From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1755417AbZBTHdj (ORCPT ); Fri, 20 Feb 2009 02:33:39 -0500 Received: (majordomo@vger.kernel.org) by vger.kernel.org id S1752552AbZBTHd2 (ORCPT ); Fri, 20 Feb 2009 02:33:28 -0500 Received: from chrocht.moloch.sk ([62.176.169.44]:42402 "EHLO mail.moloch.sk" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1752416AbZBTHd1 (ORCPT ); Fri, 20 Feb 2009 02:33:27 -0500 X-Greylist: delayed 1983 seconds by postgrey-1.27 at vger.kernel.org; Fri, 20 Feb 2009 02:33:27 EST Message-ID: <499E5506.5090406@fastmq.com> Date: Fri, 20 Feb 2009 08:00:22 +0100 From: Martin Sustrik User-Agent: Thunderbird 2.0.0.19 (X11/20090105) MIME-Version: 1.0 To: linux-kernel@vger.kernel.org Subject: Re: [patch/rfc] eventfd semaphore-like behavior References: <499E527B.2030702@fastmq.com> In-Reply-To: <499E527B.2030702@fastmq.com> Content-Type: text/plain; charset=ISO-8859-1; format=flowed Content-Transfer-Encoding: 7bit Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Hi, Speaking of eventfd use cases, we're using it as a bitmap. Everything works great, the only problem being that we have to take extra care not to set the same bit twice: fd_counter is 00000000 write (fd, 00001000) fd_counter is 00001000 write (fd, 00000001) fd_counter is 00001001 write (fd, 00001000) /* oops! */ fd_counter is 00010001 Would anyone else benefit from a new eventfd flag, say EFD_BITMAP, that would cause write(2) to do binary OR rather than + on the counter? Martin Martin Sustrik wrote: > > Enter your search termsSubmit search formWeblkml.org > Date Wed, 4 Feb 2009 14:58:39 -0800 (PST) > From Davide Libenzi <> > Subject [patch/rfc] eventfd semaphore-like behavior > > People started using eventfd in scnarios where before where using pipes. > Many of them use eventfds in a semaphore-like way, like they were before > with pipes. The problem with eventfd is that a read() on the fd returns > and wipes the whole counter, making the use of it as semaphore a little > bit more cumbersome. You can do a read() followed by a write() of > COUNTER-1, but IMO it's pretty easy and cheap to make this work w/out > extra steps. This patch introduces a new eventfd flag that tells eventfd > to only dequeue 1 from the counter, allowing simple read/write to make it > behave like a semaphore. > Simple test here: > > http://www.xmailserver.org/eventfd-sem.c > > > Signed-off-by: Davide Libenzi > > > - Davide >