From owner-freebsd-threads@FreeBSD.ORG Mon Mar 29 13:50:43 2004 Return-Path: Delivered-To: freebsd-threads@freebsd.org Received: from mx1.FreeBSD.org (mx1.freebsd.org [216.136.204.125]) by hub.freebsd.org (Postfix) with ESMTP id E714B16A4D0 for ; Mon, 29 Mar 2004 13:50:43 -0800 (PST) Received: from herring.rabson.org (mailgate.nlsystems.com [80.177.232.242]) by mx1.FreeBSD.org (Postfix) with ESMTP id 7EC3D43D45 for ; Mon, 29 Mar 2004 13:50:42 -0800 (PST) (envelope-from dfr@nlsystems.com) Received: from herring.rabson.org (herring.rabson.org [10.0.0.2]) by herring.rabson.org (8.12.11/8.12.11) with ESMTP id i2TLoVog012192; Mon, 29 Mar 2004 22:50:31 +0100 (BST) (envelope-from dfr@nlsystems.com) From: Doug Rabson To: Daniel Eischen Date: Mon, 29 Mar 2004 22:50:31 +0100 User-Agent: KMail/1.6.1 References: In-Reply-To: MIME-Version: 1.0 Content-Disposition: inline Content-Type: text/plain; charset="iso-8859-1" Content-Transfer-Encoding: 7bit Message-Id: <200403292250.31315.dfr@nlsystems.com> X-Spam-Status: No, hits=0.0 required=5.0 tests=none autolearn=no version=2.63 X-Spam-Checker-Version: SpamAssassin 2.63 (2004-01-11) on herring.rabson.org X-Virus-Scanned: ClamAV version 'clamd / ClamAV version 0.65', clamav-milter version '0.60p' cc: freebsd-threads@freebsd.org Subject: Re: Thread Local Storage X-BeenThere: freebsd-threads@freebsd.org X-Mailman-Version: 2.1.1 Precedence: list List-Id: Threading on FreeBSD List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , X-List-Received-Date: Mon, 29 Mar 2004 21:50:44 -0000 On Monday 29 March 2004 22:26, Daniel Eischen wrote: > On Mon, 29 Mar 2004, Doug Rabson wrote: > > On Monday 29 March 2004 20:36, Daniel Eischen wrote: > > > On Mon, 29 Mar 2004, Doug Rabson wrote: > > > > I've been spending a bit of time recently familiarising myself > > > > with this TLS stuff and trying out a few things. I've been > > > > playing with rtld and I have a prototype patch which implements > > > > enough TLS support to let a non-threaded program which uses > > > > static TLS work. With a tiny bit more work I can have limited > > > > support for dynamic TLS as well (not for dlopen'ed modules yet > > > > though). Is there a p4 tree for this stuff yet? I'd like to > > > > check in what I have sometime. > > > > > > > > I've also been looking at libpthread and I can see some > > > > potential problems with it. Currently libpthread on i386 uses > > > > %gs to point at a struct kcb which seems to be a per-kse > > > > structure. This structure contains a pointer to a per-thread > > > > struct tcb and this pointer is managed by the userland context > > > > switch code. Other arches are similar, e.g. ia64 uses $tp to > > > > point at struct kcb. > > > > > > > > The problem with TLS is that the i386 ABI needs %gs to point at > > > > the TLS > > > > > > There are 2 different methods allowed for the i386 ABI. > > > We want to use the other method in which there is an > > > additional indirection. The current i386 libpthread > > > stuff adheres to this method. > > > > Surely the GNU TLS ABI is preferable? It generates much smaller > > code and needs many fewer relocations. > > No, we don't want an LDT for every thread and don't want > to force a syscall for a thread switch. But the code it generates is at least twice the size for dynamic TLS. It seems that the GNU people have done a better job defining the TLS abi for i386. You don't need a syscall at thread switch if you do something like: _thread_switch(...) { if (tcb doesn't have LDT entry) { if (!free LDT entries) steal LDT entry from non-running thread; allocate LDT entry and point it at TLS goop for tcb. } load_gs(tcb's LDT sel); ... } I just have this feeling that the GNU ABI is going to get far better testing and support in the future since thats what linux uses.