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=-2.8 required=3.0 tests=BAYES_00,DKIM_SIGNED, DKIM_VALID,DKIM_VALID_AU,FREEMAIL_FORGED_FROMDOMAIN,FREEMAIL_FROM, HEADER_FROM_DIFFERENT_DOMAINS,MAILING_LIST_MULTI,SPF_HELO_NONE,SPF_PASS, URIBL_BLOCKED autolearn=no 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 36D9FC4361A for ; Fri, 4 Dec 2020 15:22:33 +0000 (UTC) Received: from lists.xenproject.org (lists.xenproject.org [192.237.175.120]) (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 E643322B42 for ; Fri, 4 Dec 2020 15:22:32 +0000 (UTC) DMARC-Filter: OpenDMARC Filter v1.3.2 mail.kernel.org E643322B42 Authentication-Results: mail.kernel.org; dmarc=fail (p=none dis=none) header.from=gmail.com Authentication-Results: mail.kernel.org; spf=pass smtp.mailfrom=xen-devel-bounces@lists.xenproject.org Received: from list by lists.xenproject.org with outflank-mailman.44892.80312 (Exim 4.92) (envelope-from ) id 1klCuT-0006IB-81; Fri, 04 Dec 2020 15:22:13 +0000 X-Outflank-Mailman: Message body and most headers restored to incoming version Received: by outflank-mailman (output) from mailman id 44892.80312; Fri, 04 Dec 2020 15:22:13 +0000 Received: from localhost ([127.0.0.1] helo=lists.xenproject.org) by lists.xenproject.org with esmtp (Exim 4.92) (envelope-from ) id 1klCuT-0006I4-3x; Fri, 04 Dec 2020 15:22:13 +0000 Received: by outflank-mailman (input) for mailman id 44892; Fri, 04 Dec 2020 15:22:11 +0000 Received: from us1-rack-iad1.inumbo.com ([172.99.69.81]) by lists.xenproject.org with esmtp (Exim 4.92) (envelope-from ) id 1klCuR-0006Hz-RP for xen-devel@lists.xenproject.org; Fri, 04 Dec 2020 15:22:11 +0000 Received: from mail-wr1-x444.google.com (unknown [2a00:1450:4864:20::444]) by us1-rack-iad1.inumbo.com (Halon) with ESMTPS id 4db20e5d-84bf-4a5f-841e-8e079aedb096; Fri, 04 Dec 2020 15:22:11 +0000 (UTC) Received: by mail-wr1-x444.google.com with SMTP id k14so5704547wrn.1 for ; Fri, 04 Dec 2020 07:22:10 -0800 (PST) X-BeenThere: xen-devel@lists.xenproject.org List-Id: Xen developer discussion List-Unsubscribe: , List-Post: List-Help: List-Subscribe: , Errors-To: xen-devel-bounces@lists.xenproject.org Precedence: list Sender: "Xen-devel" X-Inumbo-ID: 4db20e5d-84bf-4a5f-841e-8e079aedb096 DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=gmail.com; s=20161025; h=mime-version:references:in-reply-to:from:date:message-id:subject:to :cc; bh=QUeOFftGxRLW9PKuJNKArkPnQ1h/QDbCkiM1yo1Bi10=; b=I0IIGRq9limyEPu5zReHeh4uQmINXn8c79MoYumI3/PVqt33ELgb5pQJq6WbiHQ3uq vTNBWPbQvyiOurs4JbaYMe60lqVMtm/o+NStYwA9jdSgHii272MYdZK2TJKg4TuyJdo5 VaYJl9fIk0G7DVFT6KPtJvgM8CKOkbEvk7ne4ipF3R6e1whJw8sfUMYGZcq0pblA9nt0 79jQZYSALJopVyn9gFBlbM6QrcbTCy3DpaXVeYnnmXK9XrzW1EACrCaiXNJwuOGaqzir 7YZPkJNgCTYqPSzTNaEjlz41c4fvUdotBm2VBbaHHzg0BFEpyu3xzG+f870SKp9abLU0 6nhA== X-Google-DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=1e100.net; s=20161025; h=x-gm-message-state:mime-version:references:in-reply-to:from:date :message-id:subject:to:cc; bh=QUeOFftGxRLW9PKuJNKArkPnQ1h/QDbCkiM1yo1Bi10=; b=j1Wii2Hc+Rg949YC8ztsAQULxzrdtpF6XsUEi+E+WkYCy/DD8r7c2sLsiLrYcSyaKs 5K4P+xi5Vf4kw3z2AuO45P5XYkbkaY4nsBFFaeqIgnPm/JQjb2JkZ0W2Xbf5yBaZ5ufS