linux-kernel - [PATCH tip/core/rcu 07/12] stop_machine: Use {READ,WRITE)_ONCE() for multi_cpu

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Date:   Wed,  2 Oct 2019 18:38:58 -0700
From:   paulmck@...nel.org
To:     rcu@...r.kernel.org
Cc:     linux-kernel@...r.kernel.org, mingo@...nel.org,
        jiangshanlai@...il.com, dipankar@...ibm.com,
        akpm@...ux-foundation.org, mathieu.desnoyers@...icios.com,
        josh@...htriplett.org, tglx@...utronix.de, peterz@...radead.org,
        rostedt@...dmis.org, dhowells@...hat.com, edumazet@...gle.com,
        fweisbec@...il.com, oleg@...hat.com, joel@...lfernandes.org,
        "Paul E. McKenney" <paulmck@...ux.ibm.com>
Subject: [PATCH tip/core/rcu 07/12] stop_machine: Use {READ,WRITE)_ONCE() for multi_cpu_stop() ->state

From: "Paul E. McKenney" <paulmck@...ux.ibm.com>

The multi_stop_data structure's ->state field is updated and read
concurrently, so this commit replaces the current C-language accesses
with READ_ONCE() and WRITE_ONCE().

Signed-off-by: Paul E. McKenney <paulmck@...ux.ibm.com>
---
 kernel/stop_machine.c | 6 +++---
 1 file changed, 3 insertions(+), 3 deletions(-)

diff --git a/kernel/stop_machine.c b/kernel/stop_machine.c
index 34c4f11..c02c56e 100644
--- a/kernel/stop_machine.c
+++ b/kernel/stop_machine.c
@@ -167,7 +167,7 @@ static void set_state(struct multi_stop_data *msdata,
 	/* Reset ack counter. */
 	atomic_set(&msdata->thread_ack, msdata->num_threads);
 	smp_wmb();
-	msdata->state = newstate;
+	WRITE_ONCE(msdata->state, newstate);
 }
 
 /* Last one to ack a state moves to the next state. */
@@ -210,8 +210,8 @@ static int multi_cpu_stop(void *data)
 	do {
 		/* Chill out and ensure we re-read multi_stop_state. */
 		stop_machine_yield(cpumask);
-		if (msdata->state != curstate) {
-			curstate = msdata->state;
+		if (READ_ONCE(msdata->state) != curstate) {
+			curstate = READ_ONCE(msdata->state);
 			switch (curstate) {
 			case MULTI_STOP_DISABLE_IRQ:
 				local_irq_disable();
-- 
2.9.5