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: <63fc7bff-168e-478b-a3fc-4897f2ee9d69@redhat.com>
Date: Fri, 21 Feb 2025 15:29:57 -0500
From: Waiman Long <llong@...hat.com>
To: Peter Zijlstra <peterz@...radead.org>,
 "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>, 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 2/21/25 1:59 PM, Peter Zijlstra wrote:
> 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

The call chain should be

check_hung_uninterruptible_tasks()
   -> check_hung_task()
     -> debug_show_blocker()

check_hung_uninterruptible_tasks() takes rcu_read_lock() before calling 
check_hung_task(). Perhaps add a statement like

     RCU_LOCKDEP_WARN(!rcu_read_lock_held(), "no rcu lock held");

>
>   - 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.

Maybe change "owned by" to "likely owned by" :-)

Cheers,
Longman