[PATCH] printk: nbcon: check uart port is nbcon or not in nbcon_release

[PATCH] printk: nbcon: check uart port is nbcon or not in nbcon_release

[Junxiao Chang]

Different uart ports might have same console pointer, not all of
uart ports are nbcon. When uart port is shutdown, only release
nbcon if it is nbcon. There is same nbcon checking in API
nbcon_acquire.

Fixes: 6424f396c49e ("printk: nbcon: Implement processing in port->lock wrapper")
Signed-off-by: Junxiao Chang <junxiao.chang@intel.com>
---
 kernel/printk/nbcon.c | 3 +++
 1 file changed, 3 insertions(+)

diff --git a/kernel/printk/nbcon.c b/kernel/printk/nbcon.c
index 1b1b585b1675b..e53b8bebbb57e 100644
--- a/kernel/printk/nbcon.c
+++ b/kernel/printk/nbcon.c
@@ -1623,6 +1623,9 @@ void nbcon_release(struct uart_port *up)
 		.prio		= NBCON_PRIO_NORMAL,
 	};
 
+	if (!uart_is_nbcon(up))
+		return;
+
 	if (!con->locked_port)
 		return;
 
-- 
2.34.1

Re: [PATCH] printk: nbcon: check uart port is nbcon or not in nbcon_release

[John Ogness]

On 2024-01-17, Junxiao Chang <junxiao.chang@intel.com> wrote:
> Different uart ports might have same console pointer,

Please explain how this is possible. It would be a major bug.

John

RE: [PATCH] printk: nbcon: check uart port is nbcon or not in nbcon_release

[Chang, Junxiao]

There are several serial ports in one Intel ADL hardware, they are enumerated as ttyS0, ttyS1, ttyS4, and so on. Multiple console options might be appended to kernel command line. For example, "console=ttyS0,115200n8 console=ttyS4,115200n8 console=ttyS5,115200n8".

In this case, several uarts "cons" pointers are same.

During booting up process, login thread "agetty" might shutdown all uart. All uart ports could release nbcon lock even if they are "non nbcon" console. However, in "nbcon_acquire" API, if uart is not nbcon, it returns directly didn't lock anything.

This patch fixes a boot hang issue which could be reproduced every time with Intel ADL hardware/v6.6.7-rt18 kernel. BTW, this issue couldn't be reproduced with v6.6.7-rt17 kernel.

Thanks,
Junxiao

-----Original Message-----
From: John Ogness <john.ogness@linutronix.de> 
Sent: Wednesday, January 17, 2024 4:24 PM
To: Chang, Junxiao <junxiao.chang@intel.com>; bigeasy@linutronix.de; tglx@linutronix.de; rostedt@goodmis.org; linux-kernel@vger.kernel.org
Cc: Li, Hao3 <hao3.li@intel.com>; Li, Lili <lili.li@intel.com>; Gao, Jianfeng <jianfeng.gao@intel.com>; linux-rt-users@vger.kernel.org
Subject: Re: [PATCH] printk: nbcon: check uart port is nbcon or not in nbcon_release

On 2024-01-17, Junxiao Chang <junxiao.chang@intel.com> wrote:
> Different uart ports might have same console pointer,

Please explain how this is possible. It would be a major bug.

John

RE: [PATCH] printk: nbcon: check uart port is nbcon or not in nbcon_release

[John Ogness]

On 2024-01-17, "Chang, Junxiao" <junxiao.chang@intel.com> wrote:
> There are several serial ports in one Intel ADL hardware, they are
> enumerated as ttyS0, ttyS1, ttyS4, and so on. Multiple console options
> might be appended to kernel command line. For example,
> "console=ttyS0,115200n8 console=ttyS4,115200n8
> console=ttyS5,115200n8".
>
> In this case, several uarts "cons" pointers are same.

Typically a UART driver will register the console structure in the
driver's initcall(), which is only called once per driver. This is why
it is usually not possible to have multiple UART consoles.

If a driver _does_ allow registering consoles for multiple devices, then
it must allocate separate console structs for each registration.

