[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 find that the allocation of srcu_node
tree has been completed, but the ss_state is still less than SRCU_SIZE_BIG,
the mask=~0 causes delay_work timer be queued on every CPU which has
been online regardless of whether it's sdp->srcu_cblist has callbacks.
the 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]>
---
 v1->v2:
 Update commit log and modify indentation. 

 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..be420ecde851 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)))
-- 
2.48.1