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

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Message-Id: <20250220114036.e22e388402a00f7809a81dee@kernel.org>
Date: Thu, 20 Feb 2025 11:40:36 +0900
From: Masami Hiramatsu (Google) <mhiramat@...nel.org>
To: Steven Rostedt <rostedt@...dmis.org>
Cc: Waiman Long <llong@...hat.com>, "Masami Hiramatsu (Google)"
 <mhiramat@...nel.org>, Peter Zijlstra <peterz@...radead.org>, 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>, Tomasz Figa <tfiga@...omium.org>, Sergey
 Senozhatsky <senozhatsky@...omium.org>, linux-kernel@...r.kernel.org, Linux
 Memory Management List <linux-mm@...ck.org>
Subject: Re: [PATCH 1/2] hung_task: Show the blocker task if the task is
 hung on mutex

On Wed, 19 Feb 2025 20:41:53 -0500
Steven Rostedt <rostedt@...dmis.org> wrote:

> On Wed, 19 Feb 2025 20:36:13 -0500
> Waiman Long <llong@...hat.com> wrote:
> 
>  
> > >>>> this field, we don't need to take lock, though taking the wait_lock may
> > >>>> still be needed to examine other information inside the mutex.  
> > > Do we need to take it just for accessing owner, which is in an atomic?  
> > 
> > Right. I forgot it is an atomic_long_t. In that case, no lock should be 
> > needed.
> 
> Now if we have a two fields to read:
> 
> 	block_flags (for the type of lock) and blocked_on (for the lock)
> 
> We need a way to synchronize the two. What happens if we read the type, and
> the task wakes up and and then blocks on a different type of lock?

Hmm, right.
Since the blocked_on must be NULL before setting flag, if we can ensure
the writing order so that blocked_flags is always updated before
blocked_on, may it be safe?

Or, (this may introduce more memory overhead) don't use union but
use different blocked_on_mutex, blocked_on_rwsem, etc. 

Another idea is to make the owner offset same, like introducing

struct common_lock {
	atomic_long_t owner;
};

But the problem is that rt_mutex does not use atomic for storing
the owner. (we can make it atomic using wrapper)

Thank you,

> 
> Then the lock read from blocked_on could be a different type of lock than
> what is expected.
> 
> -- Steve


-- 
Masami Hiramatsu (Google) <mhiramat@...nel.org>