[PATCH 01/19] cpu/hotplug: Clean up cmpxchg() logic in cpuhp_can_boot_ap()

[Will Deacon]

cpuhp_can_boot_ap() uses atomic_try_cmpxchg() to transition the sync
state of the incoming CPU to SYNC_STATE_KICKED. However, this is
unnecessary if the state is SYNC_STATE_DEAD, since there will not be any
concurrent state modifications, and also if the state is already set to
SYNC_STATE_KICKED.

Restrict the use of cmpxchg() to the case where the existing state is
SYNC_STATE_ALIVE.

Signed-off-by: Will Deacon <will at kernel.org>
---
 kernel/cpu.c | 8 ++++----
 1 file changed, 4 insertions(+), 4 deletions(-)

diff --git a/kernel/cpu.c b/kernel/cpu.c
index b3c8553d7bd6..198c929c452a 100644
--- a/kernel/cpu.c
+++ b/kernel/cpu.c
@@ -408,22 +408,22 @@ static bool cpuhp_can_boot_ap(unsigned int cpu)
 	switch (sync) {
 	case SYNC_STATE_DEAD:
 		/* CPU is properly dead */
+		atomic_set(st, SYNC_STATE_KICKED);
 		break;
 	case SYNC_STATE_KICKED:
 		/* CPU did not come up in previous attempt */
 		break;
 	case SYNC_STATE_ALIVE:
 		/* CPU is stuck cpuhp_ap_sync_alive(). */
+		if (!atomic_try_cmpxchg_relaxed(st, &sync, SYNC_STATE_KICKED))
+			goto again;
 		break;
 	default:
 		/* CPU failed to report online or dead and is in limbo state. */
 		return false;
 	}
 
-	/* Prepare for booting */
-	if (!atomic_try_cmpxchg(st, &sync, SYNC_STATE_KICKED))
-		goto again;
-
+	/* Continue with booting */
 	return true;
 }
 
-- 
2.55.0.979.g7e5102b832-goog