Re: assertion when destroying a process shared mutex

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.

> 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;
> 


-- 
Andriy Gapon
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