[PATCH] TTY: fix UV serial console regression

[PATCH] TTY: fix UV serial console regression

[Jiri Slaby]

Commit 74c2107759d (serial: Use block_til_ready helper) and its fixup
3f582b8c110 (serial: fix termios settings in open) introduced a
regression on UV systems. The serial eventually freezes while being
used. It's completely unpredictable and sometimes needs a heap of
traffic to happen first.

To reproduce this, yast installation was used as it turned out to be
pretty reliable in reproducing. Especially during installation process
where one doesn't have an SSH daemon running. And no monitor as the HW
is completely headless. So this was fun to find. Given the machine
doesn't boot on vanilla before 2.6.36 final. (And the commits above
are older.)

Unless there is some bad race in the code, the hardware seems to be
pretty broken. Otherwise pure MSR read should not cause such a bug,
or?

So to prevent the bug, revert to the old behavior. I.e. read modem
status only if we really have to -- for non-CLOCAL set serials.
Non-CLOCAL works on this hardware OK, I tried. See? I don't.

And document that shit.

Signed-off-by: Jiri Slaby <jslaby@suse.cz>
Cc: 3.0 3.1 3.2 <stable@vger.kernel.org>
References: https://lkml.org/lkml/2011/12/6/573
References: https://bugzilla.novell.com/show_bug.cgi?id=718518
---
 drivers/tty/tty_port.c |   12 +++++++-----
 1 files changed, 7 insertions(+), 5 deletions(-)

diff --git a/drivers/tty/tty_port.c b/drivers/tty/tty_port.c
index 7a4fb1c..1109e1b 100644
--- a/drivers/tty/tty_port.c
+++ b/drivers/tty/tty_port.c
@@ -227,7 +227,6 @@ int tty_port_block_til_ready(struct tty_port *port,
 	int do_clocal = 0, retval;
 	unsigned long flags;
 	DEFINE_WAIT(wait);
-	int cd;
 
 	/* block if port is in the process of being closed */
 	if (tty_hung_up_p(filp) || port->flags & ASYNC_CLOSING) {
@@ -284,11 +283,14 @@ int tty_port_block_til_ready(struct tty_port *port,
 				retval = -ERESTARTSYS;
 			break;
 		}
-		/* Probe the carrier. For devices with no carrier detect this
-		   will always return true */
-		cd = tty_port_carrier_raised(port);
+		/*
+		 * Probe the carrier. For devices with no carrier detect
+		 * tty_port_carrier_raised will always return true.
+		 * Never ask drivers if CLOCAL is set, this causes troubles
+		 * on some hardware.
+		 */
 		if (!(port->flags & ASYNC_CLOSING) &&
-				(do_clocal || cd))
+				(do_clocal || tty_port_carrier_raised(port)))
 			break;
 		if (signal_pending(current)) {
 			retval = -ERESTARTSYS;
-- 
1.7.8.3

Re: [PATCH] TTY: fix UV serial console regression

[Greg KH]

On Thu, Jan 12, 2012 at 10:55:15PM +0100, Jiri Slaby wrote:
> Commit 74c2107759d (serial: Use block_til_ready helper) and its fixup
> 3f582b8c110 (serial: fix termios settings in open) introduced a
> regression on UV systems. The serial eventually freezes while being
> used. It's completely unpredictable and sometimes needs a heap of
> traffic to happen first.
> 
> To reproduce this, yast installation was used as it turned out to be
> pretty reliable in reproducing. Especially during installation process
> where one doesn't have an SSH daemon running. And no monitor as the HW
> is completely headless. So this was fun to find. Given the machine
> doesn't boot on vanilla before 2.6.36 final. (And the commits above
> are older.)
> 
> Unless there is some bad race in the code, the hardware seems to be
> pretty broken. Otherwise pure MSR read should not cause such a bug,
> or?
> 
> So to prevent the bug, revert to the old behavior. I.e. read modem
> status only if we really have to -- for non-CLOCAL set serials.
> Non-CLOCAL works on this hardware OK, I tried. See? I don't.
> 
> And document that shit.

Thanks for tracking this down, I'll queue it up and get it to Linus
after 3.3-rc1 is out.

greg k-h

Re: [PATCH] TTY: fix UV serial console regression

[Alan Cox]

> Unless there is some bad race in the code, the hardware seems to be
> pretty broken. Otherwise pure MSR read should not cause such a bug,
> or?

UV serial being what actual hardware and system ?

> So to prevent the bug, revert to the old behavior. I.e. read modem
> status only if we really have to -- for non-CLOCAL set serials.
> Non-CLOCAL works on this hardware OK, I tried. See? I don't.

The old behaviour is rather driver dependant eg whether it used
serial_core or not.

Doesn't look an unreasonable change though.

Alan

Re: [PATCH] TTY: fix UV serial console regression

[Jiri Slaby]

On 01/13/2012 12:41 AM, Alan Cox wrote:
>> Unless there is some bad race in the code, the hardware seems to
>> be pretty broken. Otherwise pure MSR read should not cause such a
>> bug, or?
> 
> UV serial being what actual hardware and system ?

What exact paramteres are you interested in? dmidecode on this machine
says it's an Altix XE310.

thanks,
-- 
js