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    SubjectRe: [RFC] Suspicious bug in module refcounting
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    On Tue, Feb 3, 2009 at 8:48 PM, Rusty Russell <rusty@rustcorp.com.au> wrote:
    > On Wednesday 04 February 2009 00:17:21 Karsten Keil wrote:
    >> The refcount is a per CPU atomic variable, module_refcount() simple add
    >> in a fully unprotected loop (not disabled irqs, not protected against
    >> scheduling) all per cpu values.
    >
    > Hi Karsten,
    >
    > Yes, the BUG_ON() is overly aggressive. And I really hate __module_get,
    > and it looks like most of the callers are completely bogus. The watchdog
    > drivers use it to nail themselves in place in their open routines: this is
    > OK, if a bit weird.
    >
    > We should only use __module_get() when you *can't handle* failure;
    > otherwise you should accept that the admin did rmmod --wait and don't use the
    > module any further.
    >
    > dmaengine.c seems to be taking liberties like this. AFAICT it can error
    > out, so why not just try_module_get() always?

    Currently there is no feedback loop for clients calling
    dmaengine_get(). It simply means "I want to do offload, pin any
    offload resources you may see, and don't let the resource leave until
    dmaengine_ref_count == 0". Even if we always called try_module_get()
    we would still need to wait until dmaengine_ref_count reached zero to
    be sure no transactions are in flight, effectively ignoring module_get
    failures.

    However, dma-driver module removal is still in the central control of
    the administrator as downing all network interfaces and unloading the
    async_tx api (i.e. raid456) will kill all dmaengine references. We
    just have the caveat highlighted below:

    modprobe raid456
    ifup eth0
    rmmod --wait ioat_dma &
    ifup eth1
    modprobe -r raid456
    ifdown eth0 <-- module removal succeeds here in a perfect world
    ifdown eth1 <-- module removal currently succeeds here

    Regards,
    Dan


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