Re: [RFC PATCH 03/13] futex2: Implement vectorized wait

[Zebediah Figura]

On 2/15/21 2:03 PM, Gabriel Krisman Bertazi wrote:
> André Almeida <andrealmeid@xxxxxxxxxxxxx> writes:
> 
>> Add support to wait on multiple futexes. This is the interface
>> implemented by this syscall:
>>
>> futex_waitv(struct futex_waitv *waiters, unsigned int nr_futexes,
>> 	    unsigned int flags, struct timespec *timo)
>>
>> struct futex_waitv {
>> 	void *uaddr;
>> 	unsigned int val;
>> 	unsigned int flags;
>> };
>>
>> Given an array of struct futex_waitv, wait on each uaddr. The thread
>> wakes if a futex_wake() is performed at any uaddr. The syscall returns
>> immediately if any waiter has *uaddr != val. *timo is an optional
>> timeout value for the operation. The flags argument of the syscall
>> should be used solely for specifying the timeout as realtime, if needed.
>> Flags for shared futexes, sizes, etc. should be used on the individual
>> flags of each waiter.
> 
> Given the previous proposal from Zebediah, one use case Wine has to
> support is the ability to wait on all (instead of any) of a list of
> futexes .  I suppose that could be done by a new FLAG_WAITALL passed on
> the third argument of the syscall.
> 

I don't think that such a feature would help Wine very much. It doesn't
fix any of the correctness problems with the futex-backed implementation
of NT primitives, and given the rarity of the wait-on-all operation I'm
inclined to believe it won't help performance either.

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