Re: [PATCH 1/2] ipc/sem.c: Fix complex_count vs. simple op race

[Davidlohr Bueso <dave@stgolabs.net>]

On Wed, 13 Jul 2016, Manfred Spraul wrote:

>-static void sem_wait_array(struct sem_array *sma)
>+static void complexmode_enter(struct sem_array *sma)
> {
> 	int i;
> 	struct sem *sem;
>
>-	if (sma->complex_count)  {
>-		/* The thread that increased sma->complex_count waited on
>-		 * all sem->lock locks. Thus we don't need to wait again.
>-		 */
>+	if (sma->complex_mode)  {
>+		/* We are already in complex_mode. Nothing to do */
> 		return;
> 	}
>+	WRITE_ONCE(sma->complex_mode, true);

So we can actually save those READ/WRITE_ONCE calls for complex_mode as it's
a bool and therefore tearing is not an issue.

>+
>+	/* We need a full barrier:
>+	 * The write to complex_mode must be visible
>+	 * before we read the first sem->lock spinlock state.
>+	 */
>+	smp_mb();

smp_store_mb()?

> /*
>@@ -300,56 +338,40 @@ static inline int sem_lock(struct sem_array *sma, struct sembuf *sops,
> 		/* Complex operation - acquire a full lock */
> 		ipc_lock_object(&sma->sem_perm);
>
>-		/* And wait until all simple ops that are processed
>-		 * right now have dropped their locks.
>-		 */
>-		sem_wait_array(sma);
>+		/* Prevent parallel simple ops */
>+		complexmode_enter(sma);
> 		return -1;

nit and unrelated: we should probably use some better label here than a raw
-1 (although I don't see it changing, just for nicer reading), ie: SEM_OBJECT_LOCKED

Thanks,
Davidlohr