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

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Message-ID: <477b370d-575d-717f-ee49-9f1e30c737d5@codeweavers.com>
Date:   Mon, 15 Feb 2021 14:06:49 -0600
From:   Zebediah Figura <zfigura@...eweavers.com>
To:     Gabriel Krisman Bertazi <krisman@...labora.com>,
        André Almeida <andrealmeid@...labora.com>
Cc:     Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Darren Hart <dvhart@...radead.org>,
        linux-kernel@...r.kernel.org, Steven Rostedt <rostedt@...dmis.org>,
        Sebastian Andrzej Siewior <bigeasy@...utronix.de>,
        kernel@...labora.com, pgriffais@...vesoftware.com,
        joel@...lfernandes.org, malteskarupke@...tmail.fm,
        linux-api@...r.kernel.org, fweimer@...hat.com,
        libc-alpha@...rceware.org, linux-kselftest@...r.kernel.org,
        shuah@...nel.org, acme@...nel.org, corbet@....net
Subject: Re: [RFC PATCH 03/13] futex2: Implement vectorized wait

On 2/15/21 2:03 PM, Gabriel Krisman Bertazi wrote:
> André Almeida <andrealmeid@...labora.com> 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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