[PATCH] rcu/nocb: Fix possible invalid rdp's->nocb_cb_kthread pointer access

[Zqiang]

In the preparation stage of CPU online, if the corresponding
the rdp's->nocb_cb_kthread does not exist, will be created,
there is a situation where the rdp's rcuop kthreads creation fails,
and then de-offload this CPU's rdp, does not assign this CPU's
rdp->nocb_cb_kthread pointer, but this rdp's->nocb_gp_rdp and
rdp's->rdp_gp->nocb_gp_kthread is still valid.

This will cause the subsequent re-offload operation of this offline
CPU, which will pass the conditional check and the kthread_unpark()
will access invalid rdp's->nocb_cb_kthread pointer.

This commit therefore use rdp's->nocb_gp_kthread instead of
rdp_gp's->nocb_gp_kthread for safety check.

Signed-off-by: Zqiang <qiang.zhang1...@gmail.com>
---
 kernel/rcu/tree_nocb.h | 5 ++---
 1 file changed, 2 insertions(+), 3 deletions(-)

diff --git a/kernel/rcu/tree_nocb.h b/kernel/rcu/tree_nocb.h
index 1596812f7f12..6679140bb0b5 100644
--- a/kernel/rcu/tree_nocb.h
+++ b/kernel/rcu/tree_nocb.h
@@ -1146,7 +1146,6 @@ static bool rcu_nocb_rdp_offload_wait_cond(struct 
rcu_data *rdp)
 static int rcu_nocb_rdp_offload(struct rcu_data *rdp)
 {
        int wake_gp;
-       struct rcu_data *rdp_gp = rdp->nocb_gp_rdp;
 
        WARN_ON_ONCE(cpu_online(rdp->cpu));
        /*
@@ -1156,7 +1155,7 @@ static int rcu_nocb_rdp_offload(struct rcu_data *rdp)
        if (!rdp->nocb_gp_rdp)
                return -EINVAL;
 
-       if (WARN_ON_ONCE(!rdp_gp->nocb_gp_kthread))
+       if (WARN_ON_ONCE(!rdp->nocb_gp_kthread))
                return -EINVAL;
 
        pr_info("Offloading %d\n", rdp->cpu);
@@ -1166,7 +1165,7 @@ static int rcu_nocb_rdp_offload(struct rcu_data *rdp)
 
        wake_gp = rcu_nocb_queue_toggle_rdp(rdp);
        if (wake_gp)
-               wake_up_process(rdp_gp->nocb_gp_kthread);
+               wake_up_process(rdp->nocb_gp_kthread);
 
        swait_event_exclusive(rdp->nocb_state_wq,
                              rcu_nocb_rdp_offload_wait_cond(rdp));
-- 
2.17.1