Note that register_console() will generate a warning and abort if a
driver attempts to register the same console struct twice:

    WARN(con == newcon, "console '%s%d' already registered\n",
         con->name, con->index)

So I ask again. Please explain how this is possible.

John

Re: RE: [PATCH] printk: nbcon: check uart port is nbcon or not in nbcon_release

[Sebastian Andrzej Siewior]

On 2024-01-17 11:09:24 [+0106], John Ogness wrote:
> On 2024-01-17, "Chang, Junxiao" <junxiao.chang@intel.com> wrote:
> > There are several serial ports in one Intel ADL hardware, they are
> > enumerated as ttyS0, ttyS1, ttyS4, and so on. Multiple console options
> > might be appended to kernel command line. For example,
> > "console=ttyS0,115200n8 console=ttyS4,115200n8
> > console=ttyS5,115200n8".
> >
> > In this case, several uarts "cons" pointers are same.
> 
> So I ask again. Please explain how this is possible.

I have here
| 00:03: ttyS0 at I/O 0x3f8 (irq = 4, base_baud = 115200) is a 16550A
| 00:04: ttyS1 at I/O 0x2f8 (irq = 3, base_baud = 115200) is a 16550A

and I have

| root        2315  0.0  0.0   5480  1792 ttyS0    Ss+  11:19   0:00 /sbin/agetty -o -p -- \u --keep-baud 115200,57600,38400,9600 - vt220
| root        2502  0.1  0.0   5480  1792 ttyS1    Ss+  11:20   0:00 /sbin/agetty -o -p -- \u --keep-baud 115200,57600,38400,9600 - vt220

and I can stop both of them without any trouble. 
Can this be reproduced on an ordinary x86 hardware given they have more
than one UART (up to four).
Is any of this ADL hardware upstream?

> John

Sebastian

RE: RE: [PATCH] printk: nbcon: check uart port is nbcon or not in nbcon_release

[Chang, Junxiao]

Hi John,

As you mentioned, same console driver is only registered once. 8250 console driver is registered once, its "struct console *newcon" parameter is address of "univ8250_console" which is defined in drivers\tty\serial\8250\8250_core.c.

However, in each serial port device is registered, their cons pointer( "struct console *cons;" in "struct uart_port") will be assigned with same cons in API serial_core_add_one_port:
	uport->cons = drv->cons;
That is, multiple similar 8250 uart_port devices have same console pointer which points to above univ8250_console.

Hi Sebastain,

The ADL hardware I used has two UART devices, one is lpss 8250, another is 8250_dw. Usually there is no serial port in consumer product. Maybe there is serial port in Intel ADL industrial product.
With my hardware, hang issue could be reproduced every time with 6.6.7-rt18 if serial console is enabled. If you or John need me to run some test build and get debug log with my hardware, please feel free to let me know.


The main problem is that there is "nbcon" checking in nbcon_acquire, but no this checking in nbcon_release. It makes nbcon lock not balance. My patch is just to add same "uart_is_nbcon" checking in nb_release.

void nbcon_acquire(struct uart_port *up)
{
	...
	if (!uart_is_nbcon(up))
		return;
	...

Thanks,
Junxiao

-----Original Message-----
From: Sebastian Andrzej Siewior <bigeasy@linutronix.de> 
Sent: Wednesday, January 17, 2024 6:25 PM
To: John Ogness <john.ogness@linutronix.de>
Cc: Chang, Junxiao <junxiao.chang@intel.com>; tglx@linutronix.de; rostedt@goodmis.org; linux-kernel@vger.kernel.org; Li, Hao3 <hao3.li@intel.com>; Li, Lili <lili.li@intel.com>; Gao, Jianfeng <jianfeng.gao@intel.com>; linux-rt-users@vger.kernel.org
Subject: Re: RE: [PATCH] printk: nbcon: check uart port is nbcon or not in nbcon_release

On 2024-01-17 11:09:24 [+0106], John Ogness wrote:
> On 2024-01-17, "Chang, Junxiao" <junxiao.chang@intel.com> wrote:
> > There are several serial ports in one Intel ADL hardware, they are 
> > enumerated as ttyS0, ttyS1, ttyS4, and so on. Multiple console 
> > options might be appended to kernel command line. For example,
> > "console=ttyS0,115200n8 console=ttyS4,115200n8 
> > console=ttyS5,115200n8".
> >
> > In this case, several uarts "cons" pointers are same.
> 
> So I ask again. Please explain how this is possible.

