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@cp.jp.nec.com>
Cc: Naoya Horiguchi <n-horiguchi@ah.jp.nec.com>
Reviewed-by: Peter Hurley <peter@hurleysoftware.com>
Reviewed-by: Thomas Gleixner <tglx@linutronix.de>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>