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=-15.8 required=3.0 tests=BAYES_00,DKIMWL_WL_HIGH, DKIM_SIGNED,DKIM_VALID,DKIM_VALID_AU,HEADER_FROM_DIFFERENT_DOMAINS, INCLUDES_CR_TRAILER,INCLUDES_PATCH,MAILING_LIST_MULTI,SPF_HELO_NONE,SPF_PASS 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 EA9F7C433ED for ; Fri, 16 Apr 2021 14:10:23 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id C622061184 for ; Fri, 16 Apr 2021 14:10:23 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S243285AbhDPOKr (ORCPT ); Fri, 16 Apr 2021 10:10:47 -0400 Received: from us-smtp-delivery-124.mimecast.com ([216.205.24.124]:47961 "EHLO us-smtp-delivery-124.mimecast.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S234914AbhDPOKq (ORCPT ); Fri, 16 Apr 2021 10:10:46 -0400 DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=redhat.com; s=mimecast20190719; t=1618582221; 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=w0YYzj8V73L8TZdDRT/cuzZofbVkBWAoCy5mihzJOz8=; b=LUCP3XZKU8UwzdgqG5VKW3xOvsHm/IiX8YoBFfF0dhVe4go+KrH0QMkaO9RcaqGyg2XQ6u 0xZ55kH42pM7cC289n+g3Qjswrqy2ZaCylEBr2RUao9Yq84GB51ITa7u419FLts0sF4ngY 9Wp1q7u1NSsG6wv424uJJ0PRsw4bSes= 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-557-4lNchRc0MJSUmXqjEU735g-1; Fri, 16 Apr 2021 10:10:20 -0400 X-MC-Unique: 4lNchRc0MJSUmXqjEU735g-1 Received: from smtp.corp.redhat.com (int-mx07.intmail.prod.int.phx2.redhat.com [10.5.11.22]) (using TLSv1.2 with cipher AECDH-AES256-SHA (256/256 bits)) (No client certificate requested) by mimecast-mx01.redhat.com (Postfix) with ESMTPS id 86C6883DD2A; Fri, 16 Apr 2021 14:10:18 +0000 (UTC) Received: from T590 (ovpn-12-27.pek2.redhat.com [10.72.12.27]) by smtp.corp.redhat.com (Postfix) with ESMTPS id EB3681042A90; Fri, 16 Apr 2021 14:10:11 +0000 (UTC) Date: Fri, 16 Apr 2021 22:10:07 +0800 From: Ming Lei To: Pavel Begunkov Cc: Dennis Zhou , Jens Axboe , io-uring@vger.kernel.org, Tejun Heo , Christoph Lameter , Joakim Hassila Subject: Re: [PATCH 1/2] percpu_ref: add percpu_ref_atomic_count() Message-ID: References: <10b84fd7-4c40-3fe6-6993-061b524b1487@gmail.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <10b84fd7-4c40-3fe6-6993-061b524b1487@gmail.com> X-Scanned-By: MIMEDefang 2.84 on 10.5.11.22 Precedence: bulk List-ID: X-Mailing-List: io-uring@vger.kernel.org On Fri, Apr 16, 2021 at 02:16:41PM +0100, Pavel Begunkov wrote: > On 16/04/2021 05:45, Dennis Zhou wrote: > > Hello, > > > > On Fri, Apr 16, 2021 at 01:22:51AM +0100, Pavel Begunkov wrote: > >> Add percpu_ref_atomic_count(), which returns number of references of a > >> percpu_ref switched prior into atomic mode, so the caller is responsible > >> to make sure it's in the right mode. > >> > >> Signed-off-by: Pavel Begunkov > >> --- > >> include/linux/percpu-refcount.h | 1 + > >> lib/percpu-refcount.c | 26 ++++++++++++++++++++++++++ > >> 2 files changed, 27 insertions(+) > >> > >> diff --git a/include/linux/percpu-refcount.h b/include/linux/percpu-refcount.h > >> index 16c35a728b4c..0ff40e79efa2 100644 > >> --- a/include/linux/percpu-refcount.h > >> +++ b/include/linux/percpu-refcount.h > >> @@ -131,6 +131,7 @@ void percpu_ref_kill_and_confirm(struct percpu_ref *ref, > >> void percpu_ref_resurrect(struct percpu_ref *ref); > >> void percpu_ref_reinit(struct percpu_ref *ref); > >> bool percpu_ref_is_zero(struct percpu_ref *ref); > >> +unsigned long percpu_ref_atomic_count(struct percpu_ref *ref); > >> > >> /** > >> * percpu_ref_kill - drop the initial ref > >> diff --git a/lib/percpu-refcount.c b/lib/percpu-refcount.c > >> index a1071cdefb5a..56286995e2b8 100644 > >> --- a/lib/percpu-refcount.c > >> +++ b/lib/percpu-refcount.c > >> @@ -425,6 +425,32 @@ bool percpu_ref_is_zero(struct percpu_ref *ref) > >> } > >> EXPORT_SYMBOL_GPL(percpu_ref_is_zero); > >> > >> +/** > >> + * percpu_ref_atomic_count - returns number of left references > >> + * @ref: percpu_ref to test > >> + * > >> + * This function is safe to call as long as @ref is switch into atomic mode, > >> + * and is between init and exit. > >> + */ > >> +unsigned long percpu_ref_atomic_count(struct percpu_ref *ref) > >> +{ > >> + unsigned long __percpu *percpu_count; > >> + unsigned long count, flags; > >> + > >> + if (WARN_ON_ONCE(__ref_is_percpu(ref, &percpu_count))) > >> + return -1UL; > >> + > >> + /* protect us from being destroyed */ > >> + spin_lock_irqsave(&percpu_ref_switch_lock, flags); > >> + if (ref->data) > >> + count = atomic_long_read(&ref->data->count); > >> + else > >> + count = ref->percpu_count_ptr >> __PERCPU_REF_FLAG_BITS; > > > > Sorry I missed Jens' patch before and also the update to percpu_ref. > > However, I feel like I'm missing something. This isn't entirely related > > to your patch, but I'm not following why percpu_count_ptr stores the > > excess count of an exited percpu_ref and doesn't warn when it's not > > zero. It seems like this should be an error if it's not 0? > > > > Granted we have made some contract with the user to do the right thing, > > but say someone does mess up, we don't indicate to them hey this ref is > > actually dead and if they're waiting for it to go to 0, it never will. > > fwiw, I copied is_zero, but skimming through the code don't immediately > see myself why it is so... > > Cc Ming, he split out some parts of it to dynamic allocation not too > long ago, maybe he knows the trick. I remembered that percpu_ref_is_zero() can be called even after percpu_ref_exit() returns, and looks percpu_ref_is_zero() isn't classified into 'active use'. Thanks, Ming