Re: More linux-2.6.9 module problems

From: Mike Waychison
Date: Tue Nov 09 2004 - 16:45:44 EST

Hash: SHA1

linux-os wrote:
> On Tue, 9 Nov 2004, Mike Waychison wrote:
>> Hash: SHA1
>> 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
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,
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