sunrpc: remove xpt_pool

The xpt_pool field is only used for reporting BUGs.
And it isn't used correctly.

In particular, when it is cleared in svc_xprt_received before
XPT_BUSY is cleared, there is no guarantee that either the
compiler or the CPU might not re-order to two assignments, just
setting xpt_pool to NULL after XPT_BUSY is cleared.

If a different cpu were running svc_xprt_enqueue at this moment,
it might see XPT_BUSY clear and then xpt_pool non-NULL, and
so BUG.

This could be fixed by calling
  smp_mb__before_clear_bit()
before the clear_bit.  However as xpt_pool isn't really used,
it seems safest to simply remove xpt_pool.

Another alternate would be to change the clear_bit to
clear_bit_unlock, and the test_and_set_bit to test_and_set_bit_lock.
Signed-off-by: NeilBrown <neilb@suse.de>
Signed-off-by: J. Bruce Fields <bfields@redhat.com>

include/linux/sunrpc/svc_xprt.h

@@ -63,7 +63,6 @@ struct svc_xprt {
 #define XPT_LISTENER	11		/* listening endpoint */
 #define XPT_CACHE_AUTH	12		/* cache auth info */
 
-	struct svc_pool		*xpt_pool;	/* current pool iff queued */
 	struct svc_serv		*xpt_server;	/* service for transport */
 	atomic_t    	    	xpt_reserved;	/* space on outq that is rsvd */
 	struct mutex		xpt_mutex;	/* to serialize sending data */

net/sunrpc/svc_xprt.c

@@ -351,8 +351,6 @@ void svc_xprt_enqueue(struct svc_xprt *xprt)
 		dprintk("svc: transport %p busy, not enqueued\n", xprt);
 		goto out_unlock;
 	}
-	BUG_ON(xprt->xpt_pool != NULL);
-	xprt->xpt_pool = pool;
 
 	if (!list_empty(&pool->sp_threads)) {
 		rqstp = list_entry(pool->sp_threads.next,
@@ -370,13 +368,11 @@ void svc_xprt_enqueue(struct svc_xprt *xprt)
 		rqstp->rq_reserved = serv->sv_max_mesg;
 		atomic_add(rqstp->rq_reserved, &xprt->xpt_reserved);
 		pool->sp_stats.threads_woken++;
-		BUG_ON(xprt->xpt_pool != pool);
 		wake_up(&rqstp->rq_wait);
 	} else {
 		dprintk("svc: transport %p put into queue\n", xprt);
 		list_add_tail(&xprt->xpt_ready, &pool->sp_sockets);
 		pool->sp_stats.sockets_queued++;
-		BUG_ON(xprt->xpt_pool != pool);
 	}
 
 out_unlock:
@@ -415,7 +411,6 @@ static struct svc_xprt *svc_xprt_dequeue(struct svc_pool *pool)
 void svc_xprt_received(struct svc_xprt *xprt)
 {
 	BUG_ON(!test_bit(XPT_BUSY, &xprt->xpt_flags));
-	xprt->xpt_pool = NULL;
 	/* As soon as we clear busy, the xprt could be closed and
 	 * 'put', so we need a reference to call svc_xprt_enqueue with:
 	 */