Date: Tue, 09 Nov 2004 16:43:50 -0500
From: Mike Waychison <Michael.Waychison@Sun.COM>
Subject: Re: More linux-2.6.9 module problems
In-reply-to: <Pine.LNX.4.61.0411091522440.3519@chaos.analogic.com>
To: linux-os@analogic.com
Cc: Linux kernel <linux-kernel@vger.kernel.org>
Message-id: <41913A16.5090508@sun.com>
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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.

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

~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
NOTICE:  The opinions expressed in this email are held by me,
and may not represent the views of Sun Microsystems, Inc.
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