linux-kernel - Re: [PATCH 6/6] sched: Change task

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Date:   Mon, 7 Jun 2021 11:45:00 +0100
From:   Daniel Thompson <daniel.thompson@...aro.org>
To:     Peter Zijlstra <peterz@...radead.org>
Cc:     Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>,
        Juri Lelli <juri.lelli@...hat.com>,
        Vincent Guittot <vincent.guittot@...aro.org>,
        Dietmar Eggemann <dietmar.eggemann@....com>,
        Steven Rostedt <rostedt@...dmis.org>,
        Ben Segall <bsegall@...gle.com>, Mel Gorman <mgorman@...e.de>,
        Daniel Bristot de Oliveira <bristot@...hat.com>,
        Borislav Petkov <bp@...en8.de>, x86@...nel.org,
        "H. Peter Anvin" <hpa@...or.com>, Jens Axboe <axboe@...nel.dk>,
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        "David S. Miller" <davem@...emloft.net>,
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Subject: Re: [PATCH 6/6] sched: Change task_struct::state

On Wed, Jun 02, 2021 at 03:12:31PM +0200, Peter Zijlstra wrote:
> Change the type and name of task_struct::state. Drop the volatile and
> shrink it to an 'unsigned int'. Rename it in order to find all uses
> such that we can use READ_ONCE/WRITE_ONCE as appropriate.
> 
> Signed-off-by: Peter Zijlstra (Intel) <peterz@...radead.org>
> ---
>  ...
>  kernel/debug/kdb/kdb_support.c |   18 +++++++------
>  ...
> --- a/kernel/debug/kdb/kdb_support.c
> +++ b/kernel/debug/kdb/kdb_support.c
> @@ -609,23 +609,25 @@ unsigned long kdb_task_state_string(cons
>   */
>  char kdb_task_state_char (const struct task_struct *p)
>  {
> -	int cpu;
> -	char state;
> +	unsigned int p_state;
>  	unsigned long tmp;
> +	char state;
> +	int cpu;
>  
>  	if (!p ||
>  	    copy_from_kernel_nofault(&tmp, (char *)p, sizeof(unsigned long)))
>  		return 'E';
>  
>  	cpu = kdb_process_cpu(p);
> -	state = (p->state == 0) ? 'R' :
> -		(p->state < 0) ? 'U' :
> -		(p->state & TASK_UNINTERRUPTIBLE) ? 'D' :
> -		(p->state & TASK_STOPPED) ? 'T' :
> -		(p->state & TASK_TRACED) ? 'C' :
> +	p_state = READ_ONCE(p->__state);
> +	state = (p_state == 0) ? 'R' :
> +		(p_state < 0) ? 'U' :

Looks like the U here stands for Unreachable since this patch makes it
more obvious that this clause is (and previously was) exactly that!

Dropping the U state would be good since I guess this will show up as a
"new" warning in some tools. However it was a preexisting problem so with
or without this cleaned up:
Acked-by: Daniel Thompson <daniel.thompson@...aro.org>


Daniel.

> +		(p_state & TASK_UNINTERRUPTIBLE) ? 'D' :
> +		(p_state & TASK_STOPPED) ? 'T' :
> +		(p_state & TASK_TRACED) ? 'C' :
>  		(p->exit_state & EXIT_ZOMBIE) ? 'Z' :
>  		(p->exit_state & EXIT_DEAD) ? 'E' :
> -		(p->state & TASK_INTERRUPTIBLE) ? 'S' : '?';
> +		(p_state & TASK_INTERRUPTIBLE) ? 'S' : '?';
>  	if (is_idle_task(p)) {
>  		/* Idle task.  Is it really idle, apart from the kdb
>  		 * interrupt? */