linux-kernel - Re: [PATCH 1/2] serial: 8250: Fix autoconfig

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Date:	Wed, 08 Jul 2015 19:35:26 -0400
From:	Peter Hurley <peter@...leysoftware.com>
To:	Taichi Kageyama <t-kageyama@...jp.nec.com>,
	"gregkh@...uxfoundation.org" <gregkh@...uxfoundation.org>
CC:	"linux-serial@...r.kernel.org" <linux-serial@...r.kernel.org>,
	"jslaby@...e.cz" <jslaby@...e.cz>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"prarit@...hat.com" <prarit@...hat.com>,
	Naoya Horiguchi <n-horiguchi@...jp.nec.com>
Subject: Re: [PATCH 1/2] serial: 8250: Fix autoconfig_irq() to avoid race
 conditions

On 06/05/2015 05:57 AM, Taichi Kageyama wrote:
> The following race conditions can happen if a serial is used as console.
>   Case1. CPU_B handles an interrupt from a serial
>     autoconfig_irq() fails whether the interrupt is raised or not
>     if CPU_B is disabled to handle interrupts for longer than it expects.
>   Case2. CPU_B clears UART_IER just after CPU_A sets UART_IER
>     A serial may not make an interrupt.
>     autoconfig_irq() can fail if the interrupt is not raised.
>   Case3. CPU_A sets UART_IER just after CPU_B clears UART_IER
>     This is an unexpected behavior for uart_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,)
>   -----------------------------------------------------------------
> 
> If autoconfig_irq() fails, the console doesn't work in interrupt mode,
> the mode cannot be changed anymore, and "input overrun"
> (which can make operation mistakes) happens easily.
> This problem happens with high rate every boot once it occurs
> because the boot sequence is always almost same.

Reviewed-by: Peter Hurley <peter@...leysoftware.com>


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