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 Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by smtp.lore.kernel.org (Postfix) with ESMTP id 24A34C433F5 for ; Thu, 7 Apr 2022 22:58:03 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S232365AbiDGXAC (ORCPT ); Thu, 7 Apr 2022 19:00:02 -0400 Received: from lindbergh.monkeyblade.net ([23.128.96.19]:37006 "EHLO lindbergh.monkeyblade.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S231496AbiDGW77 (ORCPT ); Thu, 7 Apr 2022 18:59:59 -0400 Received: from sin.source.kernel.org (sin.source.kernel.org [IPv6:2604:1380:40e1:4800::1]) by lindbergh.monkeyblade.net (Postfix) with ESMTPS id E7E781D08D3 for ; Thu, 7 Apr 2022 15:57:56 -0700 (PDT) Received: from smtp.kernel.org (relay.kernel.org [52.25.139.140]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by sin.source.kernel.org (Postfix) with ESMTPS id 53F50CE29E0 for ; Thu, 7 Apr 2022 22:57:55 +0000 (UTC) Received: by smtp.kernel.org (Postfix) with ESMTPSA id 3BC31C385AA; Thu, 7 Apr 2022 22:57:53 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=linux-foundation.org; s=korg; t=1649372273; bh=RN5h09BMy0SrcZOTMv0e9oxWP9f+jFVQ8ak4khF7YuA=; h=Date:From:To:Cc:Subject:In-Reply-To:References:From; b=wH+dPDwBvjM0YnQ/2yhFHLCsG2BViIutMpxZxwmpr0OiltKgRl+uEb5oStc255nm6 OhV/xRj/1T1LKg5CsfXEsSX3RbpNDKDx13xhQusGjII6ktc24soGHxmb3LXUdwNTt6 J7tsSTXqajNdgqZQ0nI+Y1zk9dGRrWqQ/vTldneU= Date: Thu, 7 Apr 2022 15:57:52 -0700 From: Andrew Morton To: Qi Zheng Cc: dennis@kernel.org, tj@kernel.org, cl@linux.com, linux-mm@kvack.org, linux-kernel@vger.kernel.org, zhouchengming@bytedance.com, songmuchun@bytedance.com, Ming Lei Subject: Re: [PATCH] percpu_ref: call wake_up_all() after percpu_ref_put() completes Message-Id: <20220407155752.769632b737f79b038cf83742@linux-foundation.org> In-Reply-To: <20220407103335.36885-1-zhengqi.arch@bytedance.com> References: <20220407103335.36885-1-zhengqi.arch@bytedance.com> X-Mailer: Sylpheed 3.7.0 (GTK+ 2.24.33; x86_64-redhat-linux-gnu) Mime-Version: 1.0 Content-Type: text/plain; charset=US-ASCII Content-Transfer-Encoding: 7bit Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org (cc Ming Lei) On Thu, 7 Apr 2022 18:33:35 +0800 Qi Zheng wrote: > In the percpu_ref_call_confirm_rcu(), we call the wake_up_all() > before calling percpu_ref_put(), which will cause the value of > percpu_ref to be unstable when percpu_ref_switch_to_atomic_sync() > returns. > > CPU0 CPU1 > > percpu_ref_switch_to_atomic_sync(&ref) > --> percpu_ref_switch_to_atomic(&ref) > --> percpu_ref_get(ref); /* put after confirmation */ > call_rcu(&ref->data->rcu, percpu_ref_switch_to_atomic_rcu); > > percpu_ref_switch_to_atomic_rcu > --> percpu_ref_call_confirm_rcu > --> data->confirm_switch = NULL; > wake_up_all(&percpu_ref_switch_waitq); > > /* here waiting to wake up */ > wait_event(percpu_ref_switch_waitq, !ref->data->confirm_switch); > (A)percpu_ref_put(ref); > /* The value of &ref is unstable! */ > percpu_ref_is_zero(&ref) > (B)percpu_ref_put(ref); > > As shown above, assuming that the counts on each cpu add up to 0 before > calling percpu_ref_switch_to_atomic_sync(), we expect that after switching > to atomic mode, percpu_ref_is_zero() can return true. But actually it will > return different values in the two cases of A and B, which is not what > we expected. > > Maybe the original purpose of percpu_ref_switch_to_atomic_sync() is > just to ensure that the conversion to atomic mode is completed, but it > should not return with an extra reference count. > > Calling wake_up_all() after percpu_ref_put() ensures that the value of > percpu_ref is stable after percpu_ref_switch_to_atomic_sync() returns. > So just do it. Thanks. I'll grab this, but shall await input from others before doing anything else with it. > Signed-off-by: Qi Zheng > +++ b/lib/percpu-refcount.c > @@ -154,13 +154,14 @@ static void percpu_ref_call_confirm_rcu(struct rcu_head *rcu) > > data->confirm_switch(ref); > data->confirm_switch = NULL; > - wake_up_all(&percpu_ref_switch_waitq); > > if (!data->allow_reinit) > __percpu_ref_exit(ref); > > /* drop ref from percpu_ref_switch_to_atomic() */ > percpu_ref_put(ref); > + > + wake_up_all(&percpu_ref_switch_waitq); > } > > static void percpu_ref_switch_to_atomic_rcu(struct rcu_head *rcu)