From owner-freebsd-threads@FreeBSD.ORG Mon Sep 1 08:08:34 2008 Return-Path: Delivered-To: freebsd-threads@freebsd.org Received: from mx1.freebsd.org (mx1.freebsd.org [IPv6:2001:4f8:fff6::34]) by hub.freebsd.org (Postfix) with ESMTP id DE5621065673; Mon, 1 Sep 2008 08:08:34 +0000 (UTC) (envelope-from davidxu@freebsd.org) Received: from freefall.freebsd.org (freefall.freebsd.org [IPv6:2001:4f8:fff6::28]) by mx1.freebsd.org (Postfix) with ESMTP id C77FD8FC16; Mon, 1 Sep 2008 08:08:34 +0000 (UTC) (envelope-from davidxu@freebsd.org) Received: from apple.my.domain (root@localhost [127.0.0.1]) by freefall.freebsd.org (8.14.2/8.14.2) with ESMTP id m8188WSL049370; Mon, 1 Sep 2008 08:08:33 GMT (envelope-from davidxu@freebsd.org) Message-ID: <48BBA369.9010108@freebsd.org> Date: Mon, 01 Sep 2008 16:10:17 +0800 From: David Xu User-Agent: Thunderbird 2.0.0.9 (X11/20080612) MIME-Version: 1.0 To: Kostik Belousov References: <48B7101E.7060203@icyb.net.ua> <48B71BA6.5040504@icyb.net.ua> <20080829141043.GX2038@deviant.kiev.zoral.com.ua> <48B8052A.6070908@icyb.net.ua> <20080829143645.GY2038@deviant.kiev.zoral.com.ua> <20080829190506.GA2038@deviant.kiev.zoral.com.ua> <20080830155622.GF2038@deviant.kiev.zoral.com.ua> <20080830184512.GH2038@deviant.kiev.zoral.com.ua> In-Reply-To: <20080830184512.GH2038@deviant.kiev.zoral.com.ua> Content-Type: text/plain; charset=UTF-8; format=flowed Content-Transfer-Encoding: 8bit Cc: Daniel Eischen , Andriy Gapon , freebsd-threads@freebsd.org Subject: Re: mysterious hang in pthread_create X-BeenThere: freebsd-threads@freebsd.org X-Mailman-Version: 2.1.5 Precedence: list List-Id: Threading on FreeBSD List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , X-List-Received-Date: Mon, 01 Sep 2008 08:08:35 -0000 Kostik Belousov wrote: > Ok, let me to tell the whole story. I am sure that in fact you know > it better then me. > > Assuming libthr is the only threading library, there are two locking > implementations for the rtld: 'default' and the one supplied by libthr. > On the first call to pthread_create(), libthr calls _rtld_thread_init() > to substitute the default by the implementation from libthr. > > In fact, default implementation is broken from my point of view. For > instance, thread_flag update is not atomic. Moreover, it does not > correctly handles sequential acquision of several locks, due > to thread_flag. > > The dl_iterate_phdr() function, called by gcc exception handling support > code, does exactly this. It acquires rtld_phdr_lock, then rtld_bind_lock. > [I shall admit it does this after my change]. In particular, this would > leave the bit for the bind lock set in the thread_flag. > > Andriy' example throw the exception and calls dl_iterate_phdr() before > first thread is created. On thread creation, _rtld_thread_init() is > called, that tries to move the locks according to thread_flag. This is > the cause for the reported wlock acquisition. > > I do not want to change anything in the default rtld locking. It is > disfunctional from the time libc_r is gone, and I think it would be > better to make it nop. My change makes the image that is linked with > libthr, to consistently use libthr locks. The ancient bug is in rtld, rlock_acquire() and wlock_acquire() test thread_flag as a boolean value, because pt_iterate_phdr() tries to lock two locks at same time, this test will always fail once it acquired first lock. The following silly patch fixes the problem Andriy encountered: Index: rtld_lock.c =================================================================== --- rtld_lock.c （版本 182594） +++ rtld_lock.c （工作副本） @@ -184,7 +184,7 @@ int rlock_acquire(rtld_lock_t lock) { - if (thread_mask_set(lock->mask)) { + if (thread_mask_set(lock->mask) & lock->mask) { dbg("rlock_acquire: recursed"); return (0); } @@ -195,7 +195,7 @@ int wlock_acquire(rtld_lock_t lock) { - if (thread_mask_set(lock->mask)) { + if (thread_mask_set(lock->mask) & lock->mask) { dbg("wlock_acquire: recursed"); return (0); } Regards, David Xu