Re: Does need memory barrier to synchronize req->result with req->iopoll_completed

[Jens Axboe <axboe@kernel.dk>]

On 6/14/20 8:10 AM, Xiaoguang Wang wrote:
> hi,
> 
> I have taken some further thoughts about previous IPOLL race fix patch,
> if io_complete_rw_iopoll() is called in interrupt context, "req->result = res"
> and "WRITE_ONCE(req->iopoll_completed, 1);" are independent store operations.
> So in io_do_iopoll(), if iopoll_completed is ture, can we make sure that
> req->result has already been perceived by the cpu executing io_do_iopoll()?

Good point, I think if we do something like the below, we should be
totally safe against an IRQ completion. Since we batch the completions,
we can get by with just a single smp_rmb() on the completion side.

diff --git a/fs/io_uring.c b/fs/io_uring.c
index 155f3d830ddb..74c2a4709b63 100644
--- a/fs/io_uring.c
+++ b/fs/io_uring.c
@@ -1736,6 +1736,9 @@ static void io_iopoll_complete(struct io_ring_ctx *ctx, unsigned int *nr_events,
 	struct req_batch rb;
 	struct io_kiocb *req;
 
+	/* order with ->result store in io_complete_rw_iopoll() */
+	smp_rmb();
+
 	rb.to_free = rb.need_iter = 0;
 	while (!list_empty(done)) {
 		int cflags = 0;
@@ -1976,6 +1979,8 @@ static void io_complete_rw_iopoll(struct kiocb *kiocb, long res, long res2)
 	if (res != req->result)
 		req_set_fail_links(req);
 	req->result = res;
+	/* order with io_poll_complete() checking ->result */
+	smp_wmb();
 	if (res != -EAGAIN)
 		WRITE_ONCE(req->iopoll_completed, 1);
 }

-- 
Jens Axboe