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To: Ian Jackson <Ian.Jackson@eu.citrix.com>
Cc: Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
       "xen-devel\@lists.xensource.com" <xen-devel@lists.xensource.com>,
       Anders Kaseorg <andersk@MIT.EDU>
Subject: Re: [Xen-devel] Re: [PATCH] IRQ handling race and spurious IIR read in serial/8250.c
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From: Markus Armbruster <armbru@redhat.com>
Date: Thu, 19 Feb 2009 19:37:53 +0100
In-Reply-To: <18845.40059.190188.291643@mariner.uk.xensource.com> (Ian Jackson's message of "Thu\, 19 Feb 2009 17\:52\:59 +0000")
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Ian Jackson <Ian.Jackson@eu.citrix.com> 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@eu.citrix.com>
>
> should be made right away.
>
> Ian.

Patch makes sense to me.

Acked-by: Markus Armbruster <armbru@redhat.com>