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Mon, 23 Mar 2020 18:08:38 +0000 (UTC) Received: from llong.remote.csb (ovpn-117-56.rdu2.redhat.com [10.10.117.56]) by smtp.corp.redhat.com (Postfix) with ESMTP id C6B519497D; Mon, 23 Mar 2020 18:08:37 +0000 (UTC) Subject: Re: [PATCH v3] ipc: use a work queue to free_ipc To: Giuseppe Scrivano , linux-kernel@vger.kernel.org Cc: rcu@vger.kernel.org, ebiederm@xmission.com, paulmck@kernel.org, viro@zeniv.linux.org.uk References: <20200225145419.527994-1-gscrivan@redhat.com> From: Waiman Long Organization: Red Hat Message-ID: <1cdee0c9-58d3-62c1-92db-3f2035fb8424@redhat.com> Date: Mon, 23 Mar 2020 14:08:37 -0400 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:60.0) Gecko/20100101 Thunderbird/60.7.2 MIME-Version: 1.0 In-Reply-To: <20200225145419.527994-1-gscrivan@redhat.com> Content-Type: text/plain; charset=utf-8 Content-Transfer-Encoding: 7bit Content-Language: en-US X-Scanned-By: MIMEDefang 2.79 on 10.5.11.15 Sender: linux-kernel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On 2/25/20 9:54 AM, Giuseppe Scrivano wrote: > the reason is to avoid a delay caused by the synchronize_rcu() call in > kern_umount() when the mqueue mount is freed. > > 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 > > Signed-off-by: Giuseppe Scrivano > --- > v3: > - added comment for free_ipc_work > - commit message rephrased > > v2: https://lkml.org/lkml/2020/2/17/839 > - comment added in free_ipc_ns() > > v1: https://lkml.org/lkml/2020/2/11/692 > > include/linux/ipc_namespace.h | 2 ++ > ipc/namespace.c | 24 ++++++++++++++++++++++-- > 2 files changed, 24 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..c0470aef41a0 100644 > --- a/ipc/namespace.c > +++ b/ipc/namespace.c > @@ -117,6 +117,10 @@ void free_ipcs(struct ipc_namespace *ns, struct ipc_ids *ids, > > static void free_ipc_ns(struct ipc_namespace *ns) > { > + /* mq_put_mnt() waits for a grace period as kern_unmount() > + * uses synchronize_rcu(). > + */ > + mq_put_mnt(ns); > sem_exit_ns(ns); > msg_exit_ns(ns); > shm_exit_ns(ns); > @@ -127,6 +131,21 @@ 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); > +} > + > +/* > + * The work queue is used to avoid the cost of synchronize_rcu in kern_unmount. > + */ > +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 +167,9 @@ 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); > } > } > I also have used llist recently. This is the standard way llist is used and I don't any problem using it to defer the freeing so that it won't block the put_ipc_ns() caller. Reviewed-by: Waiman Long