linux-kernel - Re: implementing Futex

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Date:	Fri, 14 Aug 2009 14:38:01 +0200
From:	Arnd Bergmann <arnd@...db.de>
To:	Michael Schnell <mschnell@...ino.de>
Cc:	linux-kernel@...r.kernel.org,
	Mike Frysinger <vapier.adi@...il.com>,
	Paul Mundt <lethal@...ux-sh.org>,
	Michael Kerrisk <mtk.manpages@...il.com>,
	Ingo Molnar <mingo@...e.hu>, Jakub Jelinek <jakub@...hat.com>
Subject: Re: implementing Futex

On Friday 14 August 2009, Michael Schnell wrote:
> Arnd Bergmann wrote:,
> >> From reading the code (futex_atomic_op_inuser() seems only to be called
> >> once (in futex.c) ), it seems that futex_atomic_op_inuser() is not
> >> really used at all. It seems that it'd only called for futex_wake, and
> >> here the "Operation" is derived from the last parameter of the system
> >> call, which the man page says is ignored.  So, are all the operations
> >> implemented there really necessary or just "nice to have" ?
> >
> > You misread futex_wake_op(). The operation comes from the second parameter
> > of the syscall, not the last one.
> 
> Really ?

Sorry, I misread the code as well, in a different way :(

> What am I getting wrong ?
> 
> But I also don't see how this makes any sense.

The man page says that the last parameter is ignored for op=FUTEX_WAKE,
which is correct. The code that you were looking at however is for
of=FUTEX_WAKE_OP, which is not documented as of manpages-3.22.

It was added by Jakub back in 2005, but if he wrote a documentation
for it, it never went into the man pages package. On a similar
note, Ingo also added other futex operations that are not documented
yet.

	Arnd <><
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