linux-kernel - Re: [PATCH v2 1/2] hung_task: Show the blocker task if the task is hung on mutex

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Message-ID: <20250221185939.GB7373@noisy.programming.kicks-ass.net>
Date: Fri, 21 Feb 2025 19:59:39 +0100
From: Peter Zijlstra <peterz@...radead.org>
To: "Masami Hiramatsu (Google)" <mhiramat@...nel.org>
Cc: Ingo Molnar <mingo@...hat.com>, Will Deacon <will@...nel.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Boqun Feng <boqun.feng@...il.com>, Waiman Long <longman@...hat.com>,
	Joel Granados <joel.granados@...nel.org>,
	Anna Schumaker <anna.schumaker@...cle.com>,
	Lance Yang <ioworker0@...il.com>,
	Kent Overstreet <kent.overstreet@...ux.dev>,
	Yongliang Gao <leonylgao@...cent.com>,
	Steven Rostedt <rostedt@...dmis.org>,
	Tomasz Figa <tfiga@...omium.org>,
	Sergey Senozhatsky <senozhatsky@...omium.org>,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2 1/2] hung_task: Show the blocker task if the task is
 hung on mutex

On Fri, Feb 21, 2025 at 11:30:01PM +0900, Masami Hiramatsu (Google) wrote:
> +static void debug_show_blocker(struct task_struct *task)
> +{
> +	struct task_struct *g, *t;
> +	unsigned long owner;
> +	struct mutex *lock;
> +
> +	lock = READ_ONCE(task->blocker_mutex);
> +	if (!lock)
> +		return;
> +
> +	owner = mutex_get_owner(lock);
> +	if (likely(owner)) {
> +		/* Ensure the owner information is correct. */
> +		for_each_process_thread(g, t)
> +			if ((unsigned long)t == owner) {
> +				pr_err("INFO: task %s:%d is blocked on a mutex owned by task %s:%d.\n",
> +					task->comm, task->pid, t->comm, t->pid);
> +				sched_show_task(t);
> +				return;
> +			}

 - that for_each_process_thread() scope needs { }

 - that for_each_process_thread() loop needs RCU or tasklist_lock

 - there is no saying that the owner you read with mutex_get_owner() is
   still the owner by the time you do the compare, there can have been
   owner changes.

> +	}
> +	pr_err("INFO: task %s:%d is blocked on a mutex, but the owner is not found.\n",
> +		task->comm, task->pid);
> +}