Re: [PATCH v4 0/3] hung_task: extend blocking task stacktrace dump to semaphore

[Andrew Morton]

On Thu, 20 Mar 2025 14:49:20 +0800 Lance Yang <ioworker0@xxxxxxxxx> wrote:

> Inspired by mutex blocker tracking[1], this patch series extend the
> feature to not only dump the blocker task holding a mutex but also to
> support semaphores. Unlike mutexes, semaphores lack explicit ownership
> tracking, making it challenging to identify the root cause of hangs. To
> address this, we introduce a last_holder field to the semaphore structure,
> which is updated when a task successfully calls down() and cleared during
> up().
> 
> The assumption is that if a task is blocked on a semaphore, the holders
> must not have released it. While this does not guarantee that the last
> holder is one of the current blockers, it likely provides a practical hint
> for diagnosing semaphore-related stalls.
> 
> With this change, the hung task detector can now show blocker task's info
> like below:

Seems useful, but the semaphore code isn't really an akpm thing. 
Peter, could you please comment?