linux-kernel - Re: [PATCH] tty: serial: men_z135_uart.c: Fix race between IRQ and set

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Date:	Tue, 04 Aug 2015 09:02:42 +0200
From:	Johannes Thumshirn <jthumshirn@...e.de>
To:	Peter Hurley <peter@...leysoftware.com>
Cc:	Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
	Jiri Slaby <jslaby@...e.com>, linux-serial@...r.kernel.org,
	linux-kernel@...r.kernel.org,
	Andreas Werner <andreas.werner@....de>
Subject: Re: [PATCH] tty: serial: men_z135_uart.c: Fix race between IRQ and set_termios()


Hi Peter,
Peter Hurley <peter@...leysoftware.com> writes:

> Hi Johannes,
>
> On 08/03/2015 09:58 AM, Johannes Thumshirn wrote:
>> Fix panic caused by a race between men_z135_intr() and men_z135_set_termios().
>> 
>> men_z135_intr() and men_z135_set_termios() both hold the struct uart_port::lock
>> spinlock, but men_z135_intr() does a spin_lock_irqsave() and
>> men_z135_set_termios() does a normal spin_lock(), which can lead to a deadlock
>> when an interrupt is called while the lock is being helt by
>> men_z135_set_termios().
>
>
> The irq handler can and should use normal spin_lock()/unlock().

I always thought an irq handler _must_ use the irqsave versions. Good to
know that.

>
> The set_termios() method should used spin_lock_irq(); there's no need to save the
> interrupt state because that method will never be called from interrupt context.
>
> So the 'flags' local can be dropped from the patch.

Given that the irqsave variant isn't needed that sounds reasonable.

>
> Also, the port lock is already initialized in uart_add_one_port() and should
> not be initialized by the probe() function.

OK, do you prefer (or better Greg and Jiri) prefer that change folded
into this patch or an extra patch?

>
> Regards,
> Peter Hurley
>
>> This was discovered using a insmod, hardware looppback send/receive, rmmod
>> stress test.
>> 
>> Signed-off-by: Johannes Thumshirn <jthumshirn@...e.de>
>> Cc: Andreas Werner <andreas.werner@....de>
>> ---
>>  drivers/tty/serial/men_z135_uart.c | 5 +++--
>>  1 file changed, 3 insertions(+), 2 deletions(-)
>> 
>> diff --git a/drivers/tty/serial/men_z135_uart.c b/drivers/tty/serial/men_z135_uart.c
>> index 35c5550..d020435 100644
>> --- a/drivers/tty/serial/men_z135_uart.c
>> +++ b/drivers/tty/serial/men_z135_uart.c
>> @@ -656,6 +656,7 @@ static void men_z135_set_termios(struct uart_port *port,
>>  				struct ktermios *old)
>>  {
>>  	struct men_z135_port *uart = to_men_z135(port);
>> +	unsigned long flags;
>>  	unsigned int baud;
>>  	u32 conf_reg;
>>  	u32 bd_reg;
>> @@ -717,7 +718,7 @@ static void men_z135_set_termios(struct uart_port *port,
>>  
>>  	baud = uart_get_baud_rate(port, termios, old, 0, uart_freq / 16);
>>  
>> -	spin_lock(&port->lock);
>> +	spin_lock_irqsave(&port->lock, flags);
>>  	if (tty_termios_baud_rate(termios))
>>  		tty_termios_encode_baud_rate(termios, baud, baud);
>>  
>> @@ -725,7 +726,7 @@ static void men_z135_set_termios(struct uart_port *port,
>>  	iowrite32(bd_reg, port->membase + MEN_Z135_BAUD_REG);
>>  
>>  	uart_update_timeout(port, termios->c_cflag, baud);
>> -	spin_unlock(&port->lock);
>> +	spin_unlock_irqrestore(&port->lock, flags);
>>  }
>>  
>>  static const char *men_z135_type(struct uart_port *port)
>> 
>

-- 
Johannes Thumshirn                                           Storage
jthumshirn@...e.de                                 +49 911 74053 689
SUSE LINUX GmbH, Maxfeldstr. 5, 90409 Nürnberg
GF: F. Imendörffer, J. Smithard, J. Guild, D. Upmanyu, G. Norton
HRB 21284 (AG Nürnberg)
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