[PATCH] srcu: Queue sdp->work when the delay timer is successfully deleted

[Zqiang]

In the cleanup_srcu_struct(), when iterating over per-cpu's srcu_data,
the timer_delete_sync(&sdp->delay_work) is called to cancel the delay
timer before flush_work(&sdp->work).

However, if the timer_delete_sync() returns 1 means that it successfully
deleted an pending timer before it had a chance to fire, also means that
the sdp->work cannot be queued, the subsequent flush_work(&sdp->work)
will returns immediately without waiting for anything, this causes SRCU
callbacks to not be processed.

Fix this by checking the return value of timer_delete_sync(), if it
returns 1, explicitly queue sdp->work so that the following flush_work()
can correctly wait for the work to complete.

Signed-off-by: Zqiang <qiang.zhang@xxxxxxxxx>
---
 kernel/rcu/srcutree.c | 6 +++++-
 1 file changed, 5 insertions(+), 1 deletion(-)

diff --git a/kernel/rcu/srcutree.c b/kernel/rcu/srcutree.c
index 7c2f7cc131f7..02c322b7c6f1 100644
--- a/kernel/rcu/srcutree.c
+++ b/kernel/rcu/srcutree.c
@@ -725,7 +725,11 @@ void cleanup_srcu_struct(struct srcu_struct *ssp)
 	for_each_possible_cpu(cpu) {
 		struct srcu_data *sdp = per_cpu_ptr(ssp->sda, cpu);
 
-		timer_delete_sync(&sdp->delay_work);
+		//In most scenarios, calling srcu_barrier before cleanup
+		//will not trigger WARN_ON().
+		if (WARN_ON(timer_delete_sync(&sdp->delay_work)) &&
+					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)))
 			return; /* Forgot srcu_barrier(), so just leak it! */
-- 
2.17.1