[PATCH 2/2] io_uring: add IORING_CQ_NEED_WAKEUP to the CQ ring flags

[Stefano Garzarella]

This new flag should be set/clear from the application to
enable/disable eventfd notifications when a request is completed
and queued to the CQ ring.

Before this patch, notifications were always sent if an eventfd is
registered, so to remain backwards compatible we enable them during
initialization.

It will be up to the application to disable them after initialization if
no notifications are required at the beginning.

Signed-off-by: Stefano Garzarella <sgarzare@xxxxxxxxxx>
---
 fs/io_uring.c                 | 7 +++++++
 include/uapi/linux/io_uring.h | 5 +++++
 2 files changed, 12 insertions(+)

diff --git a/fs/io_uring.c b/fs/io_uring.c
index 6e8158269f3c..7c9486ea5aa3 100644
--- a/fs/io_uring.c
+++ b/fs/io_uring.c
@@ -1152,6 +1152,8 @@ static inline bool io_should_trigger_evfd(struct io_ring_ctx *ctx)
 {
 	if (!ctx->cq_ev_fd)
 		return false;
+	if (!(READ_ONCE(ctx->rings->cq_flags) & IORING_CQ_NEED_WAKEUP))
+		return false;
 	if (!ctx->eventfd_async)
 		return true;
 	return io_wq_current_is_worker();
@@ -7684,6 +7686,11 @@ static int io_allocate_scq_urings(struct io_ring_ctx *ctx,
 	rings->cq_ring_mask = p->cq_entries - 1;
 	rings->sq_ring_entries = p->sq_entries;
 	rings->cq_ring_entries = p->cq_entries;
+	/*
+	 * For backward compatibility we start with eventfd notification
+	 * enabled.
+	 */
+	rings->cq_flags |= IORING_CQ_NEED_WAKEUP;
 	ctx->sq_mask = rings->sq_ring_mask;
 	ctx->cq_mask = rings->cq_ring_mask;
 	ctx->sq_entries = rings->sq_ring_entries;
diff --git a/include/uapi/linux/io_uring.h b/include/uapi/linux/io_uring.h
index 602bb0ece607..1d20dc61779e 100644
--- a/include/uapi/linux/io_uring.h
+++ b/include/uapi/linux/io_uring.h
@@ -209,6 +209,11 @@ struct io_cqring_offsets {
 	__u64 resv2;
 };
 
+/*
+ * cq_ring->flags
+ */
+#define IORING_CQ_NEED_WAKEUP	(1U << 0) /* needs wakeup through eventfd */
+
 /*
  * io_uring_enter(2) flags
  */
-- 
2.25.4