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

[Jens Axboe <axboe@kernel.dk>]

On 6/16/20 3:46 PM, 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) { ... }

Duh yes, that's much better. Thanks for the clue bat.

-- 
Jens Axboe