linux-kernel - Re: [PATCH printk v2 04/11] printk: nbcon: Provide functions to mark atomic write sections

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Message-ID: <ZQ1ff96wS03ctA-v@alley>
Date:   Fri, 22 Sep 2023 11:33:51 +0200
From:   Petr Mladek <pmladek@...e.com>
To:     John Ogness <john.ogness@...utronix.de>
Cc:     Sergey Senozhatsky <senozhatsky@...omium.org>,
        Steven Rostedt <rostedt@...dmis.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        linux-kernel@...r.kernel.org,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>
Subject: Re: [PATCH printk v2 04/11] printk: nbcon: Provide functions to mark
 atomic write sections

On Wed 2023-09-20 01:14:49, John Ogness wrote:
> From: Thomas Gleixner <tglx@...utronix.de>
> 
> WARN/OOPS/PANIC require printing out immediately since the
> regular printing method (and in the future, the printing
> threads) might not be able to run.
> 
> Add per-CPU state to denote the priority/urgency of the output
> and provide functions to mark the beginning and end of sections
> where the urgent messages are generated.
> 
> Note that when a CPU is in a priority elevated state, flushing
> only occurs when dropping back to a lower priority. This allows
> the full set of printk records (WARN/OOPS/PANIC output) to be
> stored in the ringbuffer before beginning to flush the backlog.

The above paragraph is a bit confusing. The code added by this patch
does not do any flushing. I guess that this last paragraph is supposed
to explain why the "nesting" array is needed. I would write
something like:

"The state also counts nesting of printing contexts per-priority.
It will be later used to prevent flushing in nested contexts."

That said, I am not sure if the special handling of nested contexts
is needed. But let's discuss it in the patch introducing the flush
funtions.

> --- a/kernel/printk/nbcon.c
> +++ b/kernel/printk/nbcon.c
> @@ -961,6 +961,95 @@ static bool nbcon_emit_next_record(struct nbcon_write_context *wctxt)
>  	return nbcon_context_exit_unsafe(ctxt);
>  }
>  
> +/**
> + * struct nbcon_cpu_state - Per CPU printk context state
> + * @prio:	The current context priority level
> + * @nesting:	Per priority nest counter
> + */
> +struct nbcon_cpu_state {
> +	enum nbcon_prio		prio;
> +	int			nesting[NBCON_PRIO_MAX];
> +};
> +
> +static DEFINE_PER_CPU(struct nbcon_cpu_state, nbcon_pcpu_state);
> +static struct nbcon_cpu_state early_nbcon_pcpu_state __initdata;
> +
> +/**
> + * nbcon_get_cpu_state - Get the per CPU console state pointer
> + *
> + * Returns either a pointer to the per CPU state of the current CPU or to
> + * the init data state during early boot.
> + */
> +static __ref struct nbcon_cpu_state *nbcon_get_cpu_state(void)
> +{
> +	if (!printk_percpu_data_ready())
> +		return &early_nbcon_pcpu_state;

My first thought, was that this was racy. I was afraid that
printk_percpu_data_ready() could change value inside
atomit_enter()/exit() area. But it actually could not happen.
Anyway, it might worth a comment. Something like:

	/*
	 * The value of __printk_percpu_data_ready is modified in normal
	 * context. As a result it could never change inside a nbcon
	 * atomic context.
	 */
	if (!printk_percpu_data_ready())
		return &early_nbcon_pcpu_state;

> +
> +	return this_cpu_ptr(&nbcon_pcpu_state);
> +}
> +
> +/**
> + * nbcon_atomic_exit - Exit a context that enforces atomic printing
> + * @prio:	Priority of the context to leave
> + * @prev_prio:	Priority of the previous context for restore
> + *
> + * Context:	Any context. Enables migration.
> + *
> + * @prev_prio is the priority returned by the corresponding
> + * nbcon_atomic_enter().
> + */
> +void nbcon_atomic_exit(enum nbcon_prio prio, enum nbcon_prio prev_prio)
> +{
> +	struct nbcon_cpu_state *cpu_state;
> +
> +	cpu_state = nbcon_get_cpu_state();

I would add a consistency check:

	WARN_ON_ONCE(cpu_state->nesting[cpu_state->prio] <= 0)

> +	/*
> +	 * Undo the nesting of nbcon_atomic_enter() at the CPU state
> +	 * priority.
> +	 */
> +	cpu_state->nesting[cpu_state->prio]--;
> +
> +	/*
> +	 * Restore the previous priority, which was returned by
> +	 * nbcon_atomic_enter().
> +	 */
> +	cpu_state->prio = prev_prio;
> +
> +	migrate_enable();
> +}

Best Regards,
Petr