From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1758831AbZBDXtA (ORCPT ); Wed, 4 Feb 2009 18:49:00 -0500 Received: (majordomo@vger.kernel.org) by vger.kernel.org id S1755365AbZBDXss (ORCPT ); Wed, 4 Feb 2009 18:48:48 -0500 Received: from wa-out-1112.google.com ([209.85.146.176]:15643 "EHLO wa-out-1112.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1752524AbZBDXsq (ORCPT ); Wed, 4 Feb 2009 18:48:46 -0500 DomainKey-Signature: a=rsa-sha1; c=nofws; d=googlemail.com; s=gamma; h=mime-version:reply-to:in-reply-to:references:date:message-id :subject:from:to:cc:content-type:content-transfer-encoding; b=kzsJwBdndS7415GqdS39NdANxgSnF2DCAaPe9db8FKNAd737A5U+aQ6nm0P6uBIT3H LvnXNXm2JqI5bFe7SNyBTX7lZCfek43dUU5NgFFDTjTNx0MgDDnA0DVsVCtSoOm5IuE7 h2pR/6zPEWgxzoimBfX+TkxGITCaAwJrb20Jk= MIME-Version: 1.0 Reply-To: mtk.manpages@gmail.com In-Reply-To: References: <20090204150507.665b5b7c.akpm@linux-foundation.org> <20090204152434.c8f65d52.akpm@linux-foundation.org> Date: Thu, 5 Feb 2009 12:48:45 +1300 Message-ID: Subject: Re: [patch/rfc] eventfd semaphore-like behavior From: Michael Kerrisk To: Davide Libenzi Cc: Andrew Morton , Linux Kernel Mailing List , Linus Torvalds , linux-api@vger.kernel.org, Ulrich Drepper Content-Type: text/plain; charset=ISO-8859-1 Content-Transfer-Encoding: 7bit Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Thu, Feb 5, 2009 at 12:27 PM, 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? Maybe I missed something, but I think we're okay, aren't we? Viz: a) The glibc eventfd() wrapper invokes sys_eventfd2() (which allows a flags arg) if it is available, and otherwise fall back to sys_eventfd() (which does not support a flags arg). b) If glibc falls back to sys_eventfd(), then it knows to reject non-zero flags. (The glibc wrapper already does this.) c) If the old sys_eventfd2() is given a flag that it doesn't recognize, then it fails with EINVAL. (That check is already in the code.) So, userspace can determine whether EFD_SEMAPHORE is supported by not getting an EINVAL error. (Okay, this falls down for binaries that bypass glibc's eventfd() wrapper, but there are unlikely to be such binaries.) 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