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    SubjectRe: Module loading/unloading and "The Stop Machine"
    Date
    On Friday 22 February 2008 22:53:50 Andi Kleen wrote