JVdBinJtnPFaJ3P4hJrHBFe1uHIQIIgXyOW9O/rn9E4niyfPvrDYu47Z46ky+TDchg0n V6hjpBo57uT4dYjKfmwPYzS/l2Yqw5NphA1KBsAr4QK5/rAXXFj0aTutusWUtiAlhyfO jStnoyz7wyFLFDimL8FN4cU7qgb2zFKccrhItNyxlXzH6Zj9pN3o5PwfNdrm5TjUTLxW p3hA== X-Gm-Message-State: AOAM532IGx4Om/486Pe1yYz1JYAKesFkKKjOrAxKR3R3FAsxbF0WliKX Fjl3oXogWy2nN1C96F5imjmD11DAoCxdB4EPyC4= X-Google-Smtp-Source: ABdhPJwNDiMyt7HazIIMyu5+0kK9tqgCNzVO1Du9W4h0ZaWZwzYOXC1KzBwtfuhxYDqfBEcLzkmJZk12AFjdF+hLtwY= X-Received: by 2002:adf:e3cf:: with SMTP id k15mr5432865wrm.259.1607095330018; Fri, 04 Dec 2020 07:22:10 -0800 (PST) MIME-Version: 1.0 References: <9d7a052a-6222-80ff-cbf1-612d4ca50c2a@suse.com> <17c90493-b438-fbc1-ca10-3bc4d89c4e5e@xen.org> <7a768bcd-80c1-d193-8796-7fb6720fa22a@suse.com> <1a8250f5-ea49-ac3a-e992-be7ec40deba9@xen.org> In-Reply-To: <1a8250f5-ea49-ac3a-e992-be7ec40deba9@xen.org> From: Tamas K Lengyel Date: Fri, 4 Dec 2020 10:21:34 -0500 Message-ID: Subject: Re: [PATCH v3 5/5] evtchn: don't call Xen consumer callback with per-channel lock held To: Julien Grall Cc: Jan Beulich , Andrew Cooper , George Dunlap , Ian Jackson , Wei Liu , Stefano Stabellini , Tamas K Lengyel , Petre Ovidiu PIRCALABU , Alexandru Isaila , "xen-devel@lists.xenproject.org" Content-Type: text/plain; charset="UTF-8" On Fri, Dec 4, 2020 at 6:29 AM Julien Grall wrote: > > Hi Jan, > > On 03/12/2020 10:09, Jan Beulich wrote: > > On 02.12.2020 22:10, Julien Grall wrote: > >> On 23/11/2020 13:30, Jan Beulich wrote: > >>> While there don't look to be any problems with this right now, the lock > >>> order implications from holding the lock can be very difficult to follow > >>> (and may be easy to violate unknowingly). The present callbacks don't > >>> (and no such callback should) have any need for the lock to be held. > >>> > >>> However, vm_event_disable() frees the structures used by respective > >>> callbacks and isn't otherwise synchronized with invocations of these > >>> callbacks, so maintain a count of in-progress calls, for evtchn_close() > >>> to wait to drop to zero before freeing the port (and dropping the lock). > >> > >> AFAICT, this callback is not the only place where the synchronization is > >> missing in the VM event code. > >> > >> For instance, vm_event_put_request() can also race against > >> vm_event_disable(). > >> > >> So shouldn't we handle this issue properly in VM event? > > > > I suppose that's a question to the VM event folks rather than me? > > Yes. From my understanding of Tamas's e-mail, they are relying on the > monitoring software to do the right thing. > > I will refrain to comment on this approach. However, given the race is > much wider than the event channel, I would recommend to not add more > code in the event channel to deal with such problem. > > Instead, this should be fixed in the VM event code when someone has time > to harden the subsystem. I double-checked and the disable route is actually more robust, we don't just rely on the toolstack doing the right thing. The domain gets paused before any calls to vm_event_disable. So I don't think there is really a race-condition here. Tamas