Skip to content

xtimer: fix mutex unlocking in _mutex_timeout #6428

New issue

Have a question about this project? Sign up for a free GitHub account to open an issue and contact its maintainers and the community.

Sign up for GitHub

By clicking “Sign up for GitHub”, you agree to our terms of service and privacy statement. We’ll occasionally send you account related emails.

Already on GitHub? Sign in to your account

Merged
OlegHahm merged 1 commit into from
Jan 20, 2017
Merged

xtimer: fix mutex unlocking in _mutex_timeout #6428

Changes from all commits
Commits
Show all changes
1 commit
Select commit
File filter

Filter by extension

Filter by extension
Conversations
Failed to load comments.
Loading
Jump to
Jump to file
Failed to load files.
Loading
Diff view
Diff view
6 changes: 5 additions & 1 deletion sys/xtimer/xtimer.c
View file
Open in desktop
Original file line number Diff line number Diff line change
Expand Up @@ -233,8 +233,12 @@ static void _mutex_timeout(void *arg)
mutex_thread_t *mt = (mutex_thread_t *)arg;

mt->timeout = 1;
list_node_t *node = list_remove(&mt->mutex->queue,
(list_node_t *)&mt->thread->rq_entry);
if ((node != NULL) && (mt->mutex->queue.next == NULL)) {
mt->mutex->queue.next = MUTEX_LOCKED;

@OlegHahm OlegHahm Jan 20, 2017

Copy link
Copy Markdown
Member

Choose a reason for hiding this comment

The reason will be displayed to describe this comment to others. Learn more.

This has to be MUTEX_LOCKED because we assume that someone still owns the mutex, right?

@lebrush lebrush Jan 20, 2017

Copy link
Copy Markdown
Member

Choose a reason for hiding this comment

The reason will be displayed to describe this comment to others. Learn more.

Exactly.

When the mutex is unlocked, queue.next is NULL. mutex -> NULL
When the mutex is locked by 1 thread queue.next is MUTEX_LOCKED. mutex -> LOCKED
When the mutex is locked by 2+ threads queue.next is the mt->thread->rq_entry of a thread and the last .next is NULL. mutex -> thread -> (thread -> ...) NULL

If node != NULL we were removed from the list, hence 2+ threads where in the list. At the same time if queue.next is NULL means that we were the last thread in the list and we need to set queue.next to MUTEX_LOCKED as there's now 1 thread still owning the mutex.

@vincent-d vincent-d Jan 20, 2017

Copy link
Copy Markdown
Member Author

Choose a reason for hiding this comment

The reason will be displayed to describe this comment to others. Learn more.

Yes, otherwise you could have locked it and then the timeout never fire.

}
sched_set_status(mt->thread, STATUS_PENDING);
list_remove(&mt->mutex->queue, (list_node_t *)&mt->thread->rq_entry);
thread_yield_higher();
}

Expand Down