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

Pavel Begunkov
2022-09-08T15:59:04Z

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