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SubjectRe: [PATCH] IRQ handling race and spurious IIR read in serial/8250.c
I wrote:
> In drivers/serial/8250.c in Linux there are two bugs:
> 1. UART_BUG_TXEN can be spuriously set, due to an IRQ race
> 2. The workaround then applied by the kernel is itself buggy

Markus Armbruster has also experienced this problem in a Xen
environment and has confirmed that my patch fixes it.

I think at the very least this change:

> Proposed initial band-aid fix (against
> Do not read IIR in serial8250_start_tx when UART_BUG_TXEN
> Reading the IIR clears some oustanding interrupts so it is not safe.
> Instead, simply transmit immediately if the buffer is empty without
> regard to IIR.
> Signed-off-by: Ian Jackson <>

should be made right away.


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