io_uring: disable drain with cqe skip

Current IOSQE_IO_DRAIN implementation doesn't work well with CQE
skipping and it's not allowed, otherwise some requests might be not
executed until the ring is destroyed and the userspace would hang.

Let's fail all drain requests after seeing IOSQE_CQE_SKIP_SUCCESS at
least once. All drained requests prior to that will get run normally,
so there should be no stalls. However, even though such mixing wouldn't
lead to issues at the moment, it's still not allowed as the behaviour
may change.

Signed-off-by: Pavel Begunkov <asml.silence@gmail.com>
Link: https://lore.kernel.org/r/bcf7164f8bf3eb54b7bb7b4fd119907fa4d4d43b.1636559119.git.asml.silence@gmail.com
Signed-off-by: Jens Axboe <axboe@kernel.dk>

fs/io_uring.c

@@ -106,10 +106,10 @@
 #define IORING_MAX_REG_BUFFERS	(1U << 14)
 
 #define SQE_COMMON_FLAGS (IOSQE_FIXED_FILE | IOSQE_IO_LINK | \
-			  IOSQE_IO_HARDLINK | IOSQE_ASYNC | \
-			  IOSQE_CQE_SKIP_SUCCESS)
+			  IOSQE_IO_HARDLINK | IOSQE_ASYNC)
 
-#define SQE_VALID_FLAGS	(SQE_COMMON_FLAGS|IOSQE_BUFFER_SELECT|IOSQE_IO_DRAIN)
+#define SQE_VALID_FLAGS	(SQE_COMMON_FLAGS | IOSQE_BUFFER_SELECT | \
+			IOSQE_IO_DRAIN | IOSQE_CQE_SKIP_SUCCESS)
 
 #define IO_REQ_CLEAN_FLAGS (REQ_F_BUFFER_SELECTED | REQ_F_NEED_CLEANUP | \
 				REQ_F_POLLED | REQ_F_INFLIGHT | REQ_F_CREDS | \
@@ -339,6 +339,7 @@ struct io_ring_ctx {
 		unsigned int		restricted: 1;
 		unsigned int		off_timeout_used: 1;
 		unsigned int		drain_active: 1;
+		unsigned int		drain_disabled: 1;
 	} ____cacheline_aligned_in_smp;
 
 	/* submission data */
@@ -7127,8 +7128,13 @@ static int io_init_req(struct io_ring_ctx *ctx, struct io_kiocb *req,
 		if ((sqe_flags & IOSQE_BUFFER_SELECT) &&
 		    !io_op_defs[opcode].buffer_select)
 			return -EOPNOTSUPP;
-		if (sqe_flags & IOSQE_IO_DRAIN)
+		if (sqe_flags & IOSQE_CQE_SKIP_SUCCESS)
+			ctx->drain_disabled = true;
+		if (sqe_flags & IOSQE_IO_DRAIN) {
+			if (ctx->drain_disabled)
+				return -EOPNOTSUPP;
 			io_init_req_drain(req);
+		}
 	}
 	if (unlikely(ctx->restricted || ctx->drain_active || ctx->drain_next)) {
 		if (ctx->restricted && !io_check_restriction(ctx, req, sqe_flags))