[PATCH for-6.2 v2 3/3] io_uring/msg_ring: optimise with correct tw notify method

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

We may want to use TWA_SIGNAL_NO_IPI instead of TWA_SIGNAL if the target
ring is configured with IORING_SETUP_COOP_TASKRUN, change
io_msg_exec_remote() to use the target ring's ->notify_method.

The caveat is that we have to set IORING_SQ_TASKRUN if the rings asks
for it. However, once task_work_add() succeeds the target ring might go
away and so we grab a ctx reference to pin the ring until we set
IORING_SQ_TASKRUN.

Signed-off-by: Pavel Begunkov <asml.silence@gmail.com>
---
 io_uring/msg_ring.c | 15 +++++++++++----
 1 file changed, 11 insertions(+), 4 deletions(-)

diff --git a/io_uring/msg_ring.c b/io_uring/msg_ring.c
index 15602a136821..af802cd645b4 100644
--- a/io_uring/msg_ring.c
+++ b/io_uring/msg_ring.c
@@ -70,15 +70,22 @@ static int io_msg_exec_remote(struct io_kiocb *req, task_work_func_t func)
 	struct io_ring_ctx *ctx = req->file->private_data;
 	struct io_msg *msg = io_kiocb_to_cmd(req, struct io_msg);
 	struct task_struct *task = READ_ONCE(ctx->submitter_task);
+	int ret = IOU_ISSUE_SKIP_COMPLETE;
 
 	if (unlikely(!task))
 		return -EOWNERDEAD;
 
+	percpu_ref_get(&ctx->refs);
 	init_task_work(&msg->tw, func);
-	if (task_work_add(ctx->submitter_task, &msg->tw, TWA_SIGNAL))
-		return -EOWNERDEAD;
-
-	return IOU_ISSUE_SKIP_COMPLETE;
+	if (task_work_add(ctx->submitter_task, &msg->tw, ctx->notify_method)) {
+		ret = -EOWNERDEAD;
+		goto out;
+	}
+	if (ctx->flags & IORING_SETUP_TASKRUN_FLAG)
+		atomic_or(IORING_SQ_TASKRUN, &ctx->rings->sq_flags);
+out:
+	percpu_ref_put(&ctx->refs);
+	return ret;
 }
 
 static void io_msg_tw_complete(struct callback_head *head)
-- 
2.38.1