[tip: core/rcu] rcu: Mark rcu_state.ncpus to detect concurrent writes

[tip-bot2 for Paul E. McKenney]

The following commit has been merged into the core/rcu branch of tip:

Commit-ID:     2f08469563550d15cb08a60898d3549720600eee
Gitweb:        https://git.kernel.org/tip/2f08469563550d15cb08a60898d3549720600eee
Author:        Paul E. McKenney <paulmck@xxxxxxxxxx>
AuthorDate:    Mon, 10 Feb 2020 05:29:58 -08:00
Committer:     Paul E. McKenney <paulmck@xxxxxxxxxx>
CommitterDate: Mon, 27 Apr 2020 11:01:15 -07:00

rcu: Mark rcu_state.ncpus to detect concurrent writes

The rcu_state structure's ncpus field is only to be modified by the
CPU-hotplug CPU-online code path, which is single-threaded.  This
commit therefore enlists KCSAN's help in enforcing this restriction.

Signed-off-by: Paul E. McKenney <paulmck@xxxxxxxxxx>
---
 kernel/rcu/tree.c | 1 +
 1 file changed, 1 insertion(+)

diff --git a/kernel/rcu/tree.c b/kernel/rcu/tree.c
index 156ac8d..c88240a 100644
--- a/kernel/rcu/tree.c
+++ b/kernel/rcu/tree.c
@@ -3612,6 +3612,7 @@ void rcu_cpu_starting(unsigned int cpu)
 	nbits = bitmap_weight(&oldmask, BITS_PER_LONG);
 	/* Allow lockless access for expedited grace periods. */
 	smp_store_release(&rcu_state.ncpus, rcu_state.ncpus + nbits); /* ^^^ */
+	ASSERT_EXCLUSIVE_WRITER(rcu_state.ncpus);
 	rcu_gpnum_ovf(rnp, rdp); /* Offline-induced counter wrap? */
 	rdp->rcu_onl_gp_seq = READ_ONCE(rcu_state.gp_seq);
 	rdp->rcu_onl_gp_flags = READ_ONCE(rcu_state.gp_flags);