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: <20250219204153.65ed1f5e@gandalf.local.home>
Date: Wed, 19 Feb 2025 20:41:53 -0500
From: Steven Rostedt <rostedt@...dmis.org>
To: Waiman Long <llong@...hat.com>
Cc: "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 20:36:13 -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?

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

-- Steve