From mboxrd@z Thu Jan 1 00:00:00 1970 From: Michael Kerrisk Subject: Re: [patch/rfc] eventfd semaphore-like behavior Date: Thu, 5 Feb 2009 12:59:07 +1300 Message-ID: References: <20090204150507.665b5b7c.akpm@linux-foundation.org> <20090204152434.c8f65d52.akpm@linux-foundation.org> <20090204155514.6abbdc8f.akpm@linux-foundation.org> Reply-To: mtk.manpages-Re5JQEeQqe8AvxtiuMwx3w@public.gmane.org Mime-Version: 1.0 Content-Type: text/plain; charset=ISO-8859-1 Content-Transfer-Encoding: 7bit Return-path: In-Reply-To: <20090204155514.6abbdc8f.akpm-de/tnXTf+JLsfHDXvbKv3WD2FQJk+8+b@public.gmane.org> Sender: linux-api-owner-u79uwXL29TY76Z2rM5mHXA@public.gmane.org To: Andrew Morton Cc: Davide Libenzi , linux-kernel-u79uwXL29TY76Z2rM5mHXA@public.gmane.org, torvalds-de/tnXTf+JLsfHDXvbKv3WD2FQJk+8+b@public.gmane.org, linux-api-u79uwXL29TY76Z2rM5mHXA@public.gmane.org List-Id: linux-api@vger.kernel.org On Thu, Feb 5, 2009 at 12:55 PM, Andrew Morton wrote: > On Wed, 4 Feb 2009 15:27:45 -0800 (PST) > Davide Libenzi wrote: > >> On Wed, 4 Feb 2009, Andrew Morton wrote: >> >> > On Wed, 4 Feb 2009 15:18:43 -0800 (PST) >> > Davide Libenzi wrote: >> > >> > > > > Simple test here: >> > > > > >> > > > > http://www.xmailserver.org/eventfd-sem.c >> > > > > >> > > > > >> > > > > Signed-off-by: Davide Libenzi >> > > > > >> > > > > +/* >> > > > > + * 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? As youll have seen by now, Ulrich and I noted that it works. > If so, it would be sensible to mention this in > the description somewhere as the approved probing method and to > maintain it. I'll add something to the man page, as this patch progresses. Cheers, Michael -- Michael Kerrisk Linux man-pages maintainer; http://www.kernel.org/doc/man-pages/ git://git.kernel.org/pub/scm/docs/man-pages/man-pages.git man-pages online: http://www.kernel.org/doc/man-pages/online_pages.html Found a bug? http://www.kernel.org/doc/man-pages/reporting_bugs.html -- To unsubscribe from this list: send the line "unsubscribe linux-api" in the body of a message to majordomo-u79uwXL29TY76Z2rM5mHXA@public.gmane.org More majordomo info at http://vger.kernel.org/majordomo-info.html