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: <9f9150b4-1cf5-4380-b431-419f70775a7d@redhat.com>
Date: Wed, 19 Feb 2025 21:15:08 -0500
From: Waiman Long <llong@...hat.com>
To: Steven Rostedt <rostedt@...dmis.org>, 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 2/19/25 8:41 PM, Steven Rostedt 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?
>
> Then the lock read from blocked_on could be a different type of lock than
> what is expected.

That is different from reading the owner. In this case, we need to use 
smp_rmb()/wmb() to sequence the read and write operations unless it is 
guaranteed that they are in the same cacheline. One possible way is as 
follows:

Writer - setting them:

     WRITE_ONCE(lock)
     smp_wmb()
     WRITE_ONCE(type)

Clearing them:

     WRITE_ONCE(type, 0)
     smp_wmb()
     WRITE_ONCE(lock, NULL)

Reader:

     READ_ONCE(type)
again:
     smp_rmb()
     READ_ONCE(lock)
     smp_rmb()
     if (READ_ONCE(type) != type)
         goto again

Cheers,
Longman