assertion when destroying a process shared mutex

[Konstantin Belousov]

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;