[PATCH v2] srcu: Fix WARN_ON() for timer_delete_sync() in cleanup_srcu_struct()

The WARN_ON() for timer_delete_sync() means that the caller forgot to
call srcu_barrier() before cleanup. however, it still can trigger even
when srcu_barrier() was properly called.

When a SRCU grace period ends and ss_state < SRCU_SIZE_BIG, the mask=~0
causes delay_work timer be queued on every online CPU regardless of
whether it's sdp->srcu_cblist has callbacks. srcu_barrier() only wait
srcu_barrier_head's callbacks to complete, however, the srcu_barrier_head
will not be inserted into an empty sdp->srcu_cblist. after that, when
cleanup_srcu_struct() finds a pending delay_work timer on a CPU with no
callbacks, triggering a false positive.

This commit therefore add rcu_segcblist_n_cbs() to WARN_ON(), make the
warning triggers only when the timer is still pending and there are actual
outstanding callbacks.

Fixes: 05c3e88488ed ("srcu: Queue sdp->work when the delay timer is successfully deleted")
Reported-by: Breno Leitao <[email protected]>
Reported-by: kernel test robot <[email protected]> 
Closes: https://lore.kernel.org/oe-lkp/[email protected]/ 
Tested-by: kernel test robot <[email protected]>
Signed-off-by: Zqiang <[email protected]>
---
 kernel/rcu/srcutree.c | 5 +++--
 1 file changed, 3 insertions(+), 2 deletions(-)

diff --git a/kernel/rcu/srcutree.c b/kernel/rcu/srcutree.c
index 35fface51d50..090dddfc7463 100644
--- a/kernel/rcu/srcutree.c
+++ b/kernel/rcu/srcutree.c
@@ -753,8 +753,9 @@ void cleanup_srcu_struct(struct srcu_struct *ssp)
 
 		// Call srcu_barrier() before this cleanup_srcu_struct()
 		// to avoid triggering this WARN_ON().
-		if (WARN_ON(timer_delete_sync(&sdp->delay_work)) &&
-					rcu_cpu_beenfullyonline(sdp->cpu))
+		if (WARN_ON(timer_delete_sync(&sdp->delay_work) &&
+			rcu_segcblist_n_cbs(&sdp->srcu_cblist)) &&
+				rcu_cpu_beenfullyonline(sdp->cpu))
 			queue_work_on(sdp->cpu, rcu_gp_wq, &sdp->work);
 		flush_work(&sdp->work);
 		if (WARN_ON(rcu_segcblist_n_cbs(&sdp->srcu_cblist)))
-- 
2.17.1