SUNRPC: make rqst_should_sleep() idempotent()

Based on its name you would think that rqst_should_sleep() would be
read-only, not changing anything.  But in fact it will clear
SP_TASK_PENDING if that was set.  This is surprising, and it blurs the
line between "check for work to do" and "dequeue work to do".

So change the "test_and_clear" to simple "test" and clear the bit once
the thread has decided to wake up and return to the caller.

With this, it makes sense to *always* set SP_TASK_PENDING when asked,
rather than to set it only if no thread could be woken up.

[ cel: Previously TASK_PENDING indicated there is work waiting but no
idle threads were found to pick up that work. After this patch, it acts
as an XPT_BUSY flag for wake-ups that have no associated xprt. ]
Signed-off-by: NeilBrown <neilb@suse.de>
Reviewed-by: Jeff Layton <jlayton@kernel.org>
Signed-off-by: Chuck Lever <chuck.lever@oracle.com>

net/sunrpc/svc_xprt.c

@@ -581,8 +581,8 @@ void svc_wake_up(struct svc_serv *serv)
 {
 	struct svc_pool *pool = &serv->sv_pools[0];
 
-	if (!svc_pool_wake_idle_thread(pool))
 	set_bit(SP_TASK_PENDING, &pool->sp_flags);
+	svc_pool_wake_idle_thread(pool);
 }
 EXPORT_SYMBOL_GPL(svc_wake_up);
 
@@ -704,7 +704,7 @@ rqst_should_sleep(struct svc_rqst *rqstp)
 	struct svc_pool		*pool = rqstp->rq_pool;
 
 	/* did someone call svc_wake_up? */
-	if (test_and_clear_bit(SP_TASK_PENDING, &pool->sp_flags))
+	if (test_bit(SP_TASK_PENDING, &pool->sp_flags))
 		return false;
 
 	/* was a socket queued? */
@@ -748,6 +748,7 @@ static struct svc_xprt *svc_get_next_xprt(struct svc_rqst *rqstp)
 
 	set_bit(RQ_BUSY, &rqstp->rq_flags);
 	smp_mb__after_atomic();
+	clear_bit(SP_TASK_PENDING, &pool->sp_flags);
 	rqstp->rq_xprt = svc_xprt_dequeue(pool);
 	if (rqstp->rq_xprt)
 		goto out_found;
@@ -756,6 +757,7 @@ static struct svc_xprt *svc_get_next_xprt(struct svc_rqst *rqstp)
 		return NULL;
 	return NULL;
 out_found:
+	clear_bit(SP_TASK_PENDING, &pool->sp_flags);
 	/* Normally we will wait up to 5 seconds for any required
 	 * cache information to be provided.
 	 */