Memory alignment issue. mutex has to ablligned?

[shlomots]

I have a class that wraps the Zephyr implementation of a mutex. For some reason, when the code runs, I get this error:

ASSERTION FAIL [mutex->lock_count > 0U] @ WEST_TOPDIR/zephyr/kernel/mutex.c:239

If I add __align(8) to the wrapper class, the problem is fixed and the code runs perfectly.
Does Zephyr's mutex implementation have to be aligned?

Thanks in advance, 
Shlomo

[Einar Thorsrud]

Hi Shlomo,

I do not see an immediate reason why you would ge tth eassert on line 249 in mutex.c due to alignment issues. I suspect that the root cause is something else, perhaps a memory corruption issue of some sort caused the mutex to be overwritten so that the lock count was set to 0 when it should not? Do you see more in the log? And can  you share more details about your project where you reproduce this?

Br,

Einar

[shlomots]

yep, I have figured it out, the mutex was inlocking a mutex in a class which was deleted by another thread.
Still, all of this saga led me to realize that there are two ways to create a mutex in zephyr, with:

K_MUTEX_DEFINE(my_mutex); 

or with 

struct k_mutex my_mutex;

k_mutex_init(&my_mutex);

but the macro opens up to:

__aligned(__alignof(struct k_mutex)) struct k_mutex my_mutex
__in_section(_k_mutex, static, _CONCAT(my_mutex,_))
__used __noasan
= {
.wait_q = Z_WAIT_Q_INIT(&my_mutex.wait_q),
.owner = NULL,
.lock_count = 0,
.owner_orig_prio= K_LOWEST_APPLICATION_THREAD_PRIO,
};

as you can see in this case the mutex is aligned.
why in one case it has to be aligned and in the other it doesn't?

 

[Einar Thorsrud]

Hi Shlomo,

Good to see you found the root caouse of the issue.

Regardign K_MUTEX_DEFINE I see that it ensures that the mutex is aligned. I am not able to see that there is a reason for this though, other than that K_MUTEX_DEFINE in turn use a sequence of macros that are more general, and that happen to use TYPE_SECTION_ITERABLE which ensures alignment. But it should not be needed.