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

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