemu-devel@nongnu.org; Tue, 26 Jan 2021 13:33:59 -0500 Received: from us-smtp-delivery-124.mimecast.com ([216.205.24.124]:33541) by eggs.gnu.org with esmtps (TLS1.2:ECDHE_RSA_AES_256_CBC_SHA1:256) (Exim 4.90_1) (envelope-from ) id 1l4TA6-0000xH-0v for qemu-devel@nongnu.org; Tue, 26 Jan 2021 13:33:59 -0500 DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=redhat.com; s=mimecast20190719; t=1611686037; 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: in-reply-to:in-reply-to:references:references; bh=TgpmBkWBVNoIZ892ZJzFn4RzfUEPBIT2DlJa+TfByXo=; b=L+G7BvlNhUNraENWUZROpTQkxCLMFodHnufdtu2ewhxX5uPxLhbkyF4uC2u677c8KaHswE ANyzRsbaPCp5FKKL6YDVtpOgSMnorIB9ZCMUiEh8ri4zUK6fHGOZZi5HVbc11kJpQvJ+DJ fMdU7fXVjOtsIcCSxDa4grVEOTvlpZk= Received: from mimecast-mx01.redhat.com (mimecast-mx01.redhat.com [209.132.183.4]) (Using TLS) by relay.mimecast.com with ESMTP id us-mta-275-qEQgX7auMUqqeVqdQDrIVA-1; Tue, 26 Jan 2021 13:33:55 -0500 X-MC-Unique: qEQgX7auMUqqeVqdQDrIVA-1 Received: from smtp.corp.redhat.com (int-mx02.intmail.prod.int.phx2.redhat.com [10.5.11.12]) (using TLSv1.2 with cipher AECDH-AES256-SHA (256/256 bits)) (No client certificate requested) by mimecast-mx01.redhat.com (Postfix) with ESMTPS id 15345801AA3; Tue, 26 Jan 2021 18:33:54 +0000 (UTC) Received: from horse.redhat.com (ovpn-118-68.rdu2.redhat.com [10.10.118.68]) by smtp.corp.redhat.com (Postfix) with ESMTP id 820B260C47; Tue, 26 Jan 2021 18:33:42 +0000 (UTC) Received: by horse.redhat.com (Postfix, from userid 10451) id F2ADF220BCF; Tue, 26 Jan 2021 13:33:36 -0500 (EST) Date: Tue, 26 Jan 2021 13:33:36 -0500 From: Vivek Goyal To: Greg Kurz Subject: Re: [PATCH 1/6] virtiofsd: Drop ->vu_dispatch_rwlock while waiting for thread to exit Message-ID: <20210126183336.GB3239@redhat.com> References: <20210125180115.22936-1-vgoyal@redhat.com> <20210125180115.22936-2-vgoyal@redhat.com> <20210126165600.7bbe369d@bahia.lan> MIME-Version: 1.0 In-Reply-To: <20210126165600.7bbe369d@bahia.lan> X-Scanned-By: MIMEDefang 2.79 on 10.5.11.12 Authentication-Results: relay.mimecast.com; auth=pass smtp.auth=CUSA124A263 smtp.mailfrom=vgoyal@redhat.com X-Mimecast-Spam-Score: 0 X-Mimecast-Originator: redhat.com Content-Type: text/plain; charset=us-ascii Content-Disposition: inline Received-SPF: pass client-ip=216.205.24.124; envelope-from=vgoyal@redhat.com; helo=us-smtp-delivery-124.mimecast.com X-Spam_score_int: -30 X-Spam_score: -3.1 X-Spam_bar: --- X-Spam_report: (-3.1 / 5.0 requ) BAYES_00=-1.9, DKIMWL_WL_HIGH=-0.255, DKIM_SIGNED=0.1, DKIM_VALID=-0.1, DKIM_VALID_AU=-0.1, DKIM_VALID_EF=-0.1, RCVD_IN_DNSWL_LOW=-0.7, RCVD_IN_MSPIKE_H3=0.001, RCVD_IN_MSPIKE_WL=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: 2.1.23 Precedence: list List-Id: List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Cc: mst@redhat.com, qemu-devel@nongnu.org, dgilbert@redhat.com, virtio-fs@redhat.com, stefanha@redhat.com, marcandre.lureau@redhat.com Errors-To: qemu-devel-bounces+qemu-devel=archiver.kernel.org@nongnu.org Sender: "Qemu-devel" On Tue, Jan 26, 2021 at 04:56:00PM +0100, Greg Kurz wrote: > On Mon, 25 Jan 2021 13:01:10 -0500 > Vivek Goyal wrote: > > > When we are shutting down virtqueues, virtio_loop() receives a message > > VHOST_USER_GET_VRING_BASE from master. We acquire ->vu_dispatch_rwlock > > and get into the process of shutting down virtqueue. In one of the > > final steps, we are waiting for fv_queue_thread() to exit/finish and > > wait with ->vu_dispatch_rwlock held. > > > > But it is possible that fv_queue_thread() itself is waiting to get > > ->vu_dispatch_rwlock (With --thread-pool=0 option). If requests > > are being processed by fv_queue_worker(), then fv_queue_worker() > > can wait for the ->vu_dispatch_rwlock, and fv_queue_thread() will > > wait for fv_queue_worker() before thread pool can be stopped. > > > > IOW, if guest is shutdown uncleanly (some sort of emergency reboot), > > it is possible that virtiofsd is processing a fs request and > > qemu initiates device shutdown sequence. In that case there seem > > to be two options. Either abort the existing request completely or > > let existing request finish. > > > > This patch is taking second approach. That is