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

[Kyle Moffett]

On Tue, Jun 7, 2011 at 15:19, Andrew Lutomirski <luto@xxxxxxx> wrote:
> On Tue, Jun 7, 2011 at 3:10 PM, David Oliver <david@xxxxxxxxxxxxxxx> 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? ÂYour
> 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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