On 07/29/2015 06:21 AM, Darren Hart wrote:
> On Tue, Jul 28, 2015 at 09:11:41PM -0700, Darren Hart wrote:
>> On Tue, Jul 28, 2015 at 10:23:51PM +0200, Thomas Gleixner wrote:
>>> On Mon, 27 Jul 2015, Michael Kerrisk (man-pages) wrote:
>>
>> ...
>>
>>>> FUTEX_REQUEUE (since Linux 2.6.0)
>>>> .\" FIXME(Torvald) Is there some indication that FUTEX_REQUEUE is broken
>>>> .\" in general, or is this comment implicitly speaking about the
>>>> .\" condvar (?) use case? If the latter we might want to weaken the
>>>> .\" advice below a little.
>>>> .\" [Anyone else have input on this?]
>>>
>>> The condvar use case exposes the flaw nicely, but that's pretty much
>>> true for everything which wants a sane requeue operation.
>>
>> In an earlier discussion I argued this point (that FUTURE_REQUEUE is broken
>> and
>> should not be used in new code) and someone argued strongly against...
>> stating
>> that there were legitimate uses for it. Of course I'm struggling to find the
>> thread and the reference at the moment - immensely useful, I know.
>>
>> I'll continue trying to find it and see if it can be useful here. I believe
>> Torvald was on the thread as well.
>>
>
> Found it on libc-alpha, here it is for reference:
>
> From: Rich Felker <dal...@libc.org>
> Date: Wed, 29 Oct 2014 22:43:17 -0400
> To: Darren Hart <dvh...@infradead.org>
> Cc: Carlos O'Donell <car...@redhat.com>, Roland McGrath
> <rol...@hack.frob.com>,
> Torvald Riegel <trie...@redhat.com>, GLIBC Devel
> <libc-al...@sourceware.org>,
> Michael Kerrisk <mtk.manpa...@gmail.com>
> Subject: Re: Add futex wrapper to glibc?
>
> On Wed, Oct 29, 2014 at 06:59:15PM -0700, Darren Hart wrote:
> > > We are IMO at the stage where futex is stable, few things are
> > > changing, and with documentation in place, I would consider adding a
> > > futex wrapper.
> >
> > Yes, at least for the defined OP codes. New OPs may be added of
> > course, but that isn't a concern for supporting what exists today, and
> > doesn't break compatibility.
> >
> > I wonder though... can we not wrap FUTEX_REQUEUE? It's fundamentally
> > broken. FUTEX_CMP_REQUEUE should *always* be used instead. The glibc
> > wrapper is one way to encourage developers to do the right thing
> > (don't expose the bad op in the header).
>
> You're mistaken here. There are plenty of valid ways to use
> FUTEX_REQUEUE - for example if the calling thread is requeuing the
> target(s) to a lock that the calling thread owns. Just because it
> doesn't meet the needs of the way glibc was using it internally
> doesn't mean it's useless for other applications.
>
> In any case, I don't think there's a proposal to intercept/modify the
> commands to futex, just to pass them through (and possibly do a
> cancellable syscall for some of them).
>
> Rich
>
>
>>>
>>>> Avoid using this operation. It is broken for its intended
>>>> purpose. Use FUTEX_CMP_REQUEUE instead.
>>>>
>>>> This operation performs the same task as
>>>> FUTEX_CMP_REQUEUE, except that no check is made using the
>>>> value in val3. (The argument val3 is ignored.)
Thanks, Darren, that's really helpful! I've removed the statement in the man
page that FUTEX_REQUEUE is broken.
By the way, Darren. There were a couple of FIXMEs in the page where you are
explicitly mentioned by name. Could you take a look at those? Specifically,
the large block of text starting at:
>> .\" FIXME XXX The following is my attempt at a definition of PI futexes,
>> .\" based on mail discussions with Darren Hart. Does it seem okay?
(tglx looked at this and blessed it, but I'd like you also to check.)
Cheers,
Michael
--
Michael Kerrisk
Linux man-pages maintainer; http://www.kernel.org/doc/man-pages/
Linux/UNIX System Programming Training: http://man7.org/training/
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