Hi Johan,

On 30/09/2021 04:54, Johan Hovold wrote:

This is just so broken; you can't just drop the lock. And you clearly
haven't even tried to understand how uart_unlock_and_check_sysrq()
works.

Please take a closer look at the commit you're trying to mimic.

Thanks for the feedback.

I have changed it to:

diff --git a/drivers/tty/serial/imx.c b/drivers/tty/serial/imx.c
index 8b121cd869e9..b7cda50602d5 100644
--- a/drivers/tty/serial/imx.c
+++ b/drivers/tty/serial/imx.c

@@ -803,7 +803,7 @@ static irqreturn_t __imx_uart_rxint(int irq, void 

*dev_id)
  				continue;
  		}

-		if (uart_handle_sysrq_char(&sport->port, (unsigned char)rx))
+		if (uart_prepare_sysrq_char(&sport->port, rx))
  			continue;

  		if (unlikely(rx & URXD_ERR)) {

@@ -844,6 +844,7 @@ static irqreturn_t __imx_uart_rxint(int irq, void 

*dev_id)
  	}

  out:
+	uart_unlock_and_check_sysrq(&sport->port);
  	tty_flip_buffer_push(port);

  	return IRQ_HANDLED;

@@ -959,6 +960,7 @@ static irqreturn_t imx_uart_int(int irq, void 

*dev_id)
  		imx_uart_writel(sport, USR1_AGTIM, USR1);

  		__imx_uart_rxint(irq, dev_id);
+		spin_lock(&sport->port.lock);
  		ret = IRQ_HANDLED;
  	}

@@ -1977,9 +1979,7 @@ imx_uart_console_write(struct console *co, const 

char *s, unsigned int count)
  	unsigned int ucr1;
  	int locked = 1;

-	if (sport->port.sysrq)
-		locked = 0;
-	else if (oops_in_progress)
+	if (oops_in_progress)
  		locked = spin_trylock_irqsave(&sport->port.lock, flags);
  	else
  		spin_lock_irqsave(&sport->port.lock, flags);

This makes the deadlock not happen after running:
echo t > /proc/sysrq-trigger

, but entering <break> + t via the console does not work anymore.


It returns the sysrq help instead:

sysrq: HELP : loglevel(0-9) reboot(b) crash(c) show-all-locks(d) 
terminate-all-tasks(e) memory-full-oom-kill(f) kill-all-tasks(i) 
thaw-filesystems(j) sak(k) show-backtrace-all-active-cpu
s(l) show-memory-usage(m) nice-all-RT-tasks(n) poweroff(o) 
show-registers(p) show-all-timers(q) unraw(r) sync(s) 
show-task-states(t) unmount(u) show-blocked-tasks(w) 
dump-ftrace-buffer(z)

Thanks