When a socket send and shutdown() happen back-to-back, both fire
wake-ups before the receiver's task_work has a chance to run. The first
wake gets poll ownership (poll_refs=1), and the second bumps it to 2.
When io_poll_check_events() runs, it calls io_poll_issue() which does a
recv that reads the data and returns IOU_RETRY. The loop then drains all
accumulated refs (atomic_sub_return(2) -> 0) and exits, even though only
the first event was consumed. Since the shutdown is a persistent state
change, no further wakeups will happen, and the multishot recv can hang
forever.

Check specifically for HUP in the poll loop, and ensure that another
loop is done to check for status if more than a single poll activation
is pending. This ensures we don't lose the shutdown event.

Cc: stable@vger.kernel.org
Fixes: dbc2564cfe0f ("io_uring: let fast poll support multishot")
Reported-by: Francis Brosseau <francis@malagauche.com>
Link: https://github.com/axboe/liburing/issues/1549
Signed-off-by: Jens Axboe <axboe@kernel.dk>

---

V3: split mshot and !mshot cases, and simply use the number of refs
    gotten in the beginning for gating retry. if one is dropped when
    we want to retry, we'll loop again as we'd still have remaining
    refs.

diff --git a/io_uring/poll.c b/io_uring/poll.c
index aac4b3b881fb..a264d73a8cbd 100644
--- a/io_uring/poll.c
+++ b/io_uring/poll.c
@@ -228,6 +228,19 @@ static inline void io_poll_execute(struct io_kiocb *req, int res)
 		__io_poll_execute(req, res);
 }
 
+static inline void io_mshot_check_retry(struct io_kiocb *req, int *v)
+{
+	/*
+	 * Release all references, retry if someone tried to restart
+	 * task_work while we were executing it.
+	 */
+	*v &= IO_POLL_REF_MASK;
+
+	/* multiple refs and HUP, ensure we loop once more */
+	if ((req->cqe.res & (POLLHUP | POLLRDHUP)) && *v != 1)
+		(*v)--;
+}
+
 /*
  * All poll tw should go through this. Checks for poll events, manages
  * references, does rewait, etc.
@@ -303,6 +316,7 @@ static int io_poll_check_events(struct io_kiocb *req, io_tw_token_t tw)
 				io_req_set_res(req, mask, 0);
 				return IOU_POLL_REMOVE_POLL_USE_RES;
 			}
+			v &= IO_POLL_REF_MASK;
 		} else {
 			int ret = io_poll_issue(req, tw);
 
@@ -312,16 +326,11 @@ static int io_poll_check_events(struct io_kiocb *req, io_tw_token_t tw)
 				return IOU_POLL_REQUEUE;
 			if (ret != IOU_RETRY && ret < 0)
 				return ret;
+			io_mshot_check_retry(req, &v);
 		}
 
 		/* force the next iteration to vfs_poll() */
 		req->cqe.res = 0;
-
-		/*
-		 * Release all references, retry if someone tried to restart
-		 * task_work while we were executing it.
-		 */
-		v &= IO_POLL_REF_MASK;
 	} while (atomic_sub_return(v, &req->poll_refs) & IO_POLL_REF_MASK);
 
 	io_napi_add(req);

-- 
Jens Axboe