Exercise 2 of Lesson 8 looks have a potential TOCTOU issue

[Rick Cai]

academy.nordicsemi.com/.../

The comparison of "increment_count + decrement_count != COMBINED_TOTAL" happens after test_mutex unlock.
It is possible that between comparison and test_mutex unlock, "increment_count" or "decrement_count" is incremented by another thread.
Then "increment_count" + "decrement_count" != "COMBINED_TOTAL".
When running in qemu, we can see  "increment_count" + "decrement_count" = 41.

It is a kind of TOCTOU.

To solve this problem, we can add a new variable sum = increment_count + decrement_count, and use sum to do comparison.

[AHaug]

Hello,

What you're observing is what is  correct. The purpose of the exercise is to show a TOCTOU i.e a race condition. 

Kind regards,
Andreas

[Rick Cai]

I observed the race condition even with mutex code enabled.

Better to add a notice that "increment_count" + "decrement_count" might not be 40 once mutex is unlocked. 

[AHaug]

Hi again,

I discussed it with the author and it looks to be as you described, yes. Printing the values by fetching values of increment and decrement count outside mutex lock will still have a race condition as you said. Getting those values added into sum inside mutex will avoid that problem, which you also said.

I will discuss it with the author if it should be updated.

Again thank you for reporting it

Kind regards,
Andreas

[Rick Cai]

I am totally fine as this is a demo to show the usage of mutex and race condition. 

Just let learners know how to use these variables correctly.