linux-kernel - Re: [PATCH] ipc: use a work queue to free

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Date:   Sun, 16 Feb 2020 06:11:51 -0800
From:   "Paul E. McKenney" <paulmck@...nel.org>
To:     Giuseppe Scrivano <gscrivan@...hat.com>
Cc:     linux-kernel@...r.kernel.org, rcu@...r.kernel.org,
        ebiederm@...ssion.com
Subject: Re: [PATCH] ipc: use a work queue to free_ipc

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 <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

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 <gscrivan@...hat.com>
> ---
>  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
>