From owner-freebsd-arch@FreeBSD.ORG Wed May 22 16:32:48 2013 Return-Path: Delivered-To: freebsd-arch@freebsd.org Received: from mx1.freebsd.org (mx1.FreeBSD.org [8.8.178.115]) by hub.freebsd.org (Postfix) with ESMTP id 6C1EE570; Wed, 22 May 2013 16:32:48 +0000 (UTC) (envelope-from bright@mu.org) Received: from elvis.mu.org (elvis.mu.org [192.203.228.196]) by mx1.freebsd.org (Postfix) with ESMTP id 5CD8C292; Wed, 22 May 2013 16:32:48 +0000 (UTC) Received: from Alfreds-MacBook-Pro-9.local (OTWAON23-1176242366.sdsl.bell.ca [70.28.8.190]) by elvis.mu.org (Postfix) with ESMTPSA id 6C9CC1A3C1B; Wed, 22 May 2013 09:32:47 -0700 (PDT) Message-ID: <519CF32D.2040609@mu.org> Date: Wed, 22 May 2013 12:32:45 -0400 From: Alfred Perlstein User-Agent: Mozilla/5.0 (Macintosh; Intel Mac OS X 10.7; rv:17.0) Gecko/20130509 Thunderbird/17.0.6 MIME-Version: 1.0 To: John Baldwin Subject: Re: FreeBSD spinlock - compatibility layer References: <981733489AB3BD4DB24B48340F53E0A55B0CFD79@MTLDAG01.mtl.com> <201305220905.57939.jhb@freebsd.org> <519CC7B4.2030208@mu.org> <201305221115.19093.jhb@freebsd.org> In-Reply-To: <201305221115.19093.jhb@freebsd.org> Content-Type: text/plain; charset=ISO-8859-1; format=flowed Content-Transfer-Encoding: 7bit Cc: Orit Moskovich , freebsd-arch@freebsd.org X-BeenThere: freebsd-arch@freebsd.org X-Mailman-Version: 2.1.14 Precedence: list List-Id: Discussion related to FreeBSD architecture List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , X-List-Received-Date: Wed, 22 May 2013 16:32:48 -0000 On 5/22/13 11:15 AM, John Baldwin wrote: > On Wednesday, May 22, 2013 9:27:16 am Alfred Perlstein wrote: >> On 5/22/13 9:05 AM, John Baldwin wrote: >>> Probably not. For example, on FreeBSD you want your driver lock to be >>> preempted by an interrupt to avoid higher interrupt latency for filter >>> handlers. Most drivers should not need temporary pinning. If they want to >>> pin work to threads they should bind threads or IRQs to specific CPUs, not >>> rely on temporary pinning. >>> >> I know how it works in FreeBSD. >> >> I think that a compatibility layer should first and foremost aim for >> compatibility, not speed at expense of expected semantics. > The problem with this is that whatever code runs under this layer also has to > cooperate with the rest of the system. Blindly using spin locks does not do > that. Also, I think my entire point is about "expected semantics". People > should think about the actual semantics they need in a driver, not just assume > that whatever side effects they get from the primitives and APIs provided on > one platform defines the semantics they need. I still assert that in terms of > what a device driver actually expects, a regular mutex will provide the correct > semantics. > I agree with your assertion that what we have MTX_DEF should work for drivers for the cases we have. I do believe though that any kernel dev outside FreeBSD will expect certain semantics from a spin mutex though. It's an interesting problem. -Alfred