linux-kernel - Re: Serial related oops

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Message-ID: <20070222172103.lspdvgh80sg804k0@webmail.inov.pt>
Date:	Thu, 22 Feb 2007 17:21:03 +0000
From:	jose.goncalves@...v.pt
To:	Russell King <rmk+lkml@....linux.org.uk>
Cc:	Frederik Deweerdt <deweerdt@...e.fr>, akpm@...ux-foundation.org,
	linux-kernel@...r.kernel.org
Subject: Re: Serial related oops

Quoting Russell King <rmk+lkml@....linux.org.uk>:

> On Thu, Feb 22, 2007 at 03:02:46PM +0000, Jose Goncalves wrote:
>> It could be a silly question (tamper with me as I'm not familiar with
>> such low level programming), but couldn't it be possible for a interrupt
>> to hit in the middle of the serial_in() calls and mess with %ebx?
>
> I'm no expert on x86, but if an interrupt was messing with %ebx, you'd
> have random crashes verywhere - userspace, kernel space in unpredicatable
> ways.
>
>> What I find real hard to understand is why a hardware fault happens
>> always in the same software instruction! I would expect a hardware fault
>> to hit randomly...
>
> Well, compared with your previous report, your latest report is different.
> Your first report had  both EIP and %ebx being zero (because they got
> corrupted when returning from serial_in).  This time only %ebx was
> corrupted.
>
> Consequently, this time we oopsed in the subsequent serial_in() rather
> than trying to return to serial8250_startup() as last time.

But there was also another difference. I CONFIGed the kernel to produce 
more debug info. This should influence the Oops report...

>
>> I left my application running this night, with a 2.6.16.41 kernel
>> unpatched  on the serial driver (my last Oops report was with Frederik
>> patch to remove the insertion made in 2.6.12) and it crashed again on
>> exactly the same point!
>
>> From that I take it that you removed the test in serial8250_startup which
> sets UART_BUG_TXEN, and the problem persisted.  That tends to suggest
> that it's not the culpret.

 From that I mean that with or without this code - 
http://lkml.org/lkml/2007/2/19/124 - the problem persisted. The 
difference is that, without it, the crashes happens more sparsly.

José Gonçalves


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