Eric Dumazet wrote:
> Davi Arnaut a écrit :
>> Eric Dumazet wrote:
>>> Davi Arnaut a écrit :
>>>> Asynchronously wait for FUTEX_WAKE operation on a futex if it still contains
>>>> a given value. There can be only one futex wait per file descriptor. However,
>>>> it can be rearmed (possibly at a different address) anytime.
>>>>
>>>> The pollable futex approach is far superior (send and receive events from
>>>> userspace or kernel) to eventfd and fixes (supercedes) FUTEX_FD at the same time.
>>>>
>>>> Building block for pollable semaphores and user-defined events.
>>>>
>>>> Signed-off-by: Davi E. M. Arnaut <[email protected]>
>>>>
<snip>
>>>> +
>>>> +struct futex_event {
>>>> + union {
>>>> + void __user *addr;
>>>> + u64 padding;
>>>> + };
>>>> + int val;
>>>> +};
>>> Hum... Here we might have a problem with 64 bit futexes, or private futexes
>>>
>>> So I believe this interface is not well defined and not expandable: in case of
>>> future additions to futexes, an old application compiled with an old pollable
>>> futex_event type might fail.
>>>
>> Hmm, how about:
>>
>> struct futex_event {
>> union {
>> void __user *addr;
>> u64 padding;
>> };
>> union {
>> int val;
>> s64 val64;
>> };
>> /* whatever room is necessary for future improvements */
>> };
>>
>> I haven't been keeping up with 64 bit or private futexes. What else
>> could probably go wrong?
>
> Well, that's the point : This interface is like an ioctl() one : pretty bad if
> not properly designed :)
I was merely mirroring the futex syscall arguments for FUTEX_WAIT. Will
those change? I hope not :)
> You probably need to stick one field containing one command or version number,
> something like that.
I'm a bit skeptical that we need versioning for such a simple operation
(command) as FUTEX_WAIT that takes an address and a value.
>
>
> struct futex_event {
> int type;
> union {
> void __user *addr;
> u64 padding;
> };
> union {
> int val;
> s64 val64;
> };
> };
>
> #define FUTEX_EVENT_SHARED32 1
> #define FUTEX_EVENT_SHARED64 2
> #define FUTEX_EVENT_PRIVATE32 (128|1)
> #define FUTEX_EVENT_PRIVATE64 (128|2)
I will take a look at the private futexes patches before commenting further.
> ...
>
> Also, you should take care of alignements constraints (a 32bit user program
> might run on a 64bit kernel)
>
Compat code? or futex alignements constraints?
--
Davi Arnaut
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