linux-kernel - Re: [patch/rfc] eventfd semaphore-like behavior

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Date:	Wed, 4 Feb 2009 16:34:34 -0800
From:	Andrew Morton <akpm@...ux-foundation.org>
To:	Davide Libenzi <davidel@...ilserver.org>
Cc:	mtk.manpages@...il.com, mtk.manpages@...glemail.com,
	linux-kernel@...r.kernel.org, torvalds@...ux-foundation.org,
	linux-api@...r.kernel.org
Subject: Re: [patch/rfc] eventfd semaphore-like behavior

On Wed, 4 Feb 2009 16:25:52 -0800 (PST)
Davide Libenzi <davidel@...ilserver.org> wrote:

> On Wed, 4 Feb 2009, Andrew Morton wrote:
> 
> > On Thu, 5 Feb 2009 12:59:07 +1300
> > Michael Kerrisk <mtk.manpages@...glemail.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.
> 
> I lost you guys. On old kernels you'd get -EINVAL when using the new flag. 
> Wasn't it clear? Or is there some side-band traffic in this conversation 
> that I missed? :)
> 

Well yes, that.  What I was trying to establish here is that we have
thought about (and preferably tested) userspace's back-compatibility
arrangements.

We have now done that.
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