Re: [PATCH 3/3] SUNRPC: svc_xprt_enqueue should not refuse to enqueue 'XPT_DEAD' transports

Trond Myklebust
2009-01-04T19:12:42Z

On Fri, 2009-01-02 at 15:44 -0600, Tom Tucker wrote:
> Bruce/Trond:
>
> This is an alternative to patches 2 and 3 from Trond's fix. I think
> Trond's fix is correct, but I believe this approach to be simpler.
>
> From: Tom Tucker <tom@opengridcomputing.com>
> Date: Wed, 31 Dec 2008 17:18:33 -0600
> Subject: [PATCH] svc: Clean up deferred requests on transport destruction
>
> A race between svc_revisit and svc_delete_xprt can result in
> deferred requests holding references on a transport that can never be
> recovered because dead transports are not enqueued for subsequent
> processing.
>
> Check for XPT_DEAD in revisit to clean up completing deferrals on a dead
> transport and sweep a transport's deferred queue to do the same for queued
> but unprocessed deferrals.
>
> Signed-off-by: Tom Tucker <tom@opengridcomputing.com>
> ---
> net/sunrpc/svc_xprt.c | 20 +++++++++++++++-----
> 1 files changed, 15 insertions(+), 5 deletions(-)
>
> diff --git a/net/sunrpc/svc_xprt.c b/net/sunrpc/svc_xprt.c
> index bf5b5cd..92ca5c6 100644
> --- a/net/sunrpc/svc_xprt.c
> +++ b/net/sunrpc/svc_xprt.c
> @@ -837,6 +837,11 @@ static void svc_age_temp_xprts(unsigned long closure)
> void svc_delete_xprt(struct svc_xprt *xprt)
> {
> struct svc_serv *serv = xprt->xpt_server;
> + struct svc_deferred_req *dr;
> +
> + /* Only do this once */
> + if (test_and_set_bit(XPT_DEAD, &xprt->xpt_flags))
> + return;
>
> dprintk("svc: svc_delete_xprt(%p)\n", xprt);
> xprt->xpt_ops->xpo_detach(xprt);
> @@ -851,12 +856,16 @@ void svc_delete_xprt(struct svc_xprt *xprt)
> * while still attached to a queue, the queue itself
> * is about to be destroyed (in svc_destroy).
> */
> - if (!test_and_set_bit(XPT_DEAD, &xprt->xpt_flags)) {
> - BUG_ON(atomic_read(&xprt->xpt_ref.refcount) < 2);
> - if (test_bit(XPT_TEMP, &xprt->xpt_flags))
> - serv->sv_tmpcnt--;
> + if (test_bit(XPT_TEMP, &xprt->xpt_flags))
> + serv->sv_tmpcnt--;
> +
> + for (dr = svc_deferred_dequeue(xprt); dr;
> + dr = svc_deferred_dequeue(xprt)) {
> svc_xprt_put(xprt);
> + kfree(dr);
> }
> +
> + svc_xprt_put(xprt);
> spin_unlock_bh(&serv->sv_lock);
> }
>
> @@ -902,7 +911,8 @@ static void svc_revisit(struct cache_deferred_req *dreq, int too_many)
> container_of(dreq, struct svc_deferred_req, handle);
> struct svc_xprt *xprt = dr->xprt;
>
> - if (too_many) {
> + if (too_many || test_bit(XPT_DEAD, &xprt->xpt_flags)) {
> + dprintk("revisit cancelled\n");
> svc_xprt_put(xprt);
> kfree(dr);
> return;
>

I see nothing that stops svc_delete_xprt() from setting XPT_DEAD after
the above test in svc_revisit(), and before the test inside
svc_xprt_enqueue(). What's preventing a race there?

Trond
--
Trond Myklebust
Linux NFS client maintainer

NetApp
Trond.Myklebust@netapp.com