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From: John Ogness <john.ogness@linutronix.de>
To: "Chang, Junxiao" <junxiao.chang@intel.com>, Sebastian Andrzej Siewior
 <bigeasy@linutronix.de>
Cc: "tglx@linutronix.de" <tglx@linutronix.de>, "rostedt@goodmis.org"
 <rostedt@goodmis.org>, "linux-kernel@vger.kernel.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" <linux-rt-users@vger.kernel.org>
Subject: RE: [PATCH] printk: nbcon: check uart port is nbcon or not in
 nbcon_release
In-Reply-To: <BN9PR11MB53709FD2BC70878B49059107EC722@BN9PR11MB5370.namprd11.prod.outlook.com>
References: <20240117065226.4166127-1-junxiao.chang@intel.com>
 <871qagtlk2.fsf@jogness.linutronix.de>
 <BN9PR11MB5370065F4E7443FEA156347EEC722@BN9PR11MB5370.namprd11.prod.outlook.com>
 <87y1cos2dv.fsf@jogness.linutronix.de>
 <20240117102446.itexUYMc@linutronix.de>
 <BN9PR11MB53709FD2BC70878B49059107EC722@BN9PR11MB5370.namprd11.prod.outlook.com>
Date: Wed, 17 Jan 2024 14:48:28 +0106
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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