Re: Serial: UART_BUG_TXEN race conditions

[Russell King <rmk+lkml@arm.linux.org.uk>]

On Mon, Jun 26, 2006 at 10:07:47PM +0200, Ingo van Lil wrote:
> Correct me if I'm mistaken, but I see two possible race conditions with 
> that piece of code:
> 
> 1. What if the IIR actually equals UART_IIR_THRI at that point? The
>    read access will clear the interrupt condition and the workaround
>    will effect the actual opposite of its intention: Neither
>    serial8250_start_tx() nor the interrupt handler will start
>    transmitting characters for the ring buffer.

Gah, looks like you're right - reading the IIR will clear the transmit
pending interrupt, so we should probably just load the transmitter up
with characters anyway if the TEMT bit is set.

> 2. What if an interrupt is triggered shortly after reading the IIR?
>    Both serial8250_start_tx() and the interrupt handler will start
>    a transmission simultaneously.

This function is run under the port spinlock, so the interrupt handler
will be held off until it completes.

-- 
Russell King
 Linux kernel    2.6 ARM Linux   - http://www.arm.linux.org.uk/
 maintainer of:  2.6 Serial core