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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