I have here
| 00:03: ttyS0 at I/O 0x3f8 (irq = 4, base_baud = 115200) is a 16550A
| 00:04: ttyS1 at I/O 0x2f8 (irq = 3, base_baud = 115200) is a 16550A

and I have

| root        2315  0.0  0.0   5480  1792 ttyS0    Ss+  11:19   0:00 /sbin/agetty -o -p -- \u --keep-baud 115200,57600,38400,9600 - vt220
| root        2502  0.1  0.0   5480  1792 ttyS1    Ss+  11:20   0:00 /sbin/agetty -o -p -- \u --keep-baud 115200,57600,38400,9600 - vt220

and I can stop both of them without any trouble. 
Can this be reproduced on an ordinary x86 hardware given they have more than one UART (up to four).
Is any of this ADL hardware upstream?

> John

Sebastian

RE: [PATCH] printk: nbcon: check uart port is nbcon or not in nbcon_release

[John Ogness]

On 2024-01-17, "Chang, Junxiao" <junxiao.chang@intel.com> wrote:
> As you mentioned, same console driver is only registered once. 8250
> console driver is registered once, its "struct console *newcon"
> parameter is address of "univ8250_console" which is defined in
> drivers\tty\serial\8250\8250_core.c.
>
> However, in each serial port device is registered, their cons pointer(
> "struct console *cons;" in "struct uart_port") will be assigned with
> same cons in API serial_core_add_one_port:
>
> 	uport->cons = drv->cons;
>
> That is, multiple similar 8250 uart_port devices have same console
> pointer which points to above univ8250_console.

Ah. This explains the "(port)->cons->index == (port)->line" check in
uart_console(). Thank you for clarifying.

> BTW, this issue couldn't be reproduced with v6.6.7-rt17 kernel.

The reason for the change is because console registration/unregistration
is not related to the port lock. There is a potential race condition if
nbcon unlocking is based on the UART port still being registered as a
console.

We could move the @locked_port flag to the struct uart_port. (Probably
renaming it to @nbcon_locked_port.) I think that would be the
appropriate fix.

John

RE: [PATCH] printk: nbcon: check uart port is nbcon or not in nbcon_release

[Chang, Junxiao]

Your idea makes sense. We tried to implement a new patch according to your idea, it could fix our booting hang problem. We will push patch for review soon. BTW, it is better to have " uart_is_nbcon " checking in nbcon_release as well as it in nbcon_acquire. 😊 Two patches will be pushed together for review.

> We could move the @locked_port flag to the struct uart_port. (Probably renaming it to @nbcon_locked_port.) I think that would be the appropriate fix.

> John

[PATCH 0/2] nbcon locking issue with v6.6.10-rt18 kernel

[Junxiao Chang]

There are two serial port devices in one Intel ADL hardware, one is 8250 lpss, another is 8250 dw. Multiple uart devices are enumerated as ttyS0, ttyS4, ttyS5,... With 6.6.10 rt18 kernel, booting hangs in nbcon_release if console is enabled by appending "console=ttySx,115200n8" to kernel command line. According to nbcon author John's suggestion, lock flag is moved from console structure to uart_port. Another patch is to add uart_is_nbcon checking in nbcon_release. 

Junxiao Chang (2):
  printk: nbcon: move locked_port flag to struct uart_port
  printk: nbcon: check uart port is nbcon or not in nbcon_release

 include/linux/console.h     |  2 --
 include/linux/serial_core.h |  1 +
 kernel/printk/nbcon.c       | 11 +++++++----
 3 files changed, 8 insertions(+), 6 deletions(-)

-- 
2.34.1

[PATCH 1/2] printk: nbcon: move locked_port flag to struct uart_port

[Junxiao Chang]

Console pointer in uart_port might be shared among multiple uart
ports. Flag port locked by nbcon should be saved in uart_port
structure instead of in console structure.

