[PATCH 1/6] io_uring: further limit non-owner defer-tw cq waiting

[Pavel Begunkov <asml.silence@gmail.com>]

In case of DEFER_TASK_WORK we try to restrict waiters to only one task,
which is also the only submitter; however, we don't do it reliably,
which might be very confusing and backfire in the future. E.g. we
currently allow multiple tasks in io_iopoll_check().

Fixes: dacbb30102689 ("io_uring: add IORING_SETUP_DEFER_TASKRUN")
Signed-off-by: Pavel Begunkov <asml.silence@gmail.com>
---
 io_uring/io_uring.c |  6 ++++++
 io_uring/io_uring.h | 11 +++++++++++
 2 files changed, 17 insertions(+)

diff --git a/io_uring/io_uring.c b/io_uring/io_uring.c
index 0482087b7c64..dc6f64ecd926 100644
--- a/io_uring/io_uring.c
+++ b/io_uring/io_uring.c
@@ -1398,6 +1398,9 @@ static int io_iopoll_check(struct io_ring_ctx *ctx, long min)
 	int ret = 0;
 	unsigned long check_cq;
 
+	if (!io_allowed_run_tw(ctx))
+		return -EEXIST;
+
 	check_cq = READ_ONCE(ctx->check_cq);
 	if (unlikely(check_cq)) {
 		if (check_cq & BIT(IO_CHECK_CQ_OVERFLOW_BIT))
@@ -2386,6 +2389,9 @@ static int io_cqring_wait(struct io_ring_ctx *ctx, int min_events,
 	ktime_t timeout = KTIME_MAX;
 	int ret;
 
+	if (!io_allowed_run_tw(ctx))
+		return -EEXIST;
+
 	do {
 		/* always run at least 1 task work to process local work */
 		ret = io_run_task_work_ctx(ctx);
diff --git a/io_uring/io_uring.h b/io_uring/io_uring.h
index 9d89425292b7..4eea0836170e 100644
--- a/io_uring/io_uring.h
+++ b/io_uring/io_uring.h
@@ -329,4 +329,15 @@ static inline struct io_kiocb *io_alloc_req(struct io_ring_ctx *ctx)
 	return container_of(node, struct io_kiocb, comp_list);
 }
 
+static inline bool io_allowed_run_tw(struct io_ring_ctx *ctx)
+{
+	if (!(ctx->flags & IORING_SETUP_DEFER_TASKRUN))
+		return true;
+	if (unlikely(ctx->submitter_task != current)) {
+		/* maybe this is before any submissions */
+		return !ctx->submitter_task;
+	}
+	return true;
+}
+
 #endif
-- 
2.37.2