lwIP mbox synchronization issue

[pillip8282]

Get a wrong event caused by sharing common variable.

Description

To signal both empty and full event of mbox, it uses same variable mail. However following specific case, it can send wrong event to the Task.

Issue procedure

1. Task A called mbox_fetch. But there is no data in mbox, task A waits an event with mbox->mail
2. Task B called mbox post. And call sys_arch_sem_signal(mbox->mail) to signal Task A
3. If context switching didn't happen then Task B is doing mbox post repeatedly.
4. mbox can be fulled because Task B posts data again and again. Then it calls sys_arch_sem_wait() to get a signal that mbox is not full
5. sys_arch_sem_wait() from Task B return immediately because task A didn't receive a signal.
6. Unexpected behavior will happen.

Solution

• Do not share the event variable between full and empty event.
• If state of mbox which is in full is changed then deliver not_full signal
• If state of mbox which is in empty is changed then deliver not_empty signal.