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=-12.3 required=3.0 tests=BAYES_00,DKIM_SIGNED, DKIM_VALID,DKIM_VALID_AU,HEADER_FROM_DIFFERENT_DOMAINS,INCLUDES_PATCH, MAILING_LIST_MULTI,NICE_REPLY_A,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 DA5DFC63777 for ; Thu, 3 Dec 2020 10:09:39 +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 5D39E20705 for ; Thu, 3 Dec 2020 10:09:39 +0000 (UTC) DMARC-Filter: OpenDMARC Filter v1.3.2 mail.kernel.org 5D39E20705 Authentication-Results: mail.kernel.org; dmarc=fail (p=quarantine dis=none) header.from=suse.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.43312.77879 (Exim 4.92) (envelope-from ) id 1kklY6-0006uW-4R; Thu, 03 Dec 2020 10:09:18 +0000 X-Outflank-Mailman: Message body and most headers restored to incoming version Received: by outflank-mailman (output) from mailman id 43312.77879; Thu, 03 Dec 2020 10:09:18 +0000 Received: from localhost ([127.0.0.1] helo=lists.xenproject.org) by lists.xenproject.org with esmtp (Exim 4.92) (envelope-from ) id 1kklY6-0006uP-1V; Thu, 03 Dec 2020 10:09:18 +0000 Received: by outflank-mailman (input) for mailman id 43312; Thu, 03 Dec 2020 10:09:15 +0000 Received: from us1-rack-iad1.inumbo.com ([172.99.69.81]) by lists.xenproject.org with esmtp (Exim 4.92) (envelope-from ) id 1kklY3-0006uK-S4 for xen-devel@lists.xenproject.org; Thu, 03 Dec 2020 10:09:15 +0000 Received: from mx2.suse.de (unknown [195.135.220.15]) by us1-rack-iad1.inumbo.com (Halon) with ESMTPS id e841fc7f-1290-4b74-abb9-3bc1fd4e123e; Thu, 03 Dec 2020 10:09:15 +0000 (UTC) Received: from relay2.suse.de (unknown [195.135.221.27]) by mx2.suse.de (Postfix) with ESMTP id 230B4AC55; Thu, 3 Dec 2020 10:09:14 +0000 (UTC) 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: e841fc7f-1290-4b74-abb9-3bc1fd4e123e X-Virus-Scanned: by amavisd-new at test-mx.suse.de DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=suse.com; s=susede1; t=1606990154; h=from:from:reply-to: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=p+MZFRaqU0qWdrdVSi3QGwDKgoH3z4XD13Kk/lwyE1k=; b=d5g0+NmI4EDvaRFtggAHhwKL/yUT7ts29tmgdBqEABtNfzwxqjn1T7XRHNZy5g+S+11aYq gtmjxaoTaSaqL+fDfR6S2agDS6CxvTe2AUE/X9ggns13W4eHncTWQtijYlCRMuyiEzeVm1 P3ykoTx7n2kMZ4tGmQVaymrqzsGbGe4= Subject: Re: [PATCH v3 5/5] evtchn: don't call Xen consumer callback with per-channel lock held To: Julien Grall Cc: Andrew Cooper , George Dunlap , Ian Jackson , Wei Liu , Stefano Stabellini , Tamas K Lengyel , Petre Ovidiu PIRCALABU , Alexandru Isaila , "xen-devel@lists.xenproject.org" References: <9d7a052a-6222-80ff-cbf1-612d4ca50c2a@suse.com> <17c90493-b438-fbc1-ca10-3bc4d89c4e5e@xen.org> From: Jan Beulich Message-ID: <7a768bcd-80c1-d193-8796-7fb6720fa22a@suse.com> Date: Thu, 3 Dec 2020 11:09:13 +0100 User-Agent: Mozilla/5.0 (Windows NT 10.0; Win64; x64; rv:78.0) Gecko/20100101 Thunderbird/78.5.1 MIME-Version: 1.0 In-Reply-To: <17c90493-b438-fbc1-ca10-3bc4d89c4e5e@xen.org> Content-Type: text/plain; charset=utf-8 Content-Language: en-US Content-Transfer-Encoding: 7bit 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? >> --- >> Should we make this accounting optional, to be requested through a new >> parameter to alloc_unbound_xen_event_channel(), or derived from other >> than the default callback being requested? > > Aside the VM event, do you see any value for the other caller? No (albeit I'm not entirely certain about vpl011_notification()'s needs), hence the consideration. It's unnecessary overhead in those cases. >> @@ -781,9 +786,15 @@ int evtchn_send(struct domain *ld, unsig >> rport = lchn->u.interdomain.remote_port; >> rchn = evtchn_from_port(rd, rport); >> if ( consumer_is_xen(rchn) ) >> + { >> + /* Don't keep holding the lock for the call below. */ >> + atomic_inc(&rchn->u.interdomain.active_calls); >> + evtchn_read_unlock(lchn); >> xen_notification_fn(rchn)(rd->vcpu[rchn->notify_vcpu_id], rport); >> - else >> - evtchn_port_set_pending(rd, rchn->notify_vcpu_id, rchn); > > atomic_dec() doesn't contain any memory barrier, so we will want one > between xen_notification_fn() and atomic_dec() to avoid re-ordering. Oh, indeed. But smp_mb() is too heavy handed here - x86 doesn't really need any barrier, yet would gain a full MFENCE that way. Actually - looks like I forgot we gained smp_mb__before_atomic() a little over half a year ago. Jan