drop the ->vu_dispatch_rwlock > > temporarily so that fv_queue_thread() can finish and deadlock does not > > happen. > > > > ->vu_dispatch_rwlock provides mutual exclusion between virtio_loop() > > (handling vhost-user protocol messages) and fv_queue_thread() (handling > > fuse filesystem requests). Rational seems to be that protocol message > > might change queue memory mappings, so we don't want both to proceed > > at the same time. > > > > In this case queue is shutting down, so I hope it is fine for fv_queue_thread() to send response back while virtio_loop() is still waiting (and not handling > > It looks this lacks a \n after "fine for" Hi Greg, Will fix. > > > any further vho-user protocol messages). > > > > IOW, assumption here is that while virto_loop is blocked processing > > VHOST_USER_GET_VRING_BASE message, it is still ok to send back the > > response on vq by fv_queue_thread(). > > > > Signed-off-by: Vivek Goyal > > --- > > tools/virtiofsd/fuse_virtio.c | 14 ++++++++++++++ > > 1 file changed, 14 insertions(+) > > > > diff --git a/tools/virtiofsd/fuse_virtio.c b/tools/virtiofsd/fuse_virtio.c > > index 9577eaa68d..6805d8ba01 100644 > > --- a/tools/virtiofsd/fuse_virtio.c > > +++ b/tools/virtiofsd/fuse_virtio.c > > @@ -813,11 +813,20 @@ static void fv_queue_cleanup_thread(struct fv_VuDev *vud, int qidx) > > fuse_log(FUSE_LOG_ERR, "Eventfd_write for queue %d: %s\n", > > qidx, strerror(errno)); > > } > > + > > + /* > > + * Drop ->vu_dispath_rwlock and reacquire. We are about to wait for > > + * for fv_queue_thread() and that might require ->vu_dispatch_rwlock > > + * to finish. > > + */ > > + pthread_rwlock_unlock(&vud->vu_dispatch_rwlock); > > ret = pthread_join(ourqi->thread, NULL); > > if (ret) { > > fuse_log(FUSE_LOG_ERR, "%s: Failed to join thread idx %d err %d\n", > > __func__, qidx, ret); > > } > > + pthread_rwlock_wrlock(&vud->vu_dispatch_rwlock); > > + > > So this is assuming that fv_queue_cleanup_thread() is called with > vu_dispatch_rwlock already taken for writing, but there are no > clear evidence in the code why it should care for the locking at > all in the first place. > > On the contrary, one of its two callers is a vhost-user callback, > in which we can reasonably have this assumption, while we can > have the opposite assumption for the other one in virtio_loop(). > > This makes me think that the drop/reacquire trick should only > be done in fv_queue_set_started(), instead of... I think this sounds reasonable. I will drop lock/re-acquire in fv_queue_set_started() around the call to fv_queue_cleanup_thread(). > > > pthread_mutex_destroy(&ourqi->vq_lock); > > close(ourqi->kill_fd); > > ourqi->kick_fd = -1; > > @@ -952,7 +961,11 @@ int virtio_loop(struct fuse_session *se) > > /* > > * Make sure all fv_queue_thread()s quit on exit, as we're about to > > * free virtio dev and fuse session, no one should access them anymore. > > + * Hold ->vu_dispatch_rwlock in write mode as fv_queue_cleanup_thread() > > + * assumes mutex is locked and unlocks/re-locks it. > > */ > > + > > + pthread_rwlock_wrlock(&se->virtio_dev->vu_dispatch_rwlock); > > > ... artificially introducing another critical section here. > > The issue isn't even specific to vu_dispatch_rwlock actually : > fv_queue_cleanup_thread() shouldn't be called with any lock > held because it might sleep in pthread_join() and cause a > deadlock all the same. So I'd rather document that instead : > drop all locks before calling fv_queue_cleanup_thread(). Sounds good. Will do. > > Also, since pthread_rwlock_wrlock() can fail, I think we should > always check it's return value, at least with an assert() like > already done elsewhere. Will check return code of pthread_rwlock_wrlock() and probably use assert(). Vivek > > > for (int i = 0; i < se->virtio_dev->nqueues; i++) { > > if (!se->virtio_dev->qi[i]) { > > continue; > > @@ -961,6 +974,7 @@ int virtio_loop(struct fuse_session *se) > > fuse_log(FUSE_LOG_INFO, "%s: Stopping queue %d thread\n", __func__, i); > > fv_queue_cleanup_thread(se->virtio_dev, i); > > } > > + pthread_rwlock_unlock(&se->virtio_dev->vu_dispatch_rwlock); > > > > fuse_log(FUSE_LOG_INFO, "%s: Exit\n", __func__); > > >