Re: [Xen-devel] Re: [PATCH] IRQ handling race and spurious IIR read in serial/8250.c

[Markus Armbruster]

Ian Jackson <Ian.Jackson@xxxxxxxxxxxxx> writes:

> 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.

Correct.

> I think at the very least this change:
>
>> Proposed initial band-aid fix (against 2.6.28.4):
>> 
>> 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 <ian.jackson@xxxxxxxxxxxxx>
>
> should be made right away.
>
> Ian.

Patch makes sense to me.

Acked-by: Markus Armbruster <armbru@xxxxxxxxxx>
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