[PATCH for-5.13] io_uring: maintain drain requests' logic

[Hao Xu <haoxu@linux.alibaba.com>]

Now that we have multishot poll requests, one sqe can emit multiple
cqes. given below example:
    sqe0(multishot poll)-->sqe1-->sqe2(drain req)
sqe2 is designed to issue after sqe0 and sqe1 completed, but since sqe0
is a multishot poll request, sqe2 may be issued after sqe0's event
triggered twice before sqe1 completed. This isn't what users leverage
drain requests for.
Here a simple solution is to ignore all multishot poll cqes, which means
drain requests  won't wait those request to be done.

Signed-off-by: Hao Xu <haoxu@linux.alibaba.com>
---
 fs/io_uring.c | 9 +++++++--
 1 file changed, 7 insertions(+), 2 deletions(-)

diff --git a/fs/io_uring.c b/fs/io_uring.c
index 513096759445..cd6d44cf5940 100644
--- a/fs/io_uring.c
+++ b/fs/io_uring.c
@@ -455,6 +455,7 @@ struct io_ring_ctx {
 	struct callback_head		*exit_task_work;
 
 	struct wait_queue_head		hash_wait;
+	unsigned                        multishot_cqes;
 
 	/* Keep this last, we don't need it for the fast path */
 	struct work_struct		exit_work;
@@ -1181,8 +1182,8 @@ static bool req_need_defer(struct io_kiocb *req, u32 seq)
 	if (unlikely(req->flags & REQ_F_IO_DRAIN)) {
 		struct io_ring_ctx *ctx = req->ctx;
 
-		return seq != ctx->cached_cq_tail
-				+ READ_ONCE(ctx->cached_cq_overflow);
+		return seq + ctx->multishot_cqes != ctx->cached_cq_tail
+			+ READ_ONCE(ctx->cached_cq_overflow);
 	}
 
 	return false;
@@ -4897,6 +4898,7 @@ static bool io_poll_complete(struct io_kiocb *req, __poll_t mask, int error)
 {
 	struct io_ring_ctx *ctx = req->ctx;
 	unsigned flags = IORING_CQE_F_MORE;
+	bool multishot_poll = !(req->poll.events & EPOLLONESHOT);
 
 	if (!error && req->poll.canceled) {
 		error = -ECANCELED;
@@ -4911,6 +4913,9 @@ static bool io_poll_complete(struct io_kiocb *req, __poll_t mask, int error)
 		req->poll.done = true;
 		flags = 0;
 	}
+	if (multishot_poll)
+		ctx->multishot_cqes++;
+
 	io_commit_cqring(ctx);
 	return !(flags & IORING_CQE_F_MORE);
 }
-- 
1.8.3.1