linux-kernel - Re: [PATCH v3] serial: 8250: Fix autoconfig

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Date:	Mon, 17 Aug 2015 00:15:50 +0000
From:	Taichi Kageyama <t-kageyama@...jp.nec.com>
To:	"gregkh@...uxfoundation.org" <gregkh@...uxfoundation.org>
CC:	"tglx@...utronix.de" <tglx@...utronix.de>,
	"peter@...leysoftware.com" <peter@...leysoftware.com>,
	"linux-serial@...r.kernel.org" <linux-serial@...r.kernel.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"jslaby@...e.cz" <jslaby@...e.cz>,
	"prarit@...hat.com" <prarit@...hat.com>,
	Naoya Horiguchi <n-horiguchi@...jp.nec.com>,
	"jiang.liu@...ux.intel.com" <jiang.liu@...ux.intel.com>
Subject: Re: [PATCH v3] serial: 8250: Fix autoconfig_irq() to avoid race
 conditions

Hi Greg,

On 2015/08/15 9:10, gregkh@...uxfoundation.org wrote:
> On Mon, Aug 10, 2015 at 12:15:40AM +0000, Taichi Kageyama wrote:
>> The following race conditions can happen when a serial port is used
>> as console.
>>
>> Case1: CPU_B is used to detect an interrupt from a serial port,
>>        but it can have interrupts disabled during the waiting time.
>> Case2: CPU_B clears UART_IER just after CPU_A sets UART_IER and then
>>        a serial port may not make an interrupt.
>> Case3: CPU_A sets UART_IER just after CPU_B clears UART_IER.
>>        This is an unexpected behavior for serial8250_console_write().
>>
>> CPU_A [autoconfig_irq]      |  CPU_B [serial8250_console_write]
>> ----------------------------|---------------------------------------
>>                             |
>> probe_irq_on()              |  spin_lock_irqsave(&port->lock,)
>> serial_outp(,UART_IER,0x0f) |  serial_out(,UART_IER,0)
>> udelay(20);                 |  uart_console_write()
>> probe_irq_off()             |
>>                             |  spin_unlock_irqrestore(&port->lock,)
>>
>> Case1 and 2 can make autoconfig_irq() failed.
>> In these cases, the console doesn't work in interrupt mode and
>> "input overrun" (which can make operation mistakes) can happen
>> on some systems. Especially in the Case1, It is known that the
>> problem happens with high rate every boot once it occurs
>> because the boot sequence is always almost same.
>>
>> port mutex makes sure that the autoconfig operation is exclusive of
>> any other concurrent HW access except by the console operation.
>> console lock is required in autoconfig_irq().
>>
>> Signed-off-by: Taichi Kageyama <t-kageyama@...jp.nec.com>
>> Cc: Naoya Horiguchi <n-horiguchi@...jp.nec.com>
>> Reviewed-by: Peter Hurley <peter@...leysoftware.com>
>> ---
>>  drivers/tty/serial/8250/8250_core.c | 6 ++++++
>>  1 file changed, 6 insertions(+)
>>
>> diff --git v4.2-rc4.org/drivers/tty/serial/8250/8250_core.c v4.2-rc4.work/drivers/tty/serial/8250/8250_core.c
>> index 37fff12..ed1e23e 100644
>> --- v4.2-rc4.org/drivers/tty/serial/8250/8250_core.c
>> +++ v4.2-rc4.work/drivers/tty/serial/8250/8250_core.c
>> @@ -1303,6 +1303,9 @@ static void autoconfig_irq(struct uart_8250_port *up)
>>  		inb_p(ICP);
>>  	}
>>  
>> +	if (uart_console(port))
>> +		console_lock();
>> +
>>  	/* forget possible initially masked and pending IRQ */
>>  	probe_irq_off(probe_irq_on());
>>  	save_mcr = serial_in(up, UART_MCR);
>> @@ -1334,6 +1337,9 @@ static void autoconfig_irq(struct uart_8250_port *up)
>>  	if (port->flags & UPF_FOURPORT)
>>  		outb_p(save_ICP, ICP);
>>  
>> +	if (uart_console(port))
>> +		console_unlock();
>> +
>>  	port->irq = (irq > 0) ? irq : 0;
>>  }
> 
> This doesn't apply to my tty-next tree at all.  Can you please rebase it
> and resend?

Oh, I didn't know the file name was changed in tty-next.
I'll rebase it.


Thanks,
Taichi
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