Fixes: 6424f396c49e ("printk: nbcon: Implement processing in port->lock wrapper")
Suggested-by: John Ogness <john.ogness@linutronix.de>
Signed-off-by: Junxiao Chang <junxiao.chang@intel.com>
---
 include/linux/console.h     | 2 --
 include/linux/serial_core.h | 1 +
 kernel/printk/nbcon.c       | 8 ++++----
 3 files changed, 5 insertions(+), 6 deletions(-)

diff --git a/include/linux/console.h b/include/linux/console.h
index f8a0628678886..1eb9580e9b18a 100644
--- a/include/linux/console.h
+++ b/include/linux/console.h
@@ -304,7 +304,6 @@ struct nbcon_write_context {
  * @nbcon_state:	State for nbcon consoles
  * @nbcon_seq:		Sequence number of the next record for nbcon to print
  * @pbufs:		Pointer to nbcon private buffer
- * @locked_port:	True, if the port lock is locked by nbcon
  * @kthread:		Printer kthread for this console
  * @rcuwait:		RCU-safe wait object for @kthread waking
  * @irq_work:		Defer @kthread waking to IRQ work context
@@ -338,7 +337,6 @@ struct console {
 	atomic_t		__private nbcon_state;
 	atomic_long_t		__private nbcon_seq;
 	struct printk_buffers	*pbufs;
-	bool			locked_port;
 	struct task_struct	*kthread;
 	struct rcuwait		rcuwait;
 	struct irq_work		irq_work;
diff --git a/include/linux/serial_core.h b/include/linux/serial_core.h
index 245c11753effd..b2221a50fcb29 100644
--- a/include/linux/serial_core.h
+++ b/include/linux/serial_core.h
@@ -488,6 +488,7 @@ struct uart_port {
 	struct uart_icount	icount;			/* statistics */
 
 	struct console		*cons;			/* struct console, if any */
+	bool			nbcon_locked_port;	/* True, if the port is locked by nbcon */
 	/* flags must be updated while holding port mutex */
 	upf_t			flags;
 
diff --git a/kernel/printk/nbcon.c b/kernel/printk/nbcon.c
index 1b1b585b1675b..b53d93585ee71 100644
--- a/kernel/printk/nbcon.c
+++ b/kernel/printk/nbcon.c
@@ -1586,7 +1586,7 @@ void nbcon_acquire(struct uart_port *up)
 	if (!uart_is_nbcon(up))
 		return;
 
-	WARN_ON_ONCE(con->locked_port);
+	WARN_ON_ONCE(up->nbcon_locked_port);
 
 	do {
 		do {
@@ -1597,7 +1597,7 @@ void nbcon_acquire(struct uart_port *up)
 
 	} while (!nbcon_context_enter_unsafe(&ctxt));
 
-	con->locked_port = true;
+	up->nbcon_locked_port = true;
 }
 EXPORT_SYMBOL_GPL(nbcon_acquire);
 
@@ -1623,13 +1623,13 @@ void nbcon_release(struct uart_port *up)
 		.prio		= NBCON_PRIO_NORMAL,
 	};
 
-	if (!con->locked_port)
+	if (!up->nbcon_locked_port)
 		return;
 
 	if (nbcon_context_exit_unsafe(&ctxt))
 		nbcon_context_release(&ctxt);
 
-	con->locked_port = false;
+	up->nbcon_locked_port = false;
 }
 EXPORT_SYMBOL_GPL(nbcon_release);
 
-- 
2.34.1

Re: [PATCH 1/2] printk: nbcon: move locked_port flag to struct uart_port

[John Ogness]

On 2024-01-23, Junxiao Chang <junxiao.chang@intel.com> wrote:
> Console pointer in uart_port might be shared among multiple uart
> ports.

I still want to investigate why the pointer is shared. This sounds
sloppy or dangerous.

> Flag port locked by nbcon should be saved in uart_port
> structure instead of in console structure.

If it turns out that the pointer sharing is necessary, this patch will
fix the reported problem.

