[PATCH v2 05/11] powerpc: Mark accesses to power_save callback in arch_cpu_idle

[Rohan McLure]

The power_save callback can be overwritten by another core at boot time.
Specifically, null values will be replaced exactly once with the callback
suitable for the particular platform (PowerNV / pseries lpars), making
this value a good candidate for __ro_after_init.

Even with this the case, KCSAN sees unmarked reads to the callback
variable, and notices that unfortunate compiler reorderings could lead
to distinct function pointers being read. In reality this is impossible,
so don't instrument at this read.

Signed-off-by: Rohan McLure <rmclure at linux.ibm.com>
---
v2: Mark instances at init where the callback is written to, and
data_race() read as there is no capacity for the value to change
underneath.
---
 arch/powerpc/kernel/idle.c | 9 ++++++---
 1 file changed, 6 insertions(+), 3 deletions(-)

diff --git a/arch/powerpc/kernel/idle.c b/arch/powerpc/kernel/idle.c
index b1c0418b25c8..43d96c0e3b96 100644
--- a/arch/powerpc/kernel/idle.c
+++ b/arch/powerpc/kernel/idle.c
@@ -35,7 +35,7 @@ EXPORT_SYMBOL(cpuidle_disable);
 
 static int __init powersave_off(char *arg)
 {
-	ppc_md.power_save = NULL;
+	WRITE_ONCE(ppc_md.power_save, NULL);
 	cpuidle_disable = IDLE_POWERSAVE_OFF;
 	return 1;
 }
@@ -43,10 +43,13 @@ __setup("powersave=off", powersave_off);
 
 void arch_cpu_idle(void)
 {
+	/* power_save callback assigned only at init so no data race */
+	void (*power_save)(void) = data_race(ppc_md.power_save);
+
 	ppc64_runlatch_off();
 
-	if (ppc_md.power_save) {
-		ppc_md.power_save();
+	if (power_save) {
+		power_save();
 		/*
 		 * Some power_save functions return with
 		 * interrupts enabled, some don't.
-- 
2.37.2