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

[Andrew Morton <akpm@linux-foundation.org>]

On Wed, 4 Feb 2009 15:18:43 -0800 (PST)
Davide Libenzi <davidel@xmailserver.org> wrote:

> > > Simple test here:
> > > 
> > > http://www.xmailserver.org/eventfd-sem.c
> > > 
> > > 
> > > Signed-off-by: Davide Libenzi <davidel@xmailserver.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.