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

[Jiufei Xue <jiufei.xue@linux.alibaba.com>]

On 2020/6/17 上午5:46, Pavel Begunkov wrote:
> 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) { ... }
>
Cool, That's clear. And we should do the reverse thing in liburing
for big endian.

I also want to add another feature bit in newer kernel, so that
the newer application should check the feature first before setting
the EPOLLEXCLUSIVE.

Thanks,
JIufei.

>> +		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;
>>
>