linux-kernel - Re: [PATCH printk v2 09/11] panic: Add atomic write enforcement to oops

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Date:   Wed, 20 Sep 2023 01:42:47 +0206
From:   John Ogness <john.ogness@...utronix.de>
To:     Petr Mladek <pmladek@...e.com>
Cc:     Sergey Senozhatsky <senozhatsky@...omium.org>,
        Steven Rostedt <rostedt@...dmis.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        linux-kernel@...r.kernel.org, Kees Cook <keescook@...omium.org>,
        Luis Chamberlain <mcgrof@...nel.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Peter Zijlstra <peterz@...radead.org>,
        Josh Poimboeuf <jpoimboe@...nel.org>,
        Arnd Bergmann <arnd@...db.de>,
        "Guilherme G. Piccoli" <gpiccoli@...lia.com>,
        Andy Shevchenko <andriy.shevchenko@...ux.intel.com>
Subject: Re: [PATCH printk v2 09/11] panic: Add atomic write enforcement to
 oops

On 2023-09-20, John Ogness <john.ogness@...utronix.de> wrote:
> diff --git a/kernel/panic.c b/kernel/panic.c
> index 86ed71ba8c4d..e2879098645d 100644
> --- a/kernel/panic.c
> +++ b/kernel/panic.c
> @@ -630,6 +634,36 @@ bool oops_may_print(void)
>   */
>  void oops_enter(void)
>  {
> +	enum nbcon_prio prev_prio;
> +	int cpu = -1;

oops_exit() calls put_cpu(). Here used to be the corresponding
get_cpu(). Somehow I managed to drop it. Here are the inline fixups.

int cur_cpu = get_cpu();
int old_cpu = -1;

> +
> +	/*
> +	 * If this turns out to be the first CPU in oops, this is the
> +	 * beginning of the outermost atomic section. Otherwise it is
> +	 * the beginning of an inner atomic section.
> +	 */
> +	prev_prio = nbcon_atomic_enter(NBCON_PRIO_EMERGENCY);
> +
> +	if (atomic_try_cmpxchg_relaxed(&oops_cpu, &cpu, smp_processor_id())) {

if (atomic_try_cmpxchg_relaxed(&oops_cpu, &old_cpu, cur_cpu)) {

> +		/*
> +		 * This is the first CPU in oops. Save the outermost
> +		 * @prev_prio in order to restore it on the outermost
> +		 * matching oops_exit(), when @oops_nesting == 0.
> +		 */
> +		oops_prev_prio = prev_prio;
> +
> +		/*
> +		 * Enter an inner atomic section that ends at the end of this
> +		 * function. In this case, the nbcon_atomic_enter() above
> +		 * began the outermost atomic section.
> +		 */
> +		prev_prio = nbcon_atomic_enter(NBCON_PRIO_EMERGENCY);
> +	}
> +
> +	/* Track nesting when this CPU is the owner. */
> +	if (cpu == -1 || cpu == smp_processor_id())

if (old_cpu == -1 || old_cpu == cur_cpu)

> +		oops_nesting++;
> +
>  	tracing_off();
>  	/* can't trust the integrity of the kernel anymore: */
>  	debug_locks_off();