[PATCH RFC 08/16] rcu: Use WRITE_ONCE() for ->rcu_need_heavy_qs

Currently raw_cpu_write() is used to clear the ->rcu_need_heavy_qs
field of the per-CPU rcu_data structure.  However, on x86 this is a
normal assignment, which does not play well with concurrent accesses.
This commit therefore upgrades the uses of raw_cpu_write() to its
concurrency-safe counterpart WRITE_ONCE() of an rdp pointer obtained
from this_cpu_ptr(&rcu_data).

KCSAN located this issue.

Signed-off-by: Paul E. McKenney <[email protected]>
---
 kernel/rcu/tree.c | 3 ++-
 1 file changed, 2 insertions(+), 1 deletion(-)

diff --git a/kernel/rcu/tree.c b/kernel/rcu/tree.c
index 03a43d3d261607..91d5b4dd08bf5c 100644
--- a/kernel/rcu/tree.c
+++ b/kernel/rcu/tree.c
@@ -357,9 +357,10 @@ bool rcu_watching_zero_in_eqs(int cpu, int *vp)
  */
 notrace void rcu_momentary_eqs(void)
 {
+	struct rcu_data *rdp = this_cpu_ptr(&rcu_data);
 	int seq;
 
-	raw_cpu_write(rcu_data.rcu_need_heavy_qs, false);
+	WRITE_ONCE(rdp->rcu_need_heavy_qs, false);
 	seq = ct_state_inc(2 * CT_RCU_WATCHING);
 	/* It is illegal to call this from idle state. */
 	WARN_ON_ONCE(!(seq & CT_RCU_WATCHING));
-- 
2.40.1