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Subject: Re: [RFC PATCH 03/13] futex2: Implement vectorized wait
To:     Gabriel Krisman Bertazi <krisman@collabora.com>,
        =?UTF-8?Q?Andr=c3=a9_Almeida?= <andrealmeid@collabora.com>
Cc:     Thomas Gleixner <tglx@linutronix.de>,
        Ingo Molnar <mingo@redhat.com>,
        Peter Zijlstra <peterz@infradead.org>,
        Darren Hart <dvhart@infradead.org>,
        linux-kernel@vger.kernel.org, Steven Rostedt <rostedt@goodmis.org>,
        Sebastian Andrzej Siewior <bigeasy@linutronix.de>,
        kernel@collabora.com, pgriffais@valvesoftware.com,
        joel@joelfernandes.org, malteskarupke@fastmail.fm,
        linux-api@vger.kernel.org, fweimer@redhat.com,
        libc-alpha@sourceware.org, linux-kselftest@vger.kernel.org,
        shuah@kernel.org, acme@kernel.org, corbet@lwn.net
References: <20210215152404.250281-1-andrealmeid@collabora.com>
 <20210215152404.250281-4-andrealmeid@collabora.com>
 <87ft1xw123.fsf@collabora.com>
From:   Zebediah Figura <zfigura@codeweavers.com>
Message-ID: <477b370d-575d-717f-ee49-9f1e30c737d5@codeweavers.com>
Date:   Mon, 15 Feb 2021 14:06:49 -0600
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From: Zebediah Figura <zfigura@codeweavers.com>
To: Gabriel Krisman Bertazi <krisman@collabora.com>,
 =?UTF-8?Q?Andr=c3=a9_Almeida?= <andrealmeid@collabora.com>
Cc: Thomas Gleixner <tglx@linutronix.de>, Ingo Molnar <mingo@redhat.com>,
 Peter Zijlstra <peterz@infradead.org>, Darren Hart <dvhart@infradead.org>,
 linux-kernel@vger.kernel.org, Steven Rostedt <rostedt@goodmis.org>,
 Sebastian Andrzej Siewior <bigeasy@linutronix.de>, kernel@collabora.com,
 pgriffais@valvesoftware.com, joel@joelfernandes.org,
 malteskarupke@fastmail.fm, linux-api@vger.kernel.org, fweimer@redhat.com,
 libc-alpha@sourceware.org, linux-kselftest@vger.kernel.org,
 shuah@kernel.org, acme@kernel.org, corbet@lwn.net
Message-ID: <477b370d-575d-717f-ee49-9f1e30c737d5@codeweavers.com>
Subject: Re: [RFC PATCH 03/13] futex2: Implement vectorized wait
References: <20210215152404.250281-1-andrealmeid@collabora.com>
 <20210215152404.250281-4-andrealmeid@collabora.com>
 <87ft1xw123.fsf@collabora.com>
In-Reply-To: <87ft1xw123.fsf@collabora.com>

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On 2/15/21 2:03 PM, Gabriel Krisman Bertazi wrote:
> Andr=C3=A9 Almeida <andrealmeid@collabora.com> writes:
>=20
>> 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 !=3D 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 neede=
d.
>> Flags for shared futexes, sizes, etc. should be used on the individual=

>> flags of each waiter.
>=20
> 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.
>=20

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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