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

Re: [PATCH 1/2] printk: nbcon: move locked_port flag to struct uart_port

[Sebastian Andrzej Siewior]

On 2024-01-24 10:53:10 [+0106], John Ogness wrote:
> On 2024-01-23, Junxiao Chang <junxiao.chang@intel.com> wrote:
> > Console pointer in uart_port might be shared among multiple uart
> > ports.
> 
> I still want to investigate why the pointer is shared. This sounds
> sloppy or dangerous.

I have x86 a server box and PNP enumerates two UARTs (8250). Only one is
wired up but both can be specified as console=.
What do I need to do to reproduce this here? Using console= twice does
not do the trick.

Sebastian

RE: [PATCH 1/2] printk: nbcon: move locked_port flag to struct uart_port

[Chang, Junxiao]

> > > Console pointer in uart_port might be shared among multiple uart 
> > > ports.
> > 
> > I still want to investigate why the pointer is shared. This sounds 
> > sloppy or dangerous.

> I have x86 a server box and PNP enumerates two UARTs (8250). Only one is wired up but both can be specified as console=.
> What do I need to do to reproduce this here? Using console= twice does not do the trick.

Issue could be reproduced with our hardware every time. My cmdline is: BOOT_IMAGE=(hd0,gpt2)/boot/bzImage-linux-intel-iot-lts-rt-6.6-kernel root=PARTLABEL=primary rootwait console=ttyS0,115200 console=tty0 init=/sbin/preinit-env console=ttyS4,115200n8 console=ttyS5,115200n8

If you would like to try any debug patch with my ADL hardware, please feel free to let me know.

For console pointer sharing issue, from code logic point of view, the call chain looks like:
serial8250_register_8250_port -> uart_add_one_port -> serial_ctrl_register_port -> serial_core_register_port -> serial_core_add_one_port

In API serial_core_add_one_port, uart_port's console pointer is assigned with driver's console pointer:
	uport->cons = drv->cons;
Driver's console pointer points to static structure "univ8250_console" which is defined in 8250_core.c

That is, all 8250 serial devices' console pointer are same, they point to univ8250_console.

Below is debug log output:
sh-5.1# dmesg | grep @@@@
[    1.687121] @@@@ univ8250_console_init univ8250_console address:ffffffff935df1e0
[    3.419880] @@@@ serial8250_register_8250_port: name:ttyS4, cons pointer:ffffffff935df1e0
[    3.534954] @@@@ serial8250_register_8250_port: name:ttyS5, cons pointer:ffffffff935df1e0
[   11.971345] @@@@ serial8250_do_shutdown, curr thread:(agetty), name:ttyS0, line 0, cons pointer:ffffffff935df1e0
[   11.971506] @@@@ serial8250_do_shutdown, curr thread:(agetty), name:ttyS4, line 4, cons pointer:ffffffff935df1e0
[   11.971849] @@@@ serial8250_do_shutdown, curr thread:(agetty), name:ttyS4, line 4, cons pointer:ffffffff935df1e0
[   11.983072] @@@@ serial8250_do_shutdown, curr thread:agetty, name:ttyS4, line 4, cons pointer:ffffffff935df1e0
[   11.983595] @@@@ serial8250_do_shutdown, curr thread:(agetty), name:ttyS5, line 5, cons pointer:ffffffff935df1e0
[   11.983895] @@@@ serial8250_do_shutdown, curr thread:(agetty), name:ttyS5, line 5, cons pointer:ffffffff935df1e0
[   12.009632] @@@@ serial8250_do_shutdown, curr thread:agetty, name:ttyS5, line 5, cons pointer:ffffffff935df1e0
sh-5.1#

All cons pointers address are same "ffffffff935df1e0".

