From owner-freebsd-arch@FreeBSD.ORG Sat Oct 25 19:53:35 2014 Return-Path: Delivered-To: freebsd-arch@freebsd.org Received: from mx1.freebsd.org (mx1.freebsd.org [8.8.178.115]) (using TLSv1.2 with cipher AECDH-AES256-SHA (256/256 bits)) (No client certificate requested) by hub.freebsd.org (Postfix) with ESMTPS id C94D38A4 for ; Sat, 25 Oct 2014 19:53:35 +0000 (UTC) Received: from h2.funkthat.com (gate2.funkthat.com [208.87.223.18]) (using TLSv1 with cipher DHE-RSA-AES256-SHA (256/256 bits)) (Client CN "funkthat.com", Issuer "funkthat.com" (not verified)) by mx1.freebsd.org (Postfix) with ESMTPS id A8B51145 for ; Sat, 25 Oct 2014 19:53:35 +0000 (UTC) Received: from h2.funkthat.com (localhost [127.0.0.1]) by h2.funkthat.com (8.14.3/8.14.3) with ESMTP id s9PJrYL7031480 (version=TLSv1/SSLv3 cipher=DHE-RSA-AES256-SHA bits=256 verify=NO); Sat, 25 Oct 2014 12:53:35 -0700 (PDT) (envelope-from jmg@h2.funkthat.com) Received: (from jmg@localhost) by h2.funkthat.com (8.14.3/8.14.3/Submit) id s9PJrYSI031479; Sat, 25 Oct 2014 12:53:34 -0700 (PDT) (envelope-from jmg) Date: Sat, 25 Oct 2014 12:53:34 -0700 From: John-Mark Gurney To: Mateusz Guzik Subject: Re: refcount_release_take_##lock Message-ID: <20141025195334.GW82214@funkthat.com> Mail-Followup-To: Mateusz Guzik , freebsd-arch@freebsd.org References: <20141025184448.GA19066@dft-labs.eu> <20141025190407.GU82214@funkthat.com> <20141025192632.GB19066@dft-labs.eu> Mime-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20141025192632.GB19066@dft-labs.eu> User-Agent: Mutt/1.4.2.3i X-Operating-System: FreeBSD 7.2-RELEASE i386 X-PGP-Fingerprint: 54BA 873B 6515 3F10 9E88 9322 9CB1 8F74 6D3F A396 X-Files: The truth is out there X-URL: http://resnet.uoregon.edu/~gurney_j/ X-Resume: http://resnet.uoregon.edu/~gurney_j/resume.html X-TipJar: bitcoin:13Qmb6AeTgQecazTWph4XasEsP7nGRbAPE X-to-the-FBI-CIA-and-NSA: HI! HOW YA DOIN? can i haz chizburger? X-Greylist: Sender IP whitelisted, not delayed by milter-greylist-4.2.2 (h2.funkthat.com [127.0.0.1]); Sat, 25 Oct 2014 12:53:35 -0700 (PDT) Cc: freebsd-arch@freebsd.org X-BeenThere: freebsd-arch@freebsd.org X-Mailman-Version: 2.1.18-1 Precedence: list List-Id: Discussion related to FreeBSD architecture List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , X-List-Received-Date: Sat, 25 Oct 2014 19:53:36 -0000 Mateusz Guzik wrote this message on Sat, Oct 25, 2014 at 21:26 +0200: > On Sat, Oct 25, 2014 at 12:04:07PM -0700, John-Mark Gurney wrote: > > Mateusz Guzik wrote this message on Sat, Oct 25, 2014 at 20:44 +0200: > > > The following idiom is used here and there: > > > > > > int old; > > > old = obj->ref; > > > if (old > 1 && atomic_cmpset_int(&obj->ref, old, old -1)) > > > return; > > > lock(&something); > > > if (refcount_release(&obj->ref) == 0) { > > > unlock(&something); > > > return; > > > } > > > free up > > > unlock(&something); > > > > > > ========== > > > > Couldn't this be better written as: > > if (__predict_false(refcount_release(&obj->ref) == 0)) { > > lock(&something); > > if (__predict_true(!obj->ref)) { > > free up > > } > > unlock(&something); > > } > > > > The reason I'm asking is that I changed how IPsec SA ref counting was > > handled, and used something similar... > > > > My code gets rid of a branch, and is better in that it uses refcount > > API properly, instead of using atomic_cmpset_int... > > This is used when given obj is kept on a list and code which traverses > it (locked) expects found objects to be valid to ref. > > If we were to reach count of 0 and then lock, it would be possible that > other thread refed + unrefed the object and is now trying to lock as > well. Per the email I wrote to Ian, this "assumption" needs to be well documented that though the "list" has a reference, and that this reference is not accounted for in the ref count... And I personally think that it's a bug for the list to not hold it's own reference... Yes, then you need to compare for when the ref count hits one, and do the lock/dec/free/unlock, but that keeps the refcount sane... > That could be remedied for type stable object by having a generation > counter, but I doubt it's worth it. Not to mention objects we lock here > are freeable :) That's too heavy weight... > > > I decided to implement it as a common function. > > > > > > We have only refcount.h and I didn't want to bloat all including code > > > with additional definitions and as such I came up with a macro that has > > > to be used in .c file and that will define appropriate inline func. > > > > > > I'm definitely looking for better names for REFCOUNT_RELEASE_TAKE_USE_ > > > macro, assuming it has to stay. > > > > You could shorten it to REFCNT_REL_TAKE_ > > > > All function use full 'refcount_release' and the like, so that would be > inconsistent. > > Losing 'take' may be an option, I don't know. Yeh, the only advantage is that it only appears once per file used, so it's not THAT long... > > > Comments? > > > > > Will you update the refcount(9) man page w/ documentation before > > committing? > > Sure. Thanks. -- John-Mark Gurney Voice: +1 415 225 5579 "All that I will do, has been done, All that I have, has not."