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

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Date:	Wed, 4 Feb 2009 15:55:14 -0800
From:	Andrew Morton <akpm@...ux-foundation.org>
To:	Davide Libenzi <davidel@...ilserver.org>
Cc:	linux-kernel@...r.kernel.org, torvalds@...ux-foundation.org,
	mtk.manpages@...il.com, linux-api@...r.kernel.org
Subject: Re: [patch/rfc] eventfd semaphore-like behavior

On Wed, 4 Feb 2009 15:27:45 -0800 (PST)
Davide Libenzi <davidel@...ilserver.org> wrote:

> 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?
> 

Dunno.  Probably try the syscall and see if it returned -EINVAL.  Does
that work in this case?  If so, it would be sensible to mention this in
the description somewhere as the approved probing method and to
maintain it.


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