Re: [PATCH] printk/nbcon: Restore IRQ in atomic flush after each emitted record

[John Ogness <john.ogness@linutronix.de>]

On 2025-12-02, Petr Mladek <pmladek@suse.com> wrote:
> diff --git a/kernel/printk/nbcon.c b/kernel/printk/nbcon.c
> index 3fa403f9831f..6b8becb6ecd9 100644
> --- a/kernel/printk/nbcon.c
> +++ b/kernel/printk/nbcon.c
> @@ -1549,6 +1549,7 @@ static int __nbcon_atomic_flush_pending_con(struct console *con, u64 stop_seq)
>  {
>  	struct nbcon_write_context wctxt = { };
>  	struct nbcon_context *ctxt = &ACCESS_PRIVATE(&wctxt, ctxt);
> +	unsigned long flags;
>  	int err = 0;
>  
>  	ctxt->console			= con;
> @@ -1557,18 +1558,31 @@ static int __nbcon_atomic_flush_pending_con(struct console *con, u64 stop_seq)
>  	ctxt->allow_unsafe_takeover	= nbcon_allow_unsafe_takeover();
>  
>  	while (nbcon_seq_read(con) < stop_seq) {
> -		if (!nbcon_context_try_acquire(ctxt, false))
> +		/*
> +		 * Atomic flushing does not use console driver synchronization
> +		 * (i.e. it does not hold the port lock for uart consoles).
> +		 * Therefore IRQs must be disabled to avoid being interrupted
> +		 * and then calling into a driver that will deadlock trying
> +		 * to acquire console ownership.
> +		 */
> +		local_irq_save(flags);
> +		if (!nbcon_context_try_acquire(ctxt, false)) {
> +			local_irq_restore(flags);
>  			return -EPERM;
> +		}
>  
>  		/*
>  		 * nbcon_emit_next_record() returns false when the console was
>  		 * handed over or taken over. In both cases the context is no
>  		 * longer valid.
>  		 */
> -		if (!nbcon_emit_next_record(&wctxt, true))
> +		if (!nbcon_emit_next_record(&wctxt, true)) {
> +			local_irq_restore(flags);
>  			return -EAGAIN;
> +		}
>  
>  		nbcon_context_release(ctxt);
> +		local_irq_restore(flags);


I am not really happy about all the local_irq_restore() usage. Using
guard syntax would be nice here, but AFAICT there is no guard for
local_irq_save()/_restore(). :-/ And I could not come up with any other
alternative that looked more elegant. So let's just keep it this way.

Reviewed-by: John Ogness <john.ogness@linutronix.de>