Debug patch:
diff --git a/drivers/tty/serial/8250/8250_core.c b/drivers/tty/serial/8250/8250_core.c
index 30434718fad80..740fd7e133a28 100644
--- a/drivers/tty/serial/8250/8250_core.c
+++ b/drivers/tty/serial/8250/8250_core.c
@@ -755,6 +755,7 @@ static int __init univ8250_console_init(void)

        serial8250_isa_init_ports();
        register_console(&univ8250_console);
+       printk("@@@@ %s univ8250_console address:%lx\n", __func__, (unsigned long)(&univ8250_console));
        return 0;
 }
 console_initcall(univ8250_console_init);
@@ -1181,6 +1182,7 @@ int serial8250_register_8250_port(const struct uart_8250_port *up)
                        if (ret)
                                goto err;

+                       printk("@@@@ %s: name:%s, cons pointer:%lx\n", __func__, uart->port.name, (unsigned long)uart->port.cons);
                        ret = uart->port.line;
                } else {
                        dev_info(uart->port.dev,
diff --git a/drivers/tty/serial/8250/8250_port.c b/drivers/tty/serial/8250/8250_port.c
index 437a7d3d34cde..93f1e548d1301 100644
--- a/drivers/tty/serial/8250/8250_port.c
+++ b/drivers/tty/serial/8250/8250_port.c
@@ -2510,6 +2510,7 @@ void serial8250_do_shutdown(struct uart_port *port)
        struct uart_8250_port *up = up_to_u8250p(port);
        unsigned long flags;

+       printk("@@@@ %s, curr thread:%s, name:%s, line %d, cons pointer:%lx\n", __func__, current->comm, port->name, port->line, (unsigned long)port->cons);
        serial8250_rpm_get(up);
        /*
         * Disable interrupts from this port

Thanks,
Junxiao

Re: RE: [PATCH 1/2] printk: nbcon: move locked_port flag to struct uart_port

[Sebastian Andrzej Siewior]

On 2024-01-25 01:08:24 [+0000], Chang, Junxiao wrote:
> 
> Issue could be reproduced with our hardware every time. My cmdline is:
> BOOT_IMAGE=(hd0,gpt2)/boot/bzImage-linux-intel-iot-lts-rt-6.6-kernel
> root=PARTLABEL=primary rootwait console=ttyS0,115200 console=tty0
> init=/sbin/preinit-env console=ttyS4,115200n8 console=ttyS5,115200n8
> 
> If you would like to try any debug patch with my ADL hardware, please feel free to let me know.
> 
> For console pointer sharing issue, from code logic point of view, the call chain looks like:
> serial8250_register_8250_port -> uart_add_one_port -> serial_ctrl_register_port -> serial_core_register_port -> serial_core_add_one_port
> 
> In API serial_core_add_one_port, uart_port's console pointer is assigned with driver's console pointer:
> 	uport->cons = drv->cons;
> Driver's console pointer points to static structure "univ8250_console" which is defined in 8250_core.c
> 
> That is, all 8250 serial devices' console pointer are same, they point to univ8250_console.

Okay, So that I see this and the unbalanced acquire/ release part with
the attached patch. I leave it to John…
Btw. You don't see kernel log output on ttyS4 + ttyS5, right? Just a
login prompt.
…
> Thanks,
> Junxiao

Sebastian

RE: RE: [PATCH 1/2] printk: nbcon: move locked_port flag to struct uart_port

[Chang, Junxiao]

> >  That is, all 8250 serial devices' console pointer are same, they point to univ8250_console.

> Okay, So that I see this and the unbalanced acquire/ release part with the attached patch. I leave it to John… Btw. You don't see kernel log output on ttyS4 + ttyS5, right? Just a login prompt.
>
> Sebastian

Right. At that time, only ttyS0 is console and nbcon.

Thanks,
Junxiao

Re: [PATCH 1/2] printk: nbcon: move locked_port flag to struct uart_port

[John Ogness]

Hi Sebastian,

On 2024-01-25, Sebastian Andrzej Siewior <bigeasy@linutronix.de> wrote:
> Okay, So that I see this and the unbalanced acquire/ release part with
> the attached patch. I leave it to John...

Please add this one patch to the 6.6-rt and later queues. The 2nd patch
in this series is not needed.

For 6.8 the fix may end up looking different, i.e. by eliminating the
struct console pointer sharing instead. But for now this patch is fully
sufficient.

Thanks.

John

Re: [PATCH 1/2] printk: nbcon: move locked_port flag to struct uart_port

[Sebastian Andrzej Siewior]

On 2024-01-26 09:04:34 [+0106], John Ogness wrote:
> Hi Sebastian,
Hi,

> Please add this one patch to the 6.6-rt and later queues. The 2nd patch
> in this series is not needed.
Okay. I just dropped a v6.8-RT with this included and I am going to poke
Clark regarding v6.6 next week.

