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Date: Wed, 04 May 2005 16:04:50 -0300
From: =?ISO-8859-1?Q?Andr=E9_Pereira_de_Almeida?= <andre@cachola.com.br>
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To: Chris Wedgwood <cw@f00f.org>
CC: linux-kernel@vger.kernel.org
Subject: Re: A patch for the file kernel/fork.c
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Chris Wedgwood wrote:

>On Wed, May 04, 2005 at 03:26:44PM -0300, Andr? Pereira de Almeida wrote:
>
>  
>
>>>>-       if (tsk->clear_child_tid && atomic_read(&mm->mm_users) > 1) {
>>>>+       if (mm && tsk->clear_child_tid && atomic_read(&mm->mm_users) > 1)
>>>>        
>>>>
>
>  
>
>>In a preemptible kernel with the serport module and a serial port try to
>>run the following program:
>>
>>int main(int argc, char **argv)
>>{
>>       int ldisc,fd;
>>
>>       fd = open("/dev/ttyS0", O_RDWR | O_NOCTTY | O_NONBLOCK);
>>       ldisc = N_MOUSE;
>>       ioctl(fd, TIOCSETD, &ldisc);
>>       read(fd, NULL, 0);
>>       return 0;
>>}
>>
>>and kill it.  In my case it will hang the computer. I think this is
>>a problem with the serport module. With this patch, the serial mouse
>>stop working, but the computer don't hang.
>>    
>>
>
>then above something like:
>
>     BUG_ON(!mm);
>
>or something might be better and eyeball the stack trace.
>  
>
In my example, a stack trace will be already generated by an oops:
[4300748.423000] Unable to handle kernel paging request at virtual 
address 6b6b6b7b
[4300748.423000]  printing eip:
[4300748.423000] d0927366
[4300748.423000] *pde = 00000000
[4300748.423000] Oops: 0000 [#1]
[4300748.423000] PREEMPT
and so on, with a call trace:
[4300748.423000] Call Trace:
[4300748.423000]  [<c0104bfa>] show_stack+0x7a/0x90
[4300748.423000]  [<c0104d7d>] show_registers+0x14d/0x1b0
[4300748.423000]  [<c0104fcc>] die+0x14c/0x2c0
[4300748.423000]  [<c0118b6f>] do_page_fault+0x31f/0x638
[4300748.423000]  [<c01046df>] error_code+0x4f/0x54
[4300748.423000]  [<c02b88fd>] tty_wakeup+0x5d/0x60

I think that maybe it's good to put a:
       WARN_ON(!mm);
but a BUG_ON or without this patch, the kernel will halt, even if the 
problem is not so severe.
Andr?.
-
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