Re: More linux-2.6.9 module problems

[Mike Waychison]

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linux-os wrote:
> On Tue, 9 Nov 2004, Mike Waychison wrote:
> 
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>> linux-os wrote:
>>
>>> On Tue, 9 Nov 2004, Mike Waychison wrote:
>>>
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>>>> linux-os wrote:
>>>>
>>>>>
>>>>> I have a memory-test procedure that tests
>>>>> memory on a board, accessed via the PCI bus.
>>>>> There is a lot of RAM and it's bank-switched
>>>>> into some 64k windows so it takes a lot of
>>>>> time to test, about 60 seconds.
>>>>>
>>>>> This is in a module, therefore inside the kernel.
>>>>> When it is invoked via an ioctl() call, the
>>>>> kernel is frozen for the whole test-time. The
>>>>> test procedure does not use any spin-locks nor
>>>>> does it even use any semaphores. It just does a
>>>>> bunch of read/write operations over the PCI/Bus.
>>>>>
>>>>> I thought that I could enable the preemptible-
>>>>> kernel option and the machine would then respond
>>>>> normally. Not so. Even with 4 CPUs, when one
>>>>> ioctl() is busy in the kernel, nothing else
>>>>> happens until its done. Even keyboard activity
>>>>> is gone, no Caps Lock and no Num Lock, no `ping`
>>>>> response over the network. However, the machine
>>>>> comes back to life when the memory-test is done.
>>>>>
>>>>> This is kernel version 2.6.9. Is it possible that
>>>>> somebody left on the BKL when calling a module
>>>>> ioctl() on this version? If not, what do I do
>>>>> to be able to execute a time-consuming procedure
>>>>> from inside the kernel? Do I break it up into
>>>>> sections and execute schedule() periodically
>>>>> (temporary work-around --works)??
>>>>>
>>>>
>>>> The BKL has always been grabbed across ioctls.  Drop the lock when you
>>>> enter your f_op->ioctl call and grab it again open completion.
>>>>
>>>
>>> Hmmm. I get 'scheduling while atomic' screaming across the screen!
>>> There are no atomic operations in my ioctl functions so I don't
>>> know what its complaining about. I think I shouldn't have tried
>>> to do anything with BKL because I (my task) doesn't own it.
>>>
>>
>> 'Scheduling while atomic' means you called some function that may
>> schedule itself out while you are holding a spinlock.  Note that the BKL
>> is not a regular spinlock, and scheduling is allowed while holding it.
>>
>> Please see
>> http://james.bond.edu.au/courses/inft73626@033/Assigs/Papers/kernel_locking_techniques.html
>>
>> by Robert Love, the section titled "The Big Kernel Lock"
>>
>> Something else is wrong with your code.
> 
> 
> Not quite. Something is wrong with the e100 network driver used in
> 2.6.9. When I do:
> 
> int ioctl(,,,,)
> {
>    int ret;
>    unlock_kernel();
>    ret = original_ioctl(...);
>    lock_kernel();
>    return ret;
> }
> In my driver,  completely unrelated to the network.... It's
> something in the e100 network driver that the kernel's
> complaining about. If I shut down the network and remove
> the network driver module I don't have any problems while
> enabling BKL. Everything runs fine.
> 

Don't do that. ioctls rightly-assume that the BKL is held when they are
called.

When I said drop the lock, I meant for _your_ ioctl code.

- --
Mike Waychison
Sun Microsystems, Inc.
1 (650) 352-5299 voice
1 (416) 202-8336 voice

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