Date: Wed, 2 Mar 2005 10:16:36 -0500 From: John Baldwin <jhb@FreeBSD.org> To: Bosko Milekic <bmilekic@technokratis.com> Cc: bmilekic@FreeBSD.org Subject: Re: Race condition in mb_free_ext()? Message-ID: <200503021016.36953.jhb@FreeBSD.org> In-Reply-To: <20050301234937.GA19502@technokratis.com> References: <20050301000436.GA33346@xor.obsecurity.org> <20050301231438.GA25573@xor.obsecurity.org> <20050301234937.GA19502@technokratis.com>
next in thread | previous in thread | raw e-mail | index | archive | help
On Tuesday 01 March 2005 06:49 pm, Bosko Milekic wrote: > You know, the more and more we run into these problems specific to how > reference counting is performed, I keep wondering whether some cleverly > defined macros for dealing with common reference counting operations > would be worthwhile. I'm not saying "introduce a refcount_t type and a > full-blown abstraction," but some template-like stuff might be useful. > It took a while just to get the refcount decrement on free path free of > races on SMP, and that's only in the mbuf code. Yeah, I have those simple refcount_foo() macros that operate on ints that would work for here and things like ucreds. Being macros, each arch can override if they have a better native instruction (xadd on x86, fetchadd on ia64, etc.) > -Bosko > > On Tue, Mar 01, 2005 at 03:14:38PM -0800, Kris Kennaway wrote: > > On Tue, Mar 01, 2005 at 06:04:27PM -0500, Bosko Milekic wrote: > > > This does not appear to explain the livelock. > > > > alc and dwhite tracked it down to a missing volatile causing gcc to > > mis-optimize the loop: > > > > - cnt = *(m->m_ext.ref_cnt); > > + cnt = *(volatile u_int *)(m->m_ext.ref_cnt); > > > > I'm currently testing that. > > > > Kris -- John Baldwin <jhb@FreeBSD.org> <>< http://www.FreeBSD.org/~jhb/ "Power Users Use the Power to Serve" = http://www.FreeBSD.org
Want to link to this message? Use this URL: <https://mail-archive.FreeBSD.org/cgi/mid.cgi?200503021016.36953.jhb>