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: <46eca355-930c-4f6a-97cc-6787b6041a0c@redhat.com>
Date: Wed, 19 Feb 2025 22:29:14 -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 9:27 PM, Steven Rostedt wrote:
> On Wed, 19 Feb 2025 21:15:08 -0500
> Waiman Long <llong@...hat.com> wrote:
>
>> 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
> Do you really need the READ/WRITE_ONCE() with the memory barriers? From
> what I understand, the compiler can't even assume what it read is the same
> after passing a memory barrier like that. So there should be no reason it
> can reread the memory location after a barrier.

You may be right. However, without using a READ_ONCE/WRITE_ONLY, a 
compiler can potentially break up the read/write into multiple smaller 
trunks resulting in partial data. So I will use them to be on the safe 
side. In this particular scenario above, we may not need to use them on 
type as we are going to reread it. I will keep them for lock though.

Cheers,
Longman


> -- Steve
>