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Date: Wed, 4 Feb 2009 16:18:16 -0800
From: Andrew Morton <akpm@linux-foundation.org>
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Subject: Re: [patch/rfc] eventfd semaphore-like behavior
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On Thu, 5 Feb 2009 12:59:07 +1300
Michael Kerrisk <mtk.manpages@googlemail.com> wrote:

> >> > > > What should be userspace's fallback strategy if that support is not
> >> > > > present?
> >> > >
> >> > > #ifdef EFD_SEMAPHORE, maybe?
> >> >
> >> > That's compile-time.  People who ship binaries will probably want
> >> > to find a runtime thing for back-compatibility.
> >>
> >> I dunno. How do they actually do when we add new flags, like the O_ ones?
> >>
> >
> > Dunno.  Probably try the syscall and see if it returned -EINVAL.  Does
> > that work in this case?
> 
> As youll have seen by now, Ulrich and I noted that it works.

I think you means "should work" ;)

We're talking about this, yes?

SYSCALL_DEFINE2(eventfd2, unsigned int, count, int, flags)
{
	int fd;
	struct eventfd_ctx *ctx;

	/* Check the EFD_* constants for consistency.  */
	BUILD_BUG_ON(EFD_CLOEXEC != O_CLOEXEC);
	BUILD_BUG_ON(EFD_NONBLOCK != O_NONBLOCK);

	if (flags & ~(EFD_CLOEXEC | EFD_NONBLOCK))
		return -EINVAL;

That looks like it should work to me.