Re: [PATCH v3 1/2] io_uring: change the poll events to be 32-bits

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

On 16/06/2020 22:21, Jens Axboe wrote:
> 
> Nah this won't work, as the BE layout will be different. So how about
> this, just add a 16-bit poll_events_hi instead. App/liburing will set
> upper bits there. Something like the below, then just needs the
> exclusive wait change on top.
> 
> Only downside I can see is that newer applications on older kernels will
> set EPOLLEXCLUSIVE but the kernel will ignore it. That's not a huge
> concern for this particular bit, but could be a concern if there are
> others that prove useful.
> 
> diff --git a/fs/io_uring.c b/fs/io_uring.c
> index de1175206807..a9d74330ad6b 100644
> --- a/fs/io_uring.c
> +++ b/fs/io_uring.c
> @@ -4809,6 +4809,9 @@ static int io_poll_add_prep(struct io_kiocb *req, const struct io_uring_sqe *sqe
>  	events = READ_ONCE(sqe->poll_events);
>  	poll->events = demangle_poll(events) | EPOLLERR | EPOLLHUP;
>  
> +	if (READ_ONCE(sqe->poll_events_hi) & EPOLLEXCLUSIVE)

poll_events_hi is 16 bit, EPOLLEXCLUSIVE is (1 << 28). It's always false.
Do you look for something like below?


union {
	...
	__u32		fsync_flags;
	__u16		poll_events;  /* compatibility */
	__u32		poll32_events; /* word-reversed for BE */
};

u32 evs = READ_ONCE(poll32_events);
if (BIG_ENDIAN)
	evs = swahw32(evs); // swap 16-bit halves

// use as always, e.g. if (evs & EPOLLEXCLUSIVE) { ... }

> +		poll->events |= EPOLLEXCLUSIVE;
> +
>  	io_get_req_task(req);
>  	return 0;
>  }
> diff --git a/include/uapi/linux/io_uring.h b/include/uapi/linux/io_uring.h
> index 92c22699a5a7..e6856d8e068f 100644
> --- a/include/uapi/linux/io_uring.h
> +++ b/include/uapi/linux/io_uring.h
> @@ -31,7 +31,10 @@ struct io_uring_sqe {
>  	union {
>  		__kernel_rwf_t	rw_flags;
>  		__u32		fsync_flags;
> -		__u16		poll_events;
> +		struct {
> +			__u16	poll_events;
> +			__u16	poll_events_hi;
> +		};
>  		__u32		sync_range_flags;
>  		__u32		msg_flags;
>  		__u32		timeout_flags;
> 

-- 
Pavel Begunkov