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 6E179C4708A for ; Thu, 27 May 2021 11:17:18 +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 25E7C610A6 for ; Thu, 27 May 2021 11:17:18 +0000 (UTC) DMARC-Filter: OpenDMARC Filter v1.3.2 mail.kernel.org 25E7C610A6 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.133006.248011 (Exim 4.92) (envelope-from ) id 1lmE0e-0006cz-DN; Thu, 27 May 2021 11:17:04 +0000 X-Outflank-Mailman: Message body and most headers restored to incoming version Received: by outflank-mailman (output) from mailman id 133006.248011; Thu, 27 May 2021 11:17:04 +0000 Received: from localhost ([127.0.0.1] helo=lists.xenproject.org) by lists.xenproject.org with esmtp (Exim 4.92) (envelope-from ) id 1lmE0e-0006cs-AU; Thu, 27 May 2021 11:17:04 +0000 Received: by outflank-mailman (input) for mailman id 133006; Thu, 27 May 2021 11:17:03 +0000 Received: from all-amaz-eas1.inumbo.com ([34.197.232.57] helo=us1-amaz-eas2.inumbo.com) by lists.xenproject.org with esmtp (Exim 4.92) (envelope-from ) id 1lmE0d-0006cm-4b for xen-devel@lists.xenproject.org; Thu, 27 May 2021 11:17:03 +0000 Received: from smtp-out1.suse.de (unknown [195.135.220.28]) by us1-amaz-eas2.inumbo.com (Halon) with ESMTPS id a6533f1b-3c6e-44e3-bc90-e1f143d4bbcf; Thu, 27 May 2021 11:17:01 +0000 (UTC) Received: from imap.suse.de (imap-alt.suse-dmz.suse.de [192.168.254.47]) (using TLSv1.2 with cipher ECDHE-ECDSA-AES128-GCM-SHA256 (128/128 bits)) (No client certificate requested) by smtp-out1.suse.de (Postfix) with ESMTPS id BF169218DD; Thu, 27 May 2021 11:17:00 +0000 (UTC) Received: from director2.suse.de (director2.suse-dmz.suse.de [192.168.254.72]) by imap.suse.de (Postfix) with ESMTPSA id 8A11611A98; Thu, 27 May 2021 11:17:00 +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: a6533f1b-3c6e-44e3-bc90-e1f143d4bbcf DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=suse.com; s=susede1; t=1622114220; 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=0WtvNwAECKle8+6jyQuA7PmY2BIdWUsLQwGx1tLe9Ns=; b=JYIEv7kr8MQ3szYMRnCH1wQVG5B7Bvhd34qGXC+FRF6tqTP0E4e86vnKSQ/H96iSLXWedV D0+a/drrixhYgVeOxcjGmNzC2pLYkk2V4DzS/m7SO+oDNavD023Y7j2TzDNwCPyBtz6P2v qIpWzHyGC0viE1OapAOJECnlS+ReyWc= Subject: Re: [PATCH v5 2/6] evtchn: convert domain event lock to an r/w one To: =?UTF-8?Q?Roger_Pau_Monn=c3=a9?= Cc: "xen-devel@lists.xenproject.org" , Andrew Cooper , George Dunlap , Ian Jackson , Julien Grall , Wei Liu , Stefano Stabellini , Kevin Tian References: <306e62e8-9070-2db9-c959-858465c50c1d@suse.com> <5f5fc6a7-6e27-8275-0f05-11ba5454156a@suse.com> From: Jan Beulich Message-ID: Date: Thu, 27 May 2021 13:16:55 +0200 User-Agent: Mozilla/5.0 (Windows NT 10.0; Win64; x64; rv:78.0) Gecko/20100101 Thunderbird/78.10.2 MIME-Version: 1.0 In-Reply-To: Content-Type: text/plain; charset=utf-8 Content-Language: en-US Content-Transfer-Encoding: 8bit On 27.05.2021 13:01, Roger Pau Monné wrote: > On Wed, Jan 27, 2021 at 09:16:07AM +0100, Jan Beulich wrote: >> Especially for the use in evtchn_move_pirqs() (called when moving a vCPU >> across pCPU-s) and the ones in EOI handling in PCI pass-through code, >> serializing perhaps an entire domain isn't helpful when no state (which >> isn't e.g. further protected by the per-channel lock) changes. > > I'm unsure this move is good from a performance PoV, as the operations > switched to use the lock in read mode is a very small subset, and then > the remaining operations get a performance penalty when compared to > using a plain spin lock. Well, yes, unfortunately review of earlier versions has resulted in there being quite a few less read_lock() uses now than I had (mistakenly) originally. There are a few worthwhile conversions, but on the whole maybe I should indeed drop this change. >> @@ -1510,9 +1509,10 @@ int evtchn_destroy(struct domain *d) >> { >> unsigned int i; >> >> - /* After this barrier no new event-channel allocations can occur. */ >> + /* After this kind-of-barrier no new event-channel allocations can occur. */ >> BUG_ON(!d->is_dying); >> - spin_barrier(&d->event_lock); >> + read_lock(&d->event_lock); >> + read_unlock(&d->event_lock); > > Don't you want to use write mode here to assure there are no read > users that have taken the lock before is_dying has been set, and thus > could make wrong assumptions? > > As I understand the point of the barrier here is to ensure there are > no lockers carrier over from before is_dying has been set. The purpose is, as the comment says, no new event channel allocations. Those happen under write lock, so a read-lock-based barrier is enough here afaict. Jan