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    SubjectQuestion regarding kernel threads and userlevel
    Hi linux-kernel,

    I have a question regarding kernel threads : Are kernel threads treated equally
    in terms of scheduling as normal userlevel processes ??

    In my test case I have a driver for a PCI card from which I want to control
    access to it's memory (prefetchable PCI space). Userlevel processes can mmap
    this PCI memory and write directly to it (via the nopage technique). This is
    also possible from the kernel thread, but to avoid trashing and short bursts on
    the PCI bus, I protect every access to the memory space with a spin lock (a
    mmapped kernel memory page which the driver initializes). That means if you
    have a SMP system and two userlevel processes wants to write to this memory,
    one will have to wait for the other before doing the memcpy (yep I'm using what
    you can call PIO). This works great for two userlevel processes.

    Now the reason for my question is; if also I have a kernel thread wanting to
    write to this memory space it will also have to wait for the same lock (though
    not mmapped since we are already in kernel space and can access the lock page
    directly). What happens, is that if a userlevel process holds this lock and the
    kernel thread gets scheduled and tries to get the same lock it will deadlock
    because the userlevel process never gets back control and releases the lock
    (kinda like when you spinlock in interrupt level on a lock wich is locked
    without spinlock_irq). Is this because the kernel thread has higher priority
    than the user level process (it has a nice level of -20) ?

    Best regards,
    Steffen Persvold Systems Engineer
    Email : Scali AS (
    Norway : Tel : (+47) 2262 8950 Olaf Helsets vei 6
    Fax : (+47) 2262 8951 N-0621 Oslo, Norway

    USA : Tel : (+1) 713 706 0544 10500 Richmond Avenue, Suite 190
    Houston, Texas 77042, USA
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