Message-ID: <45DDB096.2020807@inov.pt>
Date:	Thu, 22 Feb 2007 15:02:46 +0000
From:	Jose Goncalves <jose.goncalves@...v.pt>
To:	Frederik Deweerdt <deweerdt@...e.fr>, akpm@...ux-foundation.org,
	linux-kernel@...r.kernel.org
Subject: Re: Serial related oops

Russell King wrote:
> On Wed, Feb 21, 2007 at 02:13:15PM +0000, Jose Goncalves wrote:
>   
>> <1>[18840.304048] Unable to handle kernel NULL pointer dereference at virtual address 00000012
>> <1>[18840.313046]  printing eip:
>> <4>[18840.321687] c01bfa7a
>> <1>[18840.321714] *pde = 00000000
>> <0>[18840.331287] Oops: 0000 [#1]
>> <4>[18840.340687] Modules linked in:
>> <0>[18840.349749] CPU:    0
>> <4>[18840.349767] EIP:    0060:[<c01bfa7a>]    Not tainted VLI
>> <4>[18840.349782] EFLAGS: 00010202   (2.6.16.41-mtm5-debug1 #1) 
>> <0>[18840.377277] EIP is at serial_in+0xa/0x4a
>> <0>[18840.387221] eax: 00000060   ebx: 00000000   ecx: 00000000   edx: 00000000
>> <0>[18840.397805] esi: 00000000   edi: 00000040   ebp: c728fe1c   esp: c728fe18
>> <0>[18840.408579] ds: 007b   es: 007b   ss: 0068
>> <0>[18840.419624] Process gp_position (pid: 11629, threadinfo=c728e000 task=c7443a90)
>> <0>[18840.420509] Stack: <0>00000000 00000000 c01c0f88 00000000 00000000 c031fef0 00000005 00000202 
>> <0>[18840.445655]        c7161a1c c031fef0 c124b510 c728fe60 c01bd97d c031fef0 c124b510 c124b510 
>> <0>[18840.460540]        00000000 c773dbcc c728fe7c c01befe7 c124b510 00000000 ffffffed c773dbcc 
>>     
>
> Okay, this one is even more plainly "not a coding error".
>
>   
>> <0>[18840.566645]  [<c01c0f88>] serial8250_startup+0x28f/0x2a9
>>     
>
> The code around this point (with the return point marked) is:
>
>   
>> c01c0f78:	6a 05                	push   $0x5
>> c01c0f7a:	53                   	push   %ebx
>> c01c0f7b:	e8 f0 ea ff ff       	call   c01bfa70 <serial_in>
>> c01c0f80:	6a 00                	push   $0x0
>> c01c0f82:	53                   	push   %ebx
>> c01c0f83:	e8 e8 ea ff ff       	call   c01bfa70 <serial_in>
>> c01c0f88<<<	6a 02                	push   $0x2
>> c01c0f8a:	53                   	push   %ebx
>> c01c0f8b:	e8 e0 ea ff ff       	call   c01bfa70 <serial_in>
>>     
>
> and corresponds with this C code:
>
>         (void) serial_inp(up, UART_LSR);
>         (void) serial_inp(up, UART_RX);
>         (void) serial_inp(up, UART_IIR);
>
> Now let's look at the words pushed on the stack around this code:
>
>   00000000
>   00000000
>   c01c0f88 <- return address for serial_in (serial8250_startup+0x28f/0x2a9)
>   00000000 <- from push %ebx at c01c0f82
>   00000000 <- from push $0x0 at c01c0f80
>   c031fef0 <- from push %ebx at c01c0f7a
>   00000005 <- from push %0x5 at c01c0f78
>
> Plainly, %ebx changed across the call to serial_in() at c01c0f7b.
> First thing to notice is this violates the C code - "up" can not
> change.
>
> Now let's look at serial_in:
>
> c01bfa70:       55                      push   %ebp
> c01bfa71:       89 e5                   mov    %esp,%ebp
> c01bfa73:       53                      push   %ebx
> ...
> c01bfab7:       5b                      pop    %ebx
> c01bfab8:       5d                      pop    %ebp
> c01bfab9:       c3                      ret
>
> This code tells the CPU to preserves %ebx and %ebp.  But we know %ebx
> _wasn't_ preserved.  Ergo, your CPU is plainly not doing what the code
> told it to do.
>
> Moreover, serial_in() has preserved %ebx in the past otherwise we'd
> never got past all the other serial_in()s in serial8250_startup().
>
> So I think it's very demonstrably a hardware fault, and not software
> related.
>   

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?

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...

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!

> For all we know, it could be a one-off fault on the hardware you
> happen to have - other identical units may not behave the same (can
> you check?)
>   

Yes I have other units that I can test it. I'll do that to see if it's
really a one-off fault on the hardware.
If it continues to crash with other units I will then test with the
msleep(10) before the "And clear the interrupt registers again for
luck.", as you suggested earlier.

> If it is a one off case, you are welcome to patch that test out in
> your kernel build to remove the problem, and if it's an isolated case
> I encourage you to do this.  This is one of the great advantages of
> open source - if you hit such a problem rather than throwing the
> hardware away you can work around such issues.
>   

I didn't understand what you mean by "you are welcome to patch that test
out in your kernel build to remove the problem". Which test are you
talking about?

Regards,
José Gonçalves

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