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=-10.1 required=3.0 tests=BAYES_00,DKIM_INVALID, DKIM_SIGNED,HEADER_FROM_DIFFERENT_DOMAINS,MAILING_LIST_MULTI, MENTIONS_GIT_HOSTING,SPF_HELO_NONE,SPF_PASS,USER_AGENT_SANE_1 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 1A704C433E0 for ; Wed, 20 Jan 2021 10:55:57 +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 AA0662070A for ; Wed, 20 Jan 2021 10:55:56 +0000 (UTC) DMARC-Filter: OpenDMARC Filter v1.3.2 mail.kernel.org AA0662070A 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]:53302 helo=lists1p.gnu.org) by lists.gnu.org with esmtp (Exim 4.90_1) (envelope-from ) id 1l2B9X-0005E3-Lr for qemu-devel@archiver.kernel.org; Wed, 20 Jan 2021 05:55:55 -0500 Received: from eggs.gnu.org ([2001:470:142:3::10]:39510) by lists.gnu.org with esmtps (TLS1.2:ECDHE_RSA_AES_256_GCM_SHA384:256) (Exim 4.90_1) (envelope-from ) id 1l2AzU-0003MN-4E for qemu-devel@nongnu.org; Wed, 20 Jan 2021 05:45:32 -0500 Received: from us-smtp-delivery-124.mimecast.com ([216.205.24.124]:28227) by eggs.gnu.org with esmtps (TLS1.2:ECDHE_RSA_AES_256_CBC_SHA1:256) (Exim 4.90_1) (envelope-from ) id 1l2AzR-0006rh-Ao for qemu-devel@nongnu.org; Wed, 20 Jan 2021 05:45:31 -0500 DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=redhat.com; s=mimecast20190719; t=1611139528; 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=JDrReyzPZ7Rd59QCamdR3u8pWWFPQyKoqFGgjV17uJ0=; b=Ge4LiB45hc/QHO6yWzsesi+UEqUNOeHvfsL3N2js7tXtomO/aqqFq81gtjsPE/AZB8SwRw VuX/OJSbM5TlGg1GU12XmfnSmIG9E/JATNaQHCUt910biMMcs+48myPt+tgXsCr1iBtgw+ ADN1isJwRR58ZraQrSgEcxFt092hNvs= 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-215-1jOp5UQfNqqNlgZLjwCvVw-1; Wed, 20 Jan 2021 05:45:22 -0500 X-MC-Unique: 1jOp5UQfNqqNlgZLjwCvVw-1 Received: from smtp.corp.redhat.com (int-mx06.intmail.prod.int.phx2.redhat.com [10.5.11.16]) (using TLSv1.2 with cipher AECDH-AES256-SHA (256/256 bits)) (No client certificate requested) by mimecast-mx01.redhat.com (Postfix) with ESMTPS id 5214B802B40; Wed, 20 Jan 2021 10:45:20 +0000 (UTC) Received: from work-vm (ovpn-115-106.ams2.redhat.com [10.36.115.106]) by smtp.corp.redhat.com (Postfix) with ESMTPS id 250ED71D5F; Wed, 20 Jan 2021 10:45:03 +0000 (UTC) Date: Wed, 20 Jan 2021 10:45:01 +0000 From: "Dr. David Alan Gilbert" To: Vivek Goyal Subject: Re: What are libvhost-user locking requirements Message-ID: <20210120104501.GB2930@work-vm> References: <20210119221849.GC77840@redhat.com> MIME-Version: 1.0 In-Reply-To: <20210119221849.GC77840@redhat.com> User-Agent: Mutt/1.14.6 (2020-07-11) X-Scanned-By: MIMEDefang 2.79 on 10.5.11.16 Authentication-Results: relay.mimecast.com; auth=pass smtp.auth=CUSA124A263 smtp.mailfrom=dgilbert@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=dgilbert@redhat.com; helo=us-smtp-delivery-124.mimecast.com X-Spam_score_int: -29 X-Spam_score: -3.0 X-Spam_bar: --- X-Spam_report: (-3.0 / 5.0 requ) BAYES_00=-1.9, DKIMWL_WL_HIGH=-0.195, 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: gkurz@redhat.com, slp@redhat.com, johannes.berg@intel.com, qemu-devel@nongnu.org, virtio-fs-list , Stefan Hajnoczi , marcandre.lureau@redhat.com Errors-To: qemu-devel-bounces+qemu-devel=archiver.kernel.org@nongnu.org Sender: "Qemu-devel" * Vivek Goyal (vgoyal@redhat.com) wrote: > Hi, > > Current virtiofsd code uses libvhost-user and I am assuming virtiofsd-rs > uses it too. I am wondering what are the locking requirements for > this library. No, virtiofsd-rs uses the rust crate: https://github.com/rust-vmm/vhost-user-backend I guess that's where they get their dose of 'Fearless concurrency' > Looking at it it does not look like thread safe. Well parts of of kind > of look thread safe. For example, David Gilbert introduced a slave_mutex > to control reading/writeing on slave_fd. But dev->slave_fd can be modified > vu_set_slave_req_fd() without any locks. Similiarly _vu_queue_notify() > uses dev->slave_fd but does not take any lock. May be these are just > bugs and we can take slave_mutex in those paths so not a big deal. That would be my assumption; I don't think libvhost-user really thought about it much. > But this library does not talk about locking at all. Of course there > are many shared data structures like "struct VuDev" and helpers which > access this structure. Is client supposed to provide locking and > make sure not more than one thread is calling into the library > at one point of time. I don't think it's defined. > But in virtiofsd I see that we seem to be in mixed mode. In some cases > we are holding ->vu_dispatch_rwlock in read-only mode. So that will > allow multipler threads to call into library for one queue. I think that lock is really protecting against the queue management actions on vhost-user remapping the queue conflicting with things operating on the queue. > In other places like lo_setupmapping() and lo_removemapping(), we are > not holding ->vu_dispatch_rwlock() at all and simply call into > library vu_fs_cache_request(VHOST_USER_SLAVE_FS_MAP/...). So multiple > threads can call in. I think precisely for this use case dev->slave_mutex > has been introduced in library. Note that those calls don't actually read/write interact on the queue itself; so I don't *think* they need the vu_dispatch_rwlock. > So few queries. > > - what's the locking model needed to use libvhost-user. Is there one? I don't think it really had one. > - Is it ok to selectively add locking for some data structures in > libvhost-user. As slave_mutex has been added. So user will have to > go through the code to figure out which paths can be called without > locks and which paths can't be. Well it certainly needed something added; hence why I added slave_mutex, but the slave_mutex is mostly separate from the actual queue processing, and actually rarely used. > /me is confused and trying to wrap my head around the locking requirements > while using libvhost-user. It's not well defined at all. Dave > > Vivek -- Dr. David Alan Gilbert / dgilbert@redhat.com / Manchester, UK