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=-7.8 required=3.0 tests=DATE_IN_PAST_12_24, DKIMWL_WL_HIGH,DKIM_SIGNED,DKIM_VALID,DKIM_VALID_AU,INCLUDES_PATCH, MAILING_LIST_MULTI,SIGNED_OFF_BY,SPF_HELO_NONE,SPF_PASS,USER_AGENT_SANE_1 autolearn=unavailable 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 F3BB8C3B1BF for ; Mon, 17 Feb 2020 02:18:45 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [209.132.180.67]) by mail.kernel.org (Postfix) with ESMTP id C942120857 for ; Mon, 17 Feb 2020 02:18:45 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=kernel.org; s=default; t=1581905925; bh=DOkpBroRKhNueQVg6NXq73R0x8BJgCfT3j7Fl+cG0Yo=; h=Date:From:To:Cc:Subject:Reply-To:References:In-Reply-To:List-ID: From; b=yy48sXsNgw+HRVJOXd7FAI38bIL891Q+GNfgwDubevreB7pIc4fIukdpXhedaDaKT zJ4Mh+KIRjzPAQ0jM2/8s/SiBemGCsmLaUpqIeMjoeMQWNNQRJd/XCwdNQfRR7W9l6 nk+B3M8/qMWeY8XqbkhnZqwvfK+do2bW9bfT3GkQ= Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1727943AbgBQCSm (ORCPT ); Sun, 16 Feb 2020 21:18:42 -0500 Received: from mail.kernel.org ([198.145.29.99]:45762 "EHLO mail.kernel.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1727858AbgBQCSg (ORCPT ); Sun, 16 Feb 2020 21:18:36 -0500 Received: from paulmck-ThinkPad-P72.home (50-39-105-78.bvtn.or.frontiernet.net [50.39.105.78]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by mail.kernel.org (Postfix) with ESMTPSA id 91D102086A; Mon, 17 Feb 2020 02:18:35 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=kernel.org; s=default; t=1581905915; bh=DOkpBroRKhNueQVg6NXq73R0x8BJgCfT3j7Fl+cG0Yo=; h=Date:From:To:Cc:Subject:Reply-To:References:In-Reply-To:From; b=RLDWUDCwO9PyGREb/JfPQGLT28deo72GJeAwzSyfOWELuq9r6D7zj79uJ2qxlDD1t QCh5kz1ya3m7GmkbOCdbyXHHeyzQuCaKntUpFRK9MWAbvd/3vOZzFR/SCykp5uXNSz 59VqTwHWKXOEMivDKdQY5JjcUlv5CVc6pFpRQ6u4= Received: by paulmck-ThinkPad-P72.home (Postfix, from userid 1000) id AD97435226F4; Sun, 16 Feb 2020 06:11:51 -0800 (PST) Date: Sun, 16 Feb 2020 06:11:51 -0800 From: "Paul E. McKenney" To: Giuseppe Scrivano Cc: linux-kernel@vger.kernel.org, rcu@vger.kernel.org, ebiederm@xmission.com Subject: Re: [PATCH] ipc: use a work queue to free_ipc Message-ID: <20200216141151.GJ2935@paulmck-ThinkPad-P72> Reply-To: paulmck@kernel.org References: <20200211162408.2194037-1-gscrivan@redhat.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20200211162408.2194037-1-gscrivan@redhat.com> User-Agent: Mutt/1.9.4 (2018-02-28) Sender: linux-kernel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Tue, Feb 11, 2020 at 05:24:08PM +0100, Giuseppe Scrivano wrote: > it avoids blocking on synchronize_rcu() in kern_umount(). > > the code: > > \#define _GNU_SOURCE > \#include > \#include > \#include > \#include > int main() > { > int i; > for (i = 0; i < 1000; i++) > if (unshare (CLONE_NEWIPC) < 0) > error (EXIT_FAILURE, errno, "unshare"); > } > > gets from: > > Command being timed: "./ipc-namespace" > User time (seconds): 0.00 > System time (seconds): 0.06 > Percent of CPU this job got: 0% > Elapsed (wall clock) time (h:mm:ss or m:ss): 0:08.05 > > to: > > Command being timed: "./ipc-namespace" > User time (seconds): 0.00 > System time (seconds): 0.02 > Percent of CPU this job got: 96% > Elapsed (wall clock) time (h:mm:ss or m:ss): 0:00.03 Nice speedup! However, I am not convinced that the code shown below is safe. I believe that you need either a synchronize_rcu() in your free_ipc() function or that you need to pass free_ipc() to queue_rcu_work() instead of directly schedule_work(). As things are, I would expect you to see free_ipc_ns() being invoke too soon on heavily loaded CONFIG_PREEMPT=y kernels. Which can be quite a pain to debug! Or am I missing something? Thanx, Paul > Signed-off-by: Giuseppe Scrivano > --- > include/linux/ipc_namespace.h | 2 ++ > ipc/namespace.c | 17 +++++++++++++++-- > 2 files changed, 17 insertions(+), 2 deletions(-) > > diff --git a/include/linux/ipc_namespace.h b/include/linux/ipc_namespace.h > index c309f43bde45..a06a78c67f19 100644 > --- a/include/linux/ipc_namespace.h > +++ b/include/linux/ipc_namespace.h > @@ -68,6 +68,8 @@ struct ipc_namespace { > struct user_namespace *user_ns; > struct ucounts *ucounts; > > + struct llist_node mnt_llist; > + > struct ns_common ns; > } __randomize_layout; > > diff --git a/ipc/namespace.c b/ipc/namespace.c > index b3ca1476ca51..37d27e1b807a 100644 > --- a/ipc/namespace.c > +++ b/ipc/namespace.c > @@ -117,6 +117,7 @@ void free_ipcs(struct ipc_namespace *ns, struct ipc_ids *ids, > > static void free_ipc_ns(struct ipc_namespace *ns) > { > + mq_put_mnt(ns); > sem_exit_ns(ns); > msg_exit_ns(ns); > shm_exit_ns(ns); > @@ -127,6 +128,17 @@ static void free_ipc_ns(struct ipc_namespace *ns) > kfree(ns); > } > > +static LLIST_HEAD(free_ipc_list); > +static void free_ipc(struct work_struct *unused) > +{ > + struct llist_node *node = llist_del_all(&free_ipc_list); > + struct ipc_namespace *n, *t; > + > + llist_for_each_entry_safe(n, t, node, mnt_llist) > + free_ipc_ns(n); > +} > +static DECLARE_WORK(free_ipc_work, free_ipc); > + > /* > * put_ipc_ns - drop a reference to an ipc namespace. > * @ns: the namespace to put > @@ -148,8 +160,9 @@ void put_ipc_ns(struct ipc_namespace *ns) > if (refcount_dec_and_lock(&ns->count, &mq_lock)) { > mq_clear_sbinfo(ns); > spin_unlock(&mq_lock); > - mq_put_mnt(ns); > - free_ipc_ns(ns); > + > + if (llist_add(&ns->mnt_llist, &free_ipc_list)) > + schedule_work(&free_ipc_work); > } > } > > -- > 2.24.1 >