From owner-freebsd-hackers@FreeBSD.ORG Wed Feb 13 15:16:44 2013 Return-Path: Delivered-To: freebsd-hackers@FreeBSD.org Received: from mx1.freebsd.org (mx1.FreeBSD.org [8.8.178.115]) by hub.freebsd.org (Postfix) with ESMTP id 0AA687F4 for ; Wed, 13 Feb 2013 15:16:44 +0000 (UTC) (envelope-from ian@FreeBSD.org) Received: from duck.symmetricom.us (duck.symmetricom.us [206.168.13.214]) by mx1.freebsd.org (Postfix) with ESMTP id 8B05091F for ; Wed, 13 Feb 2013 15:16:43 +0000 (UTC) Received: from damnhippie.dyndns.org (daffy.symmetricom.us [206.168.13.218]) by duck.symmetricom.us (8.14.6/8.14.6) with ESMTP id r1DFGZBd000509 for ; Wed, 13 Feb 2013 08:16:35 -0700 (MST) (envelope-from ian@FreeBSD.org) Received: from [172.22.42.240] (revolution.hippie.lan [172.22.42.240]) by damnhippie.dyndns.org (8.14.3/8.14.3) with ESMTP id r1DFGW2Y042368; Wed, 13 Feb 2013 08:16:32 -0700 (MST) (envelope-from ian@FreeBSD.org) Subject: Re: Request for review, time_pps_fetch() enhancement From: Ian Lepore To: Konstantin Belousov In-Reply-To: <20130212203408.GM2522@kib.kiev.ua> References: <1360125698.93359.566.camel@revolution.hippie.lan> <20130206155830.GX2522@kib.kiev.ua> <20130209134706.GB19909@stack.nl> <20130210103745.GI2522@kib.kiev.ua> <1360685019.4545.170.camel@revolution.hippie.lan> <20130212203408.GM2522@kib.kiev.ua> Content-Type: text/plain; charset="us-ascii" Date: Wed, 13 Feb 2013 08:16:32 -0700 Message-ID: <1360768592.4545.209.camel@revolution.hippie.lan> Mime-Version: 1.0 X-Mailer: Evolution 2.32.1 FreeBSD GNOME Team Port Content-Transfer-Encoding: 7bit Cc: "freebsd-hackers@freebsd.org" , Jilles Tjoelker X-BeenThere: freebsd-hackers@freebsd.org X-Mailman-Version: 2.1.14 Precedence: list List-Id: Technical Discussions relating to FreeBSD List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , X-List-Received-Date: Wed, 13 Feb 2013 15:16:44 -0000 On Tue, 2013-02-12 at 22:34 +0200, Konstantin Belousov wrote: > On Tue, Feb 12, 2013 at 09:03:39AM -0700, Ian Lepore wrote: > > On Sun, 2013-02-10 at 12:37 +0200, Konstantin Belousov wrote: > > > On Sat, Feb 09, 2013 at 02:47:06PM +0100, Jilles Tjoelker wrote: > > > > On Wed, Feb 06, 2013 at 05:58:30PM +0200, Konstantin Belousov wrote: > > > > > On Tue, Feb 05, 2013 at 09:41:38PM -0700, Ian Lepore wrote: > > > > > > I'd like feedback on the attached patch, which adds support to our > > > > > > time_pps_fetch() implementation for the blocking behaviors described in > > > > > > section 3.4.3 of RFC 2783. The existing implementation can only return > > > > > > the most recently captured data without blocking. These changes add the > > > > > > ability to block (forever or with timeout) until a new event occurs. > > > > > > > > > > Index: sys/kern/kern_tc.c > > > > > > =================================================================== > > > > > > --- sys/kern/kern_tc.c (revision 246337) > > > > > > +++ sys/kern/kern_tc.c (working copy) > > > > > > @@ -1446,6 +1446,50 @@ > > > > > > * RFC 2783 PPS-API implementation. > > > > > > */ > > > > > > > > > > > > +static int > > > > > > +pps_fetch(struct pps_fetch_args *fapi, struct pps_state *pps) > > > > > > +{ > > > > > > [snip] > > > > > > + aseq = pps->ppsinfo.assert_sequence; > > > > > > + cseq = pps->ppsinfo.clear_sequence; > > > > > > + while (aseq == pps->ppsinfo.assert_sequence && > > > > > > + cseq == pps->ppsinfo.clear_sequence) { > > > > > Note that compilers are allowed to optimize these accesses even over > > > > > the sequential point, which is the tsleep() call. Only accesses to > > > > > volatile objects are forbidden to be rearranged. > > > > > > > > > I suggest to add volatile casts to pps in the loop condition. > > > > > > > > The memory pointed to by pps is global (other code may have a pointer to > > > > it); therefore, the compiler must assume that the tsleep() call (which > > > > invokes code in a different compilation unit) may modify it. > > > > > > > > Because volatile does not make concurrent access by multiple threads > > > > defined either, adding it here only seems to slow down the code > > > > (potentially). > > > The volatile guarantees that the compiler indeed reloads the value on > > > read access. Conceptually, the tsleep() does not modify or even access > > > the checked fields, and compiler is allowed to note this by whatever > > > methods (LTO ?). > > > > > > More, the standard says that an implementation is allowed to not evaluate > > > part of the expression if no side effects are produced, even by calling > > > a function. > > > > > > I agree that for practical means, the _currently_ used compilers should > > > consider the tsleep() call as the sequential point. But then the volatile > > > qualifier cast applied for the given access would not change the code as > > > well. > > > > > > > Doesn't this then imply that essentially every driver has this problem, > > and for that matter, every sequence of code anywhere in the base > > involving "loop while repeatedly sleeping, then waking and checking the > > state of some data for changes"? I sure haven't seen that many volatile > > qualifiers scattered around the code. > > No, it does not imply that every driver has this problem. > A typical driver provides the mutual exclusion for access of > the shared data, which means using locks. Locks include neccessary > barries to ensure the visibility of the changes, in particular the > compiler barriers. Ohhhh. I had never considered that using mutexes had other side effects. So is there a correct MI way to invoke the right barrier magic in a situation like this? -- Ian