From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S937372AbXG0Urm (ORCPT ); Fri, 27 Jul 2007 16:47:42 -0400 Received: (majordomo@vger.kernel.org) by vger.kernel.org id S1763544AbXG0Urc (ORCPT ); Fri, 27 Jul 2007 16:47:32 -0400 Received: from nz-out-0506.google.com ([64.233.162.224]:43979 "EHLO nz-out-0506.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1763516AbXG0Ur1 (ORCPT ); Fri, 27 Jul 2007 16:47:27 -0400 DomainKey-Signature: a=rsa-sha1; c=nofws; d=gmail.com; s=beta; h=received:message-id:date:from:to:subject:cc:in-reply-to:mime-version:content-type:content-transfer-encoding:content-disposition:references; b=ug6JrYtKi9B9PswrhY5zg5I6ONxQ4cIeNYKqkApRoGLlnBVfzR00jFPLh8mIXgQj5VtL15CN5LKyGbQVskBrR/5g8VtZDCcdhpbsE9aawPIBE30q7/HtPabmGYMas0twTrSlnCegKr27fsW8Sv/qW7o9b9j4wYd6wMyTFgKYXhI= Message-ID: Date: Sat, 28 Jul 2007 02:17:24 +0530 From: "Satyam Sharma" To: "Rodolfo Giometti" Subject: Re: LinuxPPS & spinlocks Cc: "Chris Friesen" , "David Woodhouse" , linux-kernel@vger.kernel.org, "Andrew Morton" In-Reply-To: <20070727194516.GX9840@enneenne.com> MIME-Version: 1.0 Content-Type: text/plain; charset=ISO-8859-1 Content-Transfer-Encoding: 7bit Content-Disposition: inline References: <20070717180550.GA24759@enneenne.com> <20070724080013.GA22171@gundam.enneenne.com> <1185284942.14697.319.camel@pmac.infradead.org> <20070724142050.GD4074@enneenne.com> <1185288769.14697.339.camel@pmac.infradead.org> <20070727184418.GV9840@enneenne.com> <46AA42CA.4060004@nortel.com> <20070727192848.GW9840@enneenne.com> <46AA4A1E.8040302@nortel.com> <20070727194516.GX9840@enneenne.com> Sender: linux-kernel-owner@vger.kernel.org X-Mailing-List: linux-kernel@vger.kernel.org Hi Rodolfo, On 7/28/07, Rodolfo Giometti wrote: > On Fri, Jul 27, 2007 at 01:40:14PM -0600, Chris Friesen wrote: > > > > My point is that the lock should be used to protect specific data. Thus, it > > would be more correct to say, "spinlock foo is taken because > > pps_register_source() accesses variable bar". > > > > That way, if someone else wants to access "bar", they know that they need > > to take lock "foo". > > Ah, ok! I see. :) I only glanced through the code, so could be wrong, but I noticed that the only global / shared data you have in there is a global "pps_source" array of pps_s structs. That's accessed / modified from the various syscalls introduced in the API exported to userspace, as well as the register/unregister/pps_event API exported to in-kernel client subsystems, yes? So it looks like you need to introduce proper locking for it, simply type-qualifying it as "volatile" is not enough. However, I think you've introduced two locks for it. The syscalls (that run in process context, obviously) seem to use a pps_mutex and pps_event() seems to be using the pps_lock spinlock (because that gets executed from interrupt context) -- and from the looks of it, the register/unregister functions are using /both/ the mutex and spinlock (!) This isn't quite right, (in fact there's nothing to protect pps_event from racing against a syscall), so you should use *only* the spinlock for synchronization -- the spin_lock_irqsave/restore() variants, in fact. [ Also, have you considered making pps_source a list and not an array? It'll help you lose a whole lot of MAX_SOURCES, pps_is_allocated, etc kind of gymnastics in there, and you _can_ return a pointer to the corresponding pps source struct from the register() function to the in-kernel users, so that way you get to retain the O(1) access to the corresponding source when a client calls into pps_event(), similar to how you're using the array index presently. ] I also noticed code like (from pps_event): + /* Try to grab the lock, if not we prefere loose the event... */ + if (!spin_trylock(&pps_lock)) + return; which looks worrisome and unnecessary. That spinlock looks to be of fine enough granularity to me, do you think there'd be any contention on it? I /think/ you can simply make that a spin_lock(). Overall the code looks simple / straightforward enough to me (except for the parport / uart stuff that I have no clue about), and I'll also read up on the relevant RFC for this and would hopefully try and give you a more meaningful review over the weekend. Thanks, Satyam