Date: Fri, 15 Jan 2016 01:31:49 +0100 From: Szabolcs Nagy <nsz@...t70.net> To: musl@...ts.openwall.com Subject: Re: dlopen deadlock * Rich Felker <dalias@...c.org> [2016-01-14 17:41:15 -0500]: > 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? > yes (not a common scenario) > > 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. > hm ok i thought the ctors of the same lib might end up being called twice, concurrently, but i see p->constructed protects against that > > 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. > i guess using lockfree atomics could solve the deadlock then > Rich
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