linux-kernel - Re: Change in functionality of futex() system call.

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Message-ID: <BANLkTi=vwJBgkKSY_Cq_jQS-71gH6uOPaw@mail.gmail.com>
Date:	Tue, 7 Jun 2011 18:26:10 -0400
From:	Kyle Moffett <kyle@...fetthome.net>
To:	Andrew Lutomirski <luto@....edu>
Cc:	David Oliver <david@...advisors.com>,
	Eric Dumazet <eric.dumazet@...il.com>,
	Darren Hart <dvhart@...ux.intel.com>,
	Peter Zijlstra <peterz@...radead.org>,
	linux-kernel@...r.kernel.org,
	Shawn Bohrer <sbohrer@...advisors.com>,
	Zachary Vonler <zvonler@...advisors.com>,
	KOSAKI Motohiro <kosaki.motohiro@...fujitsu.com>,
	Hugh Dickins <hughd@...gle.com>,
	Thomas Gleixner <tglx@...utronix.de>,
	Ingo Molnar <mingo@...e.hu>
Subject: Re: Change in functionality of futex() system call.

On Tue, Jun 7, 2011 at 15:19, Andrew Lutomirski <luto@....edu> wrote:
> On Tue, Jun 7, 2011 at 3:10 PM, David Oliver <david@...advisors.com> wrote:
>> I have software which currently uses shared files for a one way
>> transfer of information, which is modeled precisely by the futex (as
>> contrasted to the mutex) model. In this case, the number of receivers
>> is undetermined, so the number of wakeups is set to maxint.
>>
>> The receivers are minimally trusted: they have read access to the
>> files, so they cannot accidentally affect other processes use of the
>> data. Requiring my files to be writeable by all clients would require
>> a serious increase in the amount of software needing to be trusted.
>
> What's wrong with adding a FUTEX_WAIT_NOCONSUME flag then?  Your
> program can use it to get exactly the semantics it wants and my
> program can use it or not depending on which semantics it wants.
>
> Then we can document in the man page that, on kernels newer than
> whichever version introduced the regression, read-only mappings of a
> file cannot be used to interfere with futexes on that file.

Hmm, I would actually call it "FUTEX_POLL", since that better reflects the
operation being performed.

Certainly you would want to avoid allowing FUTEX_POLL to "steal"
limited wakeups from FUTEX_CMP_REQUEUE or whatever, so you
also need a new "FUTEX_NOTIFY".  Alternatively I guess you could just
special-case the FUTEX_WAKE && wakeups == INTMAX combination to
also notify FUTEX_POLL processes.

I almost wonder if long-term there might possibly be some decent way
to integrate this with eventfds to allow a thread to wait for notifications from
any number of memory addresses as well as other event sources.  This
would be a similar extension to signalfd, only for futexes.

Cheers,
Kyle Moffett
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