Re: More linux-2.6.9 module problems

[linux-os]

On Tue, 9 Nov 2004, linux-os wrote:

> On Tue, 9 Nov 2004, Mike Waychison wrote:
>
> > -----BEGIN PGP SIGNED MESSAGE-----
> > Hash: SHA1
> >
> > linux-os wrote:
> > > On Tue, 9 Nov 2004, Mike Waychison wrote:
> > >
> > > > -----BEGIN PGP SIGNED MESSAGE-----
> > > > Hash: SHA1
> > > >
> > > > linux-os wrote:
> > > >
> > > > > On Tue, 9 Nov 2004, Mike Waychison wrote:
> > > > >
> > > > > > -----BEGIN PGP SIGNED MESSAGE-----
> > > > > > 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
> > > > 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.
>
> Hmmm. My code didn't do any locking, therefore I don't know
> how to, as you say "drop the lock", except how other kernel drivers
> do it. If I had any semaphores (which I don't here), or spin-locks
> (which I don't), I could certainly unlock anything my code locked.
>
> However, the kernel did something before my code was called.
> Therefore, I have no way of undoing it except by calling
> unlock_kernel().
>
> Is there some other way?

I experimented with:
	release_kernel_lock(current);
        do_ioctl();
        reacquire_kernel_lock(current);

The results were truly spectacular crashes when a copy_to_user
happens in the ioctl(), returning the results. The starting
error is:
   sleeping function called from invalid context at 
arch/i386/lib/usercopy.c:599.

It says in_atomic():1, irqs_disabled():0 so something
makes __might_sleep() think that it's "in_atomic".
Looking at that, I see where !kernel_locked() is going
to cause problems in ../include/linux/hard_irq.h if
we've been preempted.

Then a few hundred thousand lines of unrelated stuff 
smp_apic_timer_interrupt, etc.

FYI, there are no spin-locks and no semaphores in the
ioctl() code, and it all works if I don't muck with the
kernel lock.

So maybe I can't do copy_to_user unless the kernel lock
is held? Seems strange.


Cheers,
Dick Johnson
Penguin : Linux version 2.6.9 on an i686 machine (5537.79 BogoMips).
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