[PATCH v4 02/11] liveupdate: Synchronize lazy initialization of FLB private state

[Pasha Tatashin]

The luo_flb_get_private() function, which is responsible for lazily
initializing the private state of FLB objects, can be called
concurrently from multiple threads. This creates a data
race on the 'initialized' flag and can lead to multiple executions of
mutex_init() and INIT_LIST_HEAD() on the same memory.

Introduce a static spinlock (luo_flb_init_lock) local to the function
to synchronize the initialization path. Use smp_load_acquire() and
smp_store_release() for memory ordering between the fast path and the
slow path.

Signed-off-by: Pasha Tatashin <pasha.tatashin@xxxxxxxxxx>
Reviewed-by: Pratyush Yadav (Google) <pratyush@xxxxxxxxxx>
---
 kernel/liveupdate/luo_flb.c | 7 ++++++-
 1 file changed, 6 insertions(+), 1 deletion(-)

diff --git a/kernel/liveupdate/luo_flb.c b/kernel/liveupdate/luo_flb.c
index f52e8114837e..cf4a8f854c83 100644
--- a/kernel/liveupdate/luo_flb.c
+++ b/kernel/liveupdate/luo_flb.c
@@ -89,13 +89,18 @@ struct luo_flb_link {
 static struct luo_flb_private *luo_flb_get_private(struct liveupdate_flb *flb)
 {
 	struct luo_flb_private *private = &ACCESS_PRIVATE(flb, private);
+	static DEFINE_SPINLOCK(luo_flb_init_lock);
 
+	if (smp_load_acquire(&private->initialized))
+		return private;
+
+	guard(spinlock)(&luo_flb_init_lock);
 	if (!private->initialized) {
 		mutex_init(&private->incoming.lock);
 		mutex_init(&private->outgoing.lock);
 		INIT_LIST_HEAD(&private->list);
 		private->users = 0;
-		private->initialized = true;
+		smp_store_release(&private->initialized, true);
 	}
 
 	return private;
-- 
2.43.0