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Subject: Re: [PATCH v3] ipc: use a work queue to free_ipc
To:     Giuseppe Scrivano <gscrivan@redhat.com>,
        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 <longman@redhat.com>
Organization: Red Hat
Message-ID: <1cdee0c9-58d3-62c1-92db-3f2035fb8424@redhat.com>
Date:   Mon, 23 Mar 2020 14:08:37 -0400
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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 <sched.h>
> \#include <error.h>
> \#include <errno.h>
> \#include <stdlib.h>
> 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 <gscrivan@redhat.com>
> ---
> 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 <longman@redhat.com>