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=-2.3 required=3.0 tests=HEADER_FROM_DIFFERENT_DOMAINS, MAILING_LIST_MULTI,SPF_PASS,USER_AGENT_MUTT 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 0C206C6778D for ; Tue, 11 Sep 2018 15:45:56 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [209.132.180.67]) by mail.kernel.org (Postfix) with ESMTP id A2D7120839 for ; Tue, 11 Sep 2018 15:45:55 +0000 (UTC) DMARC-Filter: OpenDMARC Filter v1.3.2 mail.kernel.org A2D7120839 Authentication-Results: mail.kernel.org; dmarc=fail (p=none dis=none) header.from=redhat.com Authentication-Results: mail.kernel.org; spf=none smtp.mailfrom=linux-kernel-owner@vger.kernel.org Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1727034AbeIKUpr (ORCPT ); Tue, 11 Sep 2018 16:45:47 -0400 Received: from mx3-rdu2.redhat.com ([66.187.233.73]:50430 "EHLO mx1.redhat.com" rhost-flags-OK-OK-OK-FAIL) by vger.kernel.org with ESMTP id S1726622AbeIKUpq (ORCPT ); Tue, 11 Sep 2018 16:45:46 -0400 Received: from smtp.corp.redhat.com (int-mx06.intmail.prod.int.rdu2.redhat.com [10.11.54.6]) (using TLSv1.2 with cipher AECDH-AES256-SHA (256/256 bits)) (No client certificate requested) by mx1.redhat.com (Postfix) with ESMTPS id 140CF401EF2E; Tue, 11 Sep 2018 15:45:53 +0000 (UTC) Received: from ming.t460p (ovpn-8-22.pek2.redhat.com [10.72.8.22]) by smtp.corp.redhat.com (Postfix) with ESMTPS id 639A82157F49; Tue, 11 Sep 2018 15:45:46 +0000 (UTC) Date: Tue, 11 Sep 2018 23:45:41 +0800 From: Ming Lei To: Tejun Heo Cc: linux-kernel@vger.kernel.org, Jianchao Wang , Kent Overstreet , linux-block@vger.kernel.org Subject: Re: [PATCH] percpu-refcount: relax limit on percpu_ref_reinit() Message-ID: <20180911154540.GA10082@ming.t460p> References: <20180909125824.9150-1-ming.lei@redhat.com> <20180910164920.GE1100574@devbig004.ftw2.facebook.com> <20180911000049.GB30977@ming.t460p> <20180911134836.GG1100574@devbig004.ftw2.facebook.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20180911134836.GG1100574@devbig004.ftw2.facebook.com> User-Agent: Mutt/1.9.1 (2017-09-22) X-Scanned-By: MIMEDefang 2.78 on 10.11.54.6 X-Greylist: Sender IP whitelisted, not delayed by milter-greylist-4.5.16 (mx1.redhat.com [10.11.55.5]); Tue, 11 Sep 2018 15:45:53 +0000 (UTC) X-Greylist: inspected by milter-greylist-4.5.16 (mx1.redhat.com [10.11.55.5]); Tue, 11 Sep 2018 15:45:53 +0000 (UTC) for IP:'10.11.54.6' DOMAIN:'int-mx06.intmail.prod.int.rdu2.redhat.com' HELO:'smtp.corp.redhat.com' FROM:'ming.lei@redhat.com' RCPT:'' Sender: linux-kernel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Tue, Sep 11, 2018 at 06:48:36AM -0700, Tejun Heo wrote: > Hello, Ming. > > On Tue, Sep 11, 2018 at 08:00:50AM +0800, Ming Lei wrote: > > > > @@ -357,10 +349,11 @@ EXPORT_SYMBOL_GPL(percpu_ref_kill_and_confirm); > > > > void percpu_ref_reinit(struct percpu_ref *ref) > > > > { > > > > unsigned long flags; > > > > + unsigned long __percpu *percpu_count; > > > > > > > > spin_lock_irqsave(&percpu_ref_switch_lock, flags); > > > > > > > > - WARN_ON_ONCE(!percpu_ref_is_zero(ref)); > > > > + WARN_ON_ONCE(__ref_is_percpu(ref, &percpu_count)); > > > > > > Can you elaborate this part? This doesn't seem required for the > > > described change. Why is it part of the patch? > > > > The motivation of this patch is to avoid the above warning and allow > > the ref to switch back to percpu mode without dropping to zero. > > > > That is why the check has to be changed to the above way. > > So, this part seems wrong. The function is called percpu_ref_reinit() > - the refcnt is expected to be in its initial state with just the base > ref once this function returns. If you're removing the restriction on But the comment says that 'Re-initialize @ref so that it's in the same state as when it finished', and this invariant isn't changed with this patch. > when this can be called, you should also make sure that the function > actually enforces the target state. Also, this is a separate logical > change, please put it in a separate patch. OK, will do it in V2. Thanks, Ming