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

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Message-ID: <alpine.DEB.1.10.0902041526590.6214@alien.or.mcafeemobile.com>
Date:	Wed, 4 Feb 2009 15:27:45 -0800 (PST)
From:	Davide Libenzi <davidel@...ilserver.org>
To:	Andrew Morton <akpm@...ux-foundation.org>
cc:	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Michael Kerrisk <mtk.manpages@...il.com>,
	linux-api@...r.kernel.org
Subject: Re: [patch/rfc] eventfd semaphore-like behavior

On Wed, 4 Feb 2009, Andrew Morton wrote:

> On Wed, 4 Feb 2009 15:18:43 -0800 (PST)
> Davide Libenzi <davidel@...ilserver.org> wrote:
> 
> > > > Simple test here:
> > > > 
> > > > http://www.xmailserver.org/eventfd-sem.c
> > > > 
> > > > 
> > > > Signed-off-by: Davide Libenzi <davidel@...ilserver.org>
> > > > 
> > > > +/*
> > > > + * CAREFUL: Check include/asm-generic/fcntl.h when defining
> > > > + * new flags, since they might collide with O_* ones. We want
> > > > + * to re-use O_* flags that couldn't possibly have a meaning
> > > > + * from eventfd, in order to leave a free define-space for
> > > > + * shared O_* flags.
> > > > + */
> > > > +#define EFD_SEMAPHORE (1 << 0)
> > > >  #define EFD_CLOEXEC O_CLOEXEC
> > > >  #define EFD_NONBLOCK O_NONBLOCK
> > > >  
> > > > +#define EFD_SHARED_FCNTL_FLAGS (O_CLOEXEC | O_NONBLOCK)
> > > > +#define EFD_FLAGS_SET (EFD_SHARED_FCNTL_FLAGS | EFD_SEMAPHORE)
> > > 
> > > How would you recommend that userspace determine whether its kernel
> > > supports this feature, bearing in mind that someone might backport this
> > > patch into arbitrarily earlier kernel versions?
> > > 
> > > 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?



- Davide


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