Re: [patch RFC 28/29] printk: Provide functions for atomic write enforcement

[John Ogness]

Below is a fix that was used for the LPC2022 demo so that after a
warning, the atomic printing context is not responsible for printing any
non-emergency backlog that came after the warning.

On 2022-09-11, Thomas Gleixner <tglx@xxxxxxxxxxxxx> wrote:
> --- a/kernel/printk/printk_nobkl.c
> +++ b/kernel/printk/printk_nobkl.c
> + * cons_atomic_flush_one - Flush one console in atomic mode
> + * @con:	The console to flush
> + * @prio:	The priority of the current context
> + */
> +static void cons_atomic_flush_one(struct console *con, enum cons_prio prio)
> +{
> +	struct cons_write_context *wctxt = cons_get_wctxt(con, prio);
> +	struct cons_context *ctxt = &ACCESS_PRIVATE(wctxt, ctxt);
> +
> +	if (!cons_atomic_try_acquire(con, ctxt, prio))
> +		return;
> +
> +	do {
> +		/*
> +		 * cons_emit_record() returns false when the console was
> +		 * handed over or taken over. In both cases the context is
> +		 * not longer valid.
> +		 */
> +		if (!cons_emit_record(wctxt))
> +			return;

If the CPU is no longer in an elevated priority and kthreads are
available, abort the atomic printing and let the kthreads take over. Add
the following break condition here:

		/*
		 * If the CPU is no longer in an elevated priority, let the
		 * kthreads take over, if they are available.
		 */
		if (prio <= CONS_PRIO_NORMAL && con->kthread)
			break;


> +	} while (ctxt->backlog);
> +
> +	cons_release(ctxt);
> +}

John Ogness