> Thanks.
> 
> John

Sebastian

[PATCH 2/2] printk: nbcon: check uart port is nbcon or not in nbcon_release

[Junxiao Chang]

Try to release nbcon only if current uart port is nbcon, as it does
in nbcon_acquire.

Signed-off-by: Junxiao Chang <junxiao.chang@intel.com>
---
 kernel/printk/nbcon.c | 3 +++
 1 file changed, 3 insertions(+)

diff --git a/kernel/printk/nbcon.c b/kernel/printk/nbcon.c
index b53d93585ee71..d8c6f30adde8b 100644
--- a/kernel/printk/nbcon.c
+++ b/kernel/printk/nbcon.c
@@ -1623,6 +1623,9 @@ void nbcon_release(struct uart_port *up)
 		.prio		= NBCON_PRIO_NORMAL,
 	};
 
+	if (!uart_is_nbcon(up))
+		return;
+
 	if (!up->nbcon_locked_port)
 		return;
 
-- 
2.34.1

Re: [PATCH 2/2] printk: nbcon: check uart port is nbcon or not in nbcon_release

[John Ogness]

On 2024-01-23, Junxiao Chang <junxiao.chang@intel.com> wrote:
> Try to release nbcon only if current uart port is nbcon, as it does
> in nbcon_acquire.

The release needs to undo what acquire did. Why should it have its own
checks that would cause it to _not_ undo what acquire did?

Keep in mind that an nbcon console could be unregistered while another
CPU is holding the nbcon lock. The port lock (and nbcon lock) are
protecting access to the hardware. They are not related to console
registration/unregistration.

John

RE: [PATCH 2/2] printk: nbcon: check uart port is nbcon or not in nbcon_release

[Chang, Junxiao]

> The release needs to undo what acquire did. Why should it have its own checks that would cause it to _not_ undo what acquire did?
>

I agree with you that release needs to be undo what acquire did. The problem is sometimes it might do release while it didn't acquire. In nbcon_acquire, it checks current uart_port is nbcon or not. If it is not nbcon, it returns without locking anything. So in nbcon_release, it should have this checking as well. If current uart_port is not nbcon, it should not do any lock release, this is reason I added this checking in nbcon_release.

For example, there are two uart port ttyS0 and ttyS4, ttyS0 is console and nbcon. ttyS4 is not console or nbcon. When ttyS4 uart port is shutdown, it firstly calls nbcon_acquire, then calls nbcon_release in "serial8250_do_shutdown". During nbcon_acquire, it locks nothing because current uart is not nbcon(There is uart_is_nbcon checking in nbcon_acquire). When it calls nbcon_release, it should not release anything either because it is not nbcon - it doesn't lock anything in nbcon_acquire. But with current code logic, it will release nbcon lock which should not be released.

Regards,
Junxiao

Re: [PATCH 0/2] nbcon locking issue with v6.6.10-rt18 kernel

[John Ogness]

On 2024-01-23, Junxiao Chang <junxiao.chang@intel.com> wrote:
> There are two serial port devices in one Intel ADL hardware, one is
> 8250 lpss, another is 8250 dw. Multiple uart devices are enumerated as
> ttyS0, ttyS4, ttyS5,... With 6.6.10 rt18 kernel, booting hangs in
> nbcon_release if console is enabled by appending
> "console=ttySx,115200n8" to kernel command line. According to nbcon
> author John's suggestion, lock flag is moved from console structure to
> uart_port. Another patch is to add uart_is_nbcon checking in
> nbcon_release.

Isn't the real issue that the console pointer is copied to device that
are not consoles? I am wondering why that is. Is it possible to
dynamically switch the console index during runtime? If not, I think a
proper fix would be to only assign @cons if it actually registered as a
console. This would also simplify the uart_console() macro.

It is critical that a struct console is not shared by multiple
devices. I do not like the idea (or see the point) of having non-console
devices store a struct console pointer that is registered with another
device.

I will take a closer look at that.

John