From 9a23a1d10bd0c25457d6dfda1b0e754bf8a8e973 Mon Sep 17 00:00:00 2001 From: Taichi Kageyama Date: Mon, 17 Aug 2015 02:45:29 +0000 Subject: [PATCH] serial: 8250: Fix autoconfig_irq() to avoid race conditions 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 Cc: Naoya Horiguchi Reviewed-by: Peter Hurley Reviewed-by: Thomas Gleixner Signed-off-by: Greg Kroah-Hartman --- drivers/tty/serial/8250/8250_port.c | 6 ++++++ 1 file changed, 6 insertions(+) diff --git a/drivers/tty/serial/8250/8250_port.c b/drivers/tty/serial/8250/8250_port.c index bd0c47c58285..c4dac1be3ec1 100644 --- a/drivers/tty/serial/8250/8250_port.c +++ b/drivers/tty/serial/8250/8250_port.c @@ -1238,6 +1238,9 @@ 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); @@ -1269,6 +1272,9 @@ 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; } -- 2.39.2