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

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Message-Id: <20250407130807.9d9eae28cced953cdef8b0a1@linux-foundation.org>
Date: Mon, 7 Apr 2025 13:08:07 -0700
From: Andrew Morton <akpm@...ux-foundation.org>
To: Lance Yang <ioworker0@...il.com>
Cc: will@...nel.org, peterz@...radead.org, mingo@...hat.com,
 longman@...hat.com, mhiramat@...nel.org, anna.schumaker@...cle.com,
 boqun.feng@...il.com, joel.granados@...nel.org, kent.overstreet@...ux.dev,
 leonylgao@...cent.com, linux-kernel@...r.kernel.org, rostedt@...dmis.org,
 senozhatsky@...omium.org, tfiga@...omium.org, amaindex@...look.com,
 jstultz@...gle.com
Subject: Re: [PATCH v4 0/3] hung_task: extend blocking task stacktrace dump
 to semaphore

On Thu, 20 Mar 2025 14:49:20 +0800 Lance Yang <ioworker0@...il.com> 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?