Date: Thu, 14 Jan 2016 17:41:15 -0500 From: Rich Felker <dalias@...c.org> To: musl@...ts.openwall.com Subject: Re: dlopen deadlock 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
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