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: <3c09df17-79b2-40d4-a560-f6b1ddbbb73e@redhat.com>
Date: Thu, 20 Feb 2025 08:13:31 -0500
From: Waiman Long <llong@...hat.com>
To: Steven Rostedt <rostedt@...dmis.org>,
 "Masami Hiramatsu (Google)" <mhiramat@...nel.org>
Cc: Waiman Long <llong@...hat.com>, 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 10:11 PM, Steven Rostedt wrote:
> On Thu, 20 Feb 2025 11:40:36 +0900
> Masami Hiramatsu (Google) <mhiramat@...nel.org> wrote:
>
>> 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)
> Either that, or add to the task_struct:
>
> 	struct mutex	*blocked_on_mutex;
> 	struct rwsem	*blocked_on_rwsem;
> 	struct rtlock	*blocked_on_rtlock;
>
> And just have each type assign to its own type. Then you only need to look
> at each one. But yeah, this adds even more bloat to task_struct.
>
>    :-/

Another alternative is to encode the locking type into the lowest 2 bits 
of the address and combined them into a single atomic_long_t data item. 
Of course, we can only support 4 different types with this scheme.

Cheers,
Longman

>
> -- Steve
>