Date: Mon, 23 Sep 2019 15:48:10 +0300 From: Konstantin Belousov <kostikbel@gmail.com> To: Andriy Gapon <avg@FreeBSD.org> Cc: freebsd-threads@FreeBSD.org Subject: Re: assertion when destroying a process shared mutex Message-ID: <20190923124810.GP2559@kib.kiev.ua> In-Reply-To: <a117f59a-6b36-bda7-b400-fae4280fce98@FreeBSD.org> References: <6f6a16a3-8eca-ceb0-4ca3-aadf2d926f81@FreeBSD.org> <20190920173854.GJ2559@kib.kiev.ua> <a117f59a-6b36-bda7-b400-fae4280fce98@FreeBSD.org>
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On Mon, Sep 23, 2019 at 03:09:16PM +0300, Andriy Gapon wrote: > On 20/09/2019 20:38, Konstantin Belousov wrote: > > On Fri, Sep 20, 2019 at 07:52:20PM +0300, Andriy Gapon wrote: > >> > >> Fatal error 'mutex 0x800661000 own 0x80000010 is on list 0x8006591a0 0x0' at > >> line 153 in file /usr/src/lib/libthr/thread/thr_mutex.c (errno = 0) > >> > >> This happens with a mutex initialized with PTHREAD_PROCESS_SHARED, > >> PTHREAD_MUTEX_ROBUST and PTHREAD_MUTEX_ERRORCHECK. > >> The situation that leads to the abort seems to be this: > >> - one process takes the lock and then crashes without releasing the lock > >> - some time later another process does a cleanup and attempts to destroy the mutex > >> That's where the assertion happens. > >> > >> Specifically, it seems that the assert is tripped if there are no other > >> operations on the lock between the crash of one process and the destroy in the > >> the other process. > >> > >> I wrote a small test program to demo the issue: > >> https://people.freebsd.org/~avg/shared_mtx.c > >> > >> The state of the mutex in a crash dump is this: > >> (gdb) p/x *(struct pthread_mutex *)0x800661000 > >> $6 = {m_lock = {m_owner = 0x80000010, m_flags = 0x11, m_ceilings = {0x0, 0x0}, > >> m_rb_lnk = 0x0, m_spare = {0x0, 0x0}}, m_flags = 0x1, m_count = 0x0, m_spinloops > >> = 0x0, m_yieldloops = 0x0, m_ps = 0x2, m_qe = {tqe_next = 0x0, > >> tqe_prev = 0x8006591a0}, m_pqe = {tqe_next = 0x0, tqe_prev = 0x0}, m_rb_prev > >> = 0x0} > >> > >> So, it's m_qe.tqe_prev != NULL that leads to the assert. > > > > This is only relevant for robust mutexes, otherwise the behavior is > > undefined if the owner terminates without unlocking it. I believe that > > in case of the kernel-assisted UMUTEX_RB_OWNERDEAD state, we should skip > > mutex_assert_not_owned(), same as in enqueue_mutex(). > > Thank you very much! > The patch does help. > I think that there's probably no good way to clean up m_qe. The state of robust mutexes is mostly recovered by kernel, but kernel only knows about the umutex part of the struct pthread_mutex. In fact, other parts of libthr do the same as the patch: they ignore mutex linkage if it is robust mutex recovered after the owner death.
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