[PATCH for-next] io_uring/eventfd: replace out-of-line signaling with a workqueue

[Jens Axboe <axboe@xxxxxxxxx>]

Rather than (ab)use RCU for this kind of thing, add a work queue item
and use that for the unlikely case of a notification being done inside
another notification. This cleans up the code and better adheres to
expected usage of RCU, and it means that the odd ->ops serializing
can be removed as queue_work() will handle that for us.

Signed-off-by: Jens Axboe <axboe@xxxxxxxxx>

---

diff --git a/io_uring/eventfd.c b/io_uring/eventfd.c
index 100d5da94cb9..8ecf3c106f89 100644
--- a/io_uring/eventfd.c
+++ b/io_uring/eventfd.c
@@ -17,12 +17,8 @@ struct io_ev_fd {
 	/* protected by ->completion_lock */
 	unsigned		last_cq_tail;
 	refcount_t		refs;
-	atomic_t		ops;
 	struct rcu_head		rcu;
-};
-
-enum {
-	IO_EVENTFD_OP_SIGNAL_BIT,
+	struct work_struct	work;
 };
 
 static void io_eventfd_free(struct rcu_head *rcu)
@@ -39,9 +35,9 @@ static void io_eventfd_put(struct io_ev_fd *ev_fd)
 		call_rcu(&ev_fd->rcu, io_eventfd_free);
 }
 
-static void io_eventfd_do_signal(struct rcu_head *rcu)
+static void io_eventfd_do_signal(struct work_struct *work)
 {
-	struct io_ev_fd *ev_fd = container_of(rcu, struct io_ev_fd, rcu);
+	struct io_ev_fd *ev_fd = container_of(work, struct io_ev_fd, work);
 
 	eventfd_signal_mask(ev_fd->cq_ev_fd, EPOLL_URING_WAKE);
 	io_eventfd_put(ev_fd);
@@ -63,11 +59,7 @@ static bool __io_eventfd_signal(struct io_ev_fd *ev_fd)
 		eventfd_signal_mask(ev_fd->cq_ev_fd, EPOLL_URING_WAKE);
 		return true;
 	}
-	if (!atomic_fetch_or(BIT(IO_EVENTFD_OP_SIGNAL_BIT), &ev_fd->ops)) {
-		call_rcu_hurry(&ev_fd->rcu, io_eventfd_do_signal);
-		return false;
-	}
-	return true;
+	return !queue_work(system_unbound_wq, &ev_fd->work);
 }
 
 /*
@@ -184,7 +176,7 @@ int io_eventfd_register(struct io_ring_ctx *ctx, void __user *arg,
 	ev_fd->eventfd_async = eventfd_async;
 	ctx->has_evfd = true;
 	refcount_set(&ev_fd->refs, 1);
-	atomic_set(&ev_fd->ops, 0);
+	INIT_WORK(&ev_fd->work, io_eventfd_do_signal);
 	rcu_assign_pointer(ctx->io_ev_fd, ev_fd);
 	return 0;
 }

-- 
Jens Axboe