Re: [PATCH 2/4] sunrpc: make xprt->swapper an atomic_t

[Chuck Lever <chuck.lever@oracle.com>]

Hi Jeff-

On May 30, 2015, at 8:03 AM, Jeff Layton <jlayton@poochiereds.net> wrote:

> Split xs_swapper into enable/disable functions and eliminate the
> "enable" flag.
> 
> Currently, it's racy if you have multiple swapon/swapoff operations
> running in parallel over the same xprt. Also fix it so that we only
> set it to a memalloc socket on a 0->1 transition and only clear it
> on a 1->0 transition.
> 
> Cc: Mel Gorman <mgorman@suse.de>
> Signed-off-by: Jeff Layton <jeff.layton@primarydata.com>
> ---
> include/linux/sunrpc/xprt.h |  5 +++--
> net/sunrpc/clnt.c           |  4 ++--
> net/sunrpc/xprtsock.c       | 38 +++++++++++++++++++++++++-------------
> 3 files changed, 30 insertions(+), 17 deletions(-)
> 
> diff --git a/include/linux/sunrpc/xprt.h b/include/linux/sunrpc/xprt.h
> index 8b93ef53df3c..26b1624128ec 100644
> --- a/include/linux/sunrpc/xprt.h
> +++ b/include/linux/sunrpc/xprt.h
> @@ -180,7 +180,7 @@ struct rpc_xprt {
> 	atomic_t		num_reqs;	/* total slots */
> 	unsigned long		state;		/* transport state */
> 	unsigned char		resvport   : 1; /* use a reserved port */
> -	unsigned int		swapper;	/* we're swapping over this
> +	atomic_t		swapper;	/* we're swapping over this
> 						   transport */
> 	unsigned int		bind_index;	/* bind function index */
> 
> @@ -345,7 +345,8 @@ void			xprt_release_rqst_cong(struct rpc_task *task);
> void			xprt_disconnect_done(struct rpc_xprt *xprt);
> void			xprt_force_disconnect(struct rpc_xprt *xprt);
> void			xprt_conditional_disconnect(struct rpc_xprt *xprt, unsigned int cookie);
> -int			xs_swapper(struct rpc_xprt *xprt, int enable);
> +int			xs_swapper_enable(struct rpc_xprt *xprt);
> +void			xs_swapper_disable(struct rpc_xprt *xprt);
> 
> bool			xprt_lock_connect(struct rpc_xprt *, struct rpc_task *, void *);
> void			xprt_unlock_connect(struct rpc_xprt *, void *);
> diff --git a/net/sunrpc/clnt.c b/net/sunrpc/clnt.c
> index 383cb778179f..804a75e71e84 100644
> --- a/net/sunrpc/clnt.c
> +++ b/net/sunrpc/clnt.c
> @@ -2492,7 +2492,7 @@ retry:
> 			goto retry;
> 		}
> 
> -		ret = xs_swapper(xprt, 1);
> +		ret = xs_swapper_enable(xprt);
> 		xprt_put(xprt);
> 	}
> 	return ret;
> @@ -2519,7 +2519,7 @@ retry:
> 			goto retry;
> 		}
> 
> -		xs_swapper(xprt, 0);
> +		xs_swapper_disable(xprt);
> 		xprt_put(xprt);
> 	}
> }

Seems like xs_swapper() is specific to socket-based transports.

There’s no struct sock * to use as an argument with RDMA, so xs_swapper()
would probably dereference garbage if “swapon” was invoked on a
proto=rdma mount point.

Should these new functions be made members of the rpc_xprt_ops? The
RDMA version of the methods can be made no-ops for now.


> diff --git a/net/sunrpc/xprtsock.c b/net/sunrpc/xprtsock.c
> index b29703996028..a2861bbfd319 100644
> --- a/net/sunrpc/xprtsock.c
> +++ b/net/sunrpc/xprtsock.c
> @@ -1966,31 +1966,43 @@ static void xs_set_memalloc(struct rpc_xprt *xprt)
> 	struct sock_xprt *transport = container_of(xprt, struct sock_xprt,
> 			xprt);
> 
> -	if (xprt->swapper)
> +	if (atomic_read(&xprt->swapper))
> 		sk_set_memalloc(transport->inet);
> }
> 
> /**
> - * xs_swapper - Tag this transport as being used for swap.
> + * xs_swapper_enable - Tag this transport as being used for swap.
>  * @xprt: transport to tag
> - * @enable: enable/disable
>  *
> + * Take a reference to this transport on behalf of the rpc_clnt, and
> + * optionally mark it for swapping if it wasn't already.
>  */
> -int xs_swapper(struct rpc_xprt *xprt, int enable)
> +int
> +xs_swapper_enable(struct rpc_xprt *xprt)
> {
> 	struct sock_xprt *transport = container_of(xprt, struct sock_xprt,
> 			xprt);
> -	int err = 0;
> 
> -	if (enable) {
> -		xprt->swapper++;
> -		xs_set_memalloc(xprt);
> -	} else if (xprt->swapper) {
> -		xprt->swapper--;
> -		sk_clear_memalloc(transport->inet);
> -	}
> +	if (atomic_inc_return(&xprt->swapper) == 1)
> +		sk_set_memalloc(transport->inet);
> +	return 0;
> +}
> 
> -	return err;
> +/**
> + * xs_swapper_disable - Untag this transport as being used for swap.
> + * @xprt: transport to tag
> + *
> + * Drop a "swapper" reference to this xprt on behalf of the rpc_clnt. If the
> + * swapper refcount goes to 0, untag the socket as a memalloc socket.
> + */
> +void
> +xs_swapper_disable(struct rpc_xprt *xprt)
> +{
> +	struct sock_xprt *transport = container_of(xprt, struct sock_xprt,
> +			xprt);
> +
> +	if (atomic_dec_and_test(&xprt->swapper))
> +		sk_clear_memalloc(transport->inet);
> }
> #else
> static void xs_set_memalloc(struct rpc_xprt *xprt)
> -- 
> 2.4.1
> 
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--
Chuck Lever
chuck[dot]lever[at]oracle[dot]com



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