staging/rdma/hfi1: fix pio progress routine race with allocator

The allocation code assumes that the shadow ring cannot
be overrun because the credits will limit the allocation.

Unfortuately, the progress mechanism in sc_release_update() updates
the free count prior to processing the shadow ring, allowing the
shadow ring to be overrun by an allocation.
Reviewed-by: Mark Debbage <mark.debbage@intel.com>
Signed-off-by: Mike Marciniszyn <mike.marciniszyn@intel.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

drivers/staging/rdma/hfi1/pio.c

@@ -1565,6 +1565,7 @@ void sc_release_update(struct send_context *sc)
 	u64 hw_free;
 	u32 head, tail;
 	unsigned long old_free;
+	unsigned long free;
 	unsigned long extra;
 	unsigned long flags;
 	int code;
@@ -1579,7 +1580,7 @@ void sc_release_update(struct send_context *sc)
 	extra = (((hw_free & CR_COUNTER_SMASK) >> CR_COUNTER_SHIFT)
 			- (old_free & CR_COUNTER_MASK))
 				& CR_COUNTER_MASK;
-	sc->free = old_free + extra;
+	free = old_free + extra;
 	trace_hfi1_piofree(sc, extra);
 
 	/* call sent buffer callbacks */
@@ -1589,7 +1590,7 @@ void sc_release_update(struct send_context *sc)
 	while (head != tail) {
 		pbuf = &sc->sr[tail].pbuf;
 
-		if (sent_before(sc->free, pbuf->sent_at)) {
+		if (sent_before(free, pbuf->sent_at)) {
 			/* not sent yet */
 			break;
 		}
@@ -1603,8 +1604,10 @@ void sc_release_update(struct send_context *sc)
 		if (tail >= sc->sr_size)
 			tail = 0;
 	}
-	/* update tail, in case we moved it */
 	sc->sr_tail = tail;
+	/* make sure tail is updated before free */
+	smp_wmb();
+	sc->free = free;
 	spin_unlock_irqrestore(&sc->release_lock, flags);
 	sc_piobufavail(sc);
 }