Re: [RFC PATCH] SUNRPC: protect service sockets lists during per-net shutdown

["J. Bruce Fields" <bfields@fieldses.org>]

On Fri, May 11, 2012 at 03:41:56PM +0400, Stanislav Kinsbursky wrote:
> Service sv_tempsocks and sv_permsocks lists are accessible by tasks with
> different network namespaces, and thus per-net service destruction must be
> protected.
> These lists are protected by service sv_lock. So lets wrap list munipulations
> with this lock and move tranports destruction outside wrapped area to prevent
> deadlocks.

The comment I originally quoted is still wrong now:

	/*
	 * The set of xprts (contained in the sv_tempsocks and
	 * sv_permsocks lists) is now constant, since it is modified
	 * only by accepting new sockets (done by service threads in
	 * svc_recv) or aging old ones (done by sv_temptimer), or
	 * configuration changes (excluded by whatever locking the
	 * caller is using--nfsd_mutex in the case of nfsd).  So it's
	 * safe to traverse those lists and shut everything down:
	 */

And I think that's still a problem.

A server thread could be running svc_recv(), handling a new connection
on a listening socket in the network namespace that we're shutting down.

And then I'm not sure exactly what happens, but it doesn't look right.
At best we end up adding a connection from the new network namespace
after we thought we'd got rid of them all.  More likely we crash
somewhere.

--b.


> 
> Signed-off-by: Stanislav Kinsbursky <skinsbursky@parallels.com>
> ---
>  net/sunrpc/svc_xprt.c |   29 +++++++++++++++++++++++++----
>  1 files changed, 25 insertions(+), 4 deletions(-)
> 
> diff --git a/net/sunrpc/svc_xprt.c b/net/sunrpc/svc_xprt.c
> index 8195c6a..233f993 100644
> --- a/net/sunrpc/svc_xprt.c
> +++ b/net/sunrpc/svc_xprt.c
> @@ -954,7 +954,8 @@ static void svc_clear_pools(struct svc_serv *serv, struct net *net)
>  	}
>  }
>  
> -static void svc_clear_list(struct list_head *xprt_list, struct net *net)
> +static void svc_clear_list(struct list_head *xprt_list, struct net *net,
> +			   struct list_head *kill_list)
>  {
>  	struct svc_xprt *xprt;
>  	struct svc_xprt *tmp;
> @@ -962,7 +963,8 @@ static void svc_clear_list(struct list_head *xprt_list, struct net *net)
>  	list_for_each_entry_safe(xprt, tmp, xprt_list, xpt_list) {
>  		if (xprt->xpt_net != net)
>  			continue;
> -		svc_delete_xprt(xprt);
> +		list_move(&xprt->xpt_list, kill_list);
> +		set_bit(XPT_DETACHED, &xprt->xpt_flags);
>  	}
>  	list_for_each_entry(xprt, xprt_list, xpt_list)
>  		BUG_ON(xprt->xpt_net == net);
> @@ -970,6 +972,15 @@ static void svc_clear_list(struct list_head *xprt_list, struct net *net)
>  
>  void svc_close_net(struct svc_serv *serv, struct net *net)
>  {
> +	struct svc_xprt *xprt;
> +	LIST_HEAD(kill_list);
> +
> +	/*
> +	 * Protect the lists, since they can be by tasks with different network
> +	 * namespace contexts.
> +	 */
> +	spin_lock(&serv->sv_lock);
> +
>  	svc_close_list(&serv->sv_tempsocks, net);
>  	svc_close_list(&serv->sv_permsocks, net);
>  
> @@ -979,8 +990,18 @@ void svc_close_net(struct svc_serv *serv, struct net *net)
>  	 * svc_enqueue will not add new entries without taking the
>  	 * sp_lock and checking XPT_BUSY.
>  	 */
> -	svc_clear_list(&serv->sv_tempsocks, net);
> -	svc_clear_list(&serv->sv_permsocks, net);
> +	svc_clear_list(&serv->sv_tempsocks, net, &kill_list);
> +	svc_clear_list(&serv->sv_permsocks, net, &kill_list);
> +
> +	spin_unlock(&serv->sv_lock);
> +
> +	/*
> +	 * Destroy collected transports.
> +	 * Note: tranports has been marked as XPT_DETACHED on svc_clear_list(),
> +	 * so no need to protect againt list_del() in svc_delete_xprt().
> +	 */
> +	list_for_each_entry(xprt, &kill_list, xpt_list)
> +		svc_delete_xprt(xprt);
>  }
>  
>  /*
>