From owner-svn-src-head@FreeBSD.ORG Fri Jul 27 12:45:37 2012 Return-Path: Delivered-To: svn-src-head@FreeBSD.org Received: from mx1.freebsd.org (mx1.freebsd.org [IPv6:2001:4f8:fff6::34]) by hub.freebsd.org (Postfix) with ESMTP id 06133106566C; Fri, 27 Jul 2012 12:45:37 +0000 (UTC) (envelope-from glebius@FreeBSD.org) Received: from cell.glebius.int.ru (glebius.int.ru [81.19.64.117]) by mx1.freebsd.org (Postfix) with ESMTP id 79DE28FC12; Fri, 27 Jul 2012 12:45:36 +0000 (UTC) Received: from cell.glebius.int.ru (localhost [127.0.0.1]) by cell.glebius.int.ru (8.14.5/8.14.5) with ESMTP id q6RCjYOs035964; Fri, 27 Jul 2012 16:45:34 +0400 (MSK) (envelope-from glebius@FreeBSD.org) Received: (from glebius@localhost) by cell.glebius.int.ru (8.14.5/8.14.5/Submit) id q6RCjYeH035963; Fri, 27 Jul 2012 16:45:34 +0400 (MSK) (envelope-from glebius@FreeBSD.org) X-Authentication-Warning: cell.glebius.int.ru: glebius set sender to glebius@FreeBSD.org using -f Date: Fri, 27 Jul 2012 16:45:34 +0400 From: Gleb Smirnoff To: Bruce Evans Message-ID: <20120727124534.GT14135@FreeBSD.org> References: <201207270916.q6R9Gm23086648@svn.freebsd.org> <20120727111237.GC2676@deviant.kiev.zoral.com.ua> <20120727111904.GQ14135@FreeBSD.org> <20120727221529.K7360@besplex.bde.org> MIME-Version: 1.0 Content-Type: text/plain; charset=koi8-r Content-Disposition: inline In-Reply-To: <20120727221529.K7360@besplex.bde.org> User-Agent: Mutt/1.5.21 (2010-09-15) Cc: Konstantin Belousov , svn-src-head@FreeBSD.org, svn-src-all@FreeBSD.org, src-committers@FreeBSD.org Subject: Re: svn commit: r238828 - head/sys/sys X-BeenThere: svn-src-head@freebsd.org X-Mailman-Version: 2.1.5 Precedence: list List-Id: SVN commit messages for the src tree for head/-current List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , X-List-Received-Date: Fri, 27 Jul 2012 12:45:37 -0000 On Fri, Jul 27, 2012 at 10:32:55PM +1000, Bruce Evans wrote: B> > On Fri, Jul 27, 2012 at 02:12:37PM +0300, Konstantin Belousov wrote: B> > K> On Fri, Jul 27, 2012 at 09:16:48AM +0000, Gleb Smirnoff wrote: B> > K> > ... B> > K> > Log: B> > K> > Add assertion for refcount overflow. B> > K> > B> > K> > Submitted by: Andrey Zonov B> > K> > Reviewed by: kib B> > K> It was discussed rather then reviewed. B> > K> B> > K> I suggest that the assert may be expressed as a check after the increment, B> > K> which verifies that counter is != 0. This allows to avoid namespace B> > K> pollution due to limits.h. B> > B> > Hmm, overflowing unsigned is a defined behavior in C. If Bruce agrees, B> > then I'm happy with KASSERT after increment. B> B> Comparing with (uint_t)-1 before is equivalent. You can even omit the B> cast (depend on the default promotion). B> B> I just noticed that there is a technical problem -- the count is read B> unlocked in the KASSERT. And since the comparision is for equality, B> if you lose the race reading the count when it reaches the overflow B> threshold, then you won't see it overflow unless it wraps again and B> you win the race next time (or later). atomic_cmpset could be used B> to clamp the value at the max, but that is too much for an assertion. We have discussed that. As alternative I proposed: @@ -50,8 +51,14 @@ static __inline void refcount_acquire(volatile u_int *count) { +#ifdef INVARIANTS + u_int old; + old = atomic_fetchadd_int(count, 1); + KASSERT(old < UINT_MAX, ("refcount %p overflowed", count)); +#else atomic_add_acq_int(count, 1); +#endif } Konstantin didn't like that production code differs from INVARIANTS. So we ended with what I committed, advocating to the fact that although assertion is racy and bad panics still can occur, the "good panics" would occur much more often, and a single "good panic" is enough to show what's going on. -- Totus tuus, Glebius.