eventfd/kaio integration fix

Jeff Roberson discovered a race when using kaio eventfd based notifications.
When it occurs it can lead tomissed wakeups and hung userspace.

This patch fixes the race by moving the notification inside the spinlocked
section of kaio. The operation is safe since eventfd spinlock and kaio one
are unrelated.

Signed-off-by: Davide Libenzi <[e-mail]>
Cc: Zach Brown <[e-mail]>
Cc: Jeff Roberson <[e-mail]>
Signed-off-by: Andrew Morton <[e-mail]>
Signed-off-by: Linus Torvalds <[e-mail]>

fs/aio.c

···
 		return 1;
 	}
 
-	/*
-	 * Check if the user asked us to deliver the result through an
-	 * eventfd. The eventfd_signal() function is safe to be called
-	 * from IRQ context.
-	 */
-	if (!IS_ERR(iocb->ki_eventfd))
-		eventfd_signal(iocb->ki_eventfd, 1);
-
 	info = &ctx->ring_info;
 
 	/* add a completion event to the ring buffer.
···
 	kunmap_atomic(ring, KM_IRQ1);
 
 	pr_debug("added to ring %p at [%lu]\n", iocb, tail);
+
+	/*
+	 * Check if the user asked us to deliver the result through an
+	 * eventfd. The eventfd_signal() function is safe to be called
+	 * from IRQ context.
+	 */
+	if (!IS_ERR(iocb->ki_eventfd))
+		eventfd_signal(iocb->ki_eventfd, 1);
+
 put_rq:
 	/* everything turned out well, dispose of the aiocb. */
 	ret = __aio_put_req(ctx, iocb);