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

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Message-ID: <87ikenyclh.fsf@jogness.linutronix.de>
Date: Wed, 03 Dec 2025 13:33:38 +0106
From: John Ogness <john.ogness@...utronix.de>
To: Leo Yan <leo.yan@....com>
Cc: Petr Mladek <pmladek@...e.com>, Sergey Senozhatsky
 <senozhatsky@...omium.org>, Steven Rostedt <rostedt@...dmis.org>, Breno
 Leitao <leitao@...ian.org>, linux@...linux.org.uk, paulmck@...nel.org,
 usamaarif642@...il.com, linux-arm-kernel@...ts.infradead.org,
 linux-kernel@...r.kernel.org, kernel-team@...a.com, rmikey@...a.com
Subject: Re: [PATCH] printk/nbcon: Restore IRQ in atomic flush after each
 emitted record

On 2025-12-02, Leo Yan <leo.yan@....com> wrote:
>> 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().
>
> Sorry for suddenly jumping in.  Wouldn't guard(irqsave)() be helpful
> here?

Thanks, I was not aware of the irqsave variant. We would want the scoped
version. So something like this?

$ git diff -w
diff --git a/kernel/printk/nbcon.c b/kernel/printk/nbcon.c
index 3fa403f9831f..55beb64c4f4a 100644
--- a/kernel/printk/nbcon.c
+++ b/kernel/printk/nbcon.c
@@ -1557,6 +1557,14 @@ 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) {
+		/*
+		 * 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.
+		 */
+		scoped_guard(irqsave) {
 			if (!nbcon_context_try_acquire(ctxt, false))
 				return -EPERM;
 
@@ -1569,6 +1577,7 @@ static int __nbcon_atomic_flush_pending_con(struct console *con, u64 stop_seq)
 				return -EAGAIN;
 
 			nbcon_context_release(ctxt);
+		}
 
 		if (!ctxt->backlog) {
 			/* Are there reserved but not yet finalized records? */

John