[ 3.5.y.z extended stable ] Patch "serial: imx: Fix recursive locking bug" has been added to staging queue

Luis Henriques luis.henriques at canonical.com
Tue Feb 26 16:14:31 UTC 2013

This is a note to let you know that I have just added a patch titled

    serial: imx: Fix recursive locking bug

to the linux-3.5.y-queue branch of the 3.5.y.z extended stable tree 
which can be found at:


If you, or anyone else, feels it should not be added to this tree, please 
reply to this email.

For more information about the 3.5.y.z tree, see



>From 79c17ebdad8566e8f2c3370c29febe04d606fe68 Mon Sep 17 00:00:00 2001
From: Thomas Gleixner <tglx at linutronix.de>
Date: Thu, 14 Feb 2013 21:01:06 +0100
Subject: [PATCH] serial: imx: Fix recursive locking bug

commit 677fe555cbfb188af58cce105f4dae9505e58c31 upstream.

commit 9ec1882df2 (tty: serial: imx: console write routing is unsafe
on SMP) introduced a recursive locking bug in imx_console_write().

The callchain is:

          spin_lock_irqsave(&sport->port.lock,flags); <--- DEAD

The bad news is that the kernel debugging facilities can dectect the
problem, but the printks never surface on the serial console for
obvious reasons.

There is a similar issue with oops_in_progress. If the kernel crashes
we really don't want to be stuck on the lock and unable to tell what

In general most UP originated drivers miss these checks and nobody
ever notices because CONFIG_PROVE_LOCKING seems to be still ignored by
a large number of developers.

The solution is to avoid locking in the sysrq case and trylock in the
oops_in_progress case.

This scheme is used in other drivers as well and it would be nice if
we could move this to a common place, so the usual copy/paste/modify
bugs can be avoided.

Now there is another issue with this scheme:

CPU0 	    	     	 CPU1
			   sysrq_detection() -> sets port->sysrq
			 return from interrupt
     if (port->sysrq)
     	avoid locking

port->sysrq is reset with the next receive character. So as long as
the port->sysrq is not reset and this can take an endless amount of
time if after the break no futher receive character follows, all
console writes happen unlocked.

While the current writer is protected against other console writers by
the console sem, it's unprotected against open/close or other
operations which fiddle with the port. That's what the above mentioned
commit tried to solve.

That's an issue in all drivers which use that scheme and unfortunately
there is no easy workaround. The only solution is to have a separate
indicator port->sysrq_cpu. uart_handle_sysrq_char() then sets it to
smp_processor_id() before calling into handle_sysrq() and resets it to
-1 after that. Then change the locking check to:

     if (port->sysrq_cpu == smp_processor_id())
     	 locked = 0;
     else if (oops_in_progress)
         locked = spin_trylock_irqsave(port->lock, flags);
  	 spin_lock_irqsave(port->lock, flags);

That would force all other cpus into the spin_lock path. Problem
solved, but that's way beyond the scope of this fix and really wants
to be implemented in a common function which calls the uart specific
write function to avoid another gazillion of hard to debug
copy/paste/modify bugs.

Reported-and-tested-by: Tim Sander <tim at krieglstein.org>
Signed-off-by: Thomas Gleixner <tglx at linutronix.de>
Signed-off-by: Greg Kroah-Hartman <gregkh at linuxfoundation.org>
Signed-off-by: Luis Henriques <luis.henriques at canonical.com>
 drivers/tty/serial/imx.c | 11 +++++++++--
 1 file changed, 9 insertions(+), 2 deletions(-)

diff --git a/drivers/tty/serial/imx.c b/drivers/tty/serial/imx.c
index e983331..616f8c5 100644
--- a/drivers/tty/serial/imx.c
+++ b/drivers/tty/serial/imx.c
@@ -1217,8 +1217,14 @@ imx_console_write(struct console *co, const char *s, unsigned int count)
 	struct imx_port_ucrs old_ucr;
 	unsigned int ucr1;
 	unsigned long flags;
+	int locked = 1;

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

 	 *	First, save UCR1/2/3 and then disable interrupts
@@ -1245,7 +1251,8 @@ imx_console_write(struct console *co, const char *s, unsigned int count)

 	imx_port_ucrs_restore(&sport->port, &old_ucr);

-	spin_unlock_irqrestore(&sport->port.lock, flags);
+	if (locked)
+		spin_unlock_irqrestore(&sport->port.lock, flags);


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