Date: Fri, 20 Sep 2019 20:38:54 +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: <20190920173854.GJ2559@kib.kiev.ua> In-Reply-To: <6f6a16a3-8eca-ceb0-4ca3-aadf2d926f81@FreeBSD.org> References: <6f6a16a3-8eca-ceb0-4ca3-aadf2d926f81@FreeBSD.org>
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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(). diff --git a/lib/libthr/thread/thr_mutex.c b/lib/libthr/thread/thr_mutex.c index dc09f539add..57984ef6d0e 100644 --- a/lib/libthr/thread/thr_mutex.c +++ b/lib/libthr/thread/thr_mutex.c @@ -474,7 +474,11 @@ _thr_mutex_destroy(pthread_mutex_t *mutex) if (m == THR_PSHARED_PTR) { m1 = __thr_pshared_offpage(mutex, 0); if (m1 != NULL) { - mutex_assert_not_owned(_get_curthread(), m1); + if ((uint32_t)m1->m_lock.m_owner != + UMUTEX_RB_OWNERDEAD) { + mutex_assert_not_owned( + _get_curthread(), m1); + } __thr_pshared_destroy(mutex); } *mutex = THR_MUTEX_DESTROYED;
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