From mboxrd@z Thu Jan 1 00:00:00 1970 X-Msuck: nntp://news.gmane.org/gmane.linux.lib.musl.general/9115 Path: news.gmane.org!not-for-mail From: Rich Felker Newsgroups: gmane.linux.lib.musl.general Subject: Re: dlopen deadlock Date: Thu, 14 Jan 2016 17:41:15 -0500 Message-ID: <20160114224115.GW238@brightrain.aerifal.cx> References: <20160113110937.GE13558@port70.net> Reply-To: musl@lists.openwall.com NNTP-Posting-Host: plane.gmane.org Mime-Version: 1.0 Content-Type: text/plain; charset=us-ascii X-Trace: ger.gmane.org 1452811298 29576 80.91.229.3 (14 Jan 2016 22:41:38 GMT) X-Complaints-To: usenet@ger.gmane.org NNTP-Posting-Date: Thu, 14 Jan 2016 22:41:38 +0000 (UTC) To: musl@lists.openwall.com Original-X-From: musl-return-9128-gllmg-musl=m.gmane.org@lists.openwall.com Thu Jan 14 23:41:32 2016 Return-path: Envelope-to: gllmg-musl@m.gmane.org Original-Received: from mother.openwall.net ([195.42.179.200]) by plane.gmane.org with smtp (Exim 4.69) (envelope-from ) id 1aJqa9-0003gO-T8 for gllmg-musl@m.gmane.org; Thu, 14 Jan 2016 23:41:29 +0100 Original-Received: (qmail 26318 invoked by uid 550); 14 Jan 2016 22:41:27 -0000 Mailing-List: contact musl-help@lists.openwall.com; run by ezmlm Precedence: bulk List-Post: List-Help: List-Unsubscribe: List-Subscribe: List-ID: Original-Received: (qmail 26296 invoked from network); 14 Jan 2016 22:41:27 -0000 Content-Disposition: inline In-Reply-To: <20160113110937.GE13558@port70.net> User-Agent: Mutt/1.5.21 (2010-09-15) Original-Sender: Rich Felker Xref: news.gmane.org gmane.linux.lib.musl.general:9115 Archived-At: On Wed, Jan 13, 2016 at 12:09:37PM +0100, Szabolcs Nagy wrote: > This bug i reported against glibc also affects musl: > https://sourceware.org/bugzilla/show_bug.cgi?id=19448 > > in case of musl it's not the global load lock, but the > init_fini_lock that causes the problem. The deadlock happens when a ctor makes a thread that calls dlopen and does not return until the new thread's dlopen returns, right? > the multi-threadedness detection is also problematic in > do_init_fini: > > need_locking = has_threads > if (need_locking) > lock(init_fini_lock) > for all deps > run_ctors(dep) > if (!need_locking && has_threads) > need_locking = 1 > lock(init_fini_lock) > if (need_locking) > unlock(init_fini_lock) > > checking for threads after ctors are run is too late if > the ctors may start new threads that can dlopen libs with > common deps with the currently loaded lib. The logic seems unnecessary now that there's no lazy/optional thread pointer initialization (originally it was a problem because pthread_mutex_lock with a recursive mutex needed to access TLS for the owner tid, but TLS might not have been initialized when the ctors ran) but I don't immediately see how it's harmful. The only state the lock protects is p->constructed and the fini chain (fini_head, p->fini_next) which are all used before the ctors run. The need for locking is re-evaluated after the ctors run. > one solution i can think of is to have an init_fini_lock > for each dso, then the deadlock only happens if a ctor > tries to dlopen its own lib (directly or indirectly) > which is nonsense (the library depends on itself being > loaded) The lock has to protect the fini chain linked list (used to control order of dtors) so I don't think having it be per-dso is a possibility. Rich