From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1760957AbYDNP7Q (ORCPT ); Mon, 14 Apr 2008 11:59:16 -0400 Received: (majordomo@vger.kernel.org) by vger.kernel.org id S1755112AbYDNP7B (ORCPT ); Mon, 14 Apr 2008 11:59:01 -0400 Received: from sj-iport-6.cisco.com ([171.71.176.117]:17268 "EHLO sj-iport-6.cisco.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1755064AbYDNP7A (ORCPT ); Mon, 14 Apr 2008 11:59:00 -0400 From: Roland Dreier To: Ingo Molnar Cc: Matthew Wilcox , Peter Zijlstra , Ingo Oeser , Daniel Walker , linux-kernel@vger.kernel.org, Linus Torvalds Subject: Re: [PATCH] Replace completions with semaphores References: <20080411210022.GJ11962@parisc-linux.org> <1208003081.7427.7.camel@twins> <20080412172606.GL11962@parisc-linux.org> <20080412204748.GN11962@parisc-linux.org> <20080413070833.GC19773@elte.hu> <20080413125758.GQ11962@parisc-linux.org> <20080414153928.GA22259@elte.hu> X-Message-Flag: Warning: May contain useful information Date: Mon, 14 Apr 2008 08:58:54 -0700 In-Reply-To: <20080414153928.GA22259@elte.hu> (Ingo Molnar's message of "Mon, 14 Apr 2008 17:39:28 +0200") Message-ID: User-Agent: Gnus/5.13 (Gnus v5.13) Emacs/23.0.60 (gnu/linux) MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii X-OriginalArrivalTime: 14 Apr 2008 15:58:55.0358 (UTC) FILETIME=[712BA9E0:01C89E48] Authentication-Results: sj-dkim-1; header.From=rdreier@cisco.com; dkim=pass ( sig from cisco.com/sjdkim1004 verified; ); Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org > which ones exactly are these places that demand the use of a counting > semaphore? I cannot think of a single place where it's the best choice, > let alone one where it's the only choice. Two of the places that use semaphores are drivers/infiniband/hw/mthca and drivers/net/mlx4 -- in both cases, the device firmware allows up to "N" outstanding firmware commands to be in flight, and the driver uses a semaphore to handle issuing firmware commands. That is, down() when we want to issue a command, and up() when the firmware responds that the command is complete. What would you suggest as a better way to code this? This is an honest question -- there probably is a more elegant way to handle this situation and I really would like to learn about it. Also, the argument that removing semaphores makes the kernel as a whole better does make sense to me; I wouldn't be opposed to basically open-coding semaphores in terms of wait_event() in the driver or something like that, but I wouldn't say that such an implementation is locally more readable or maintainable if we look only at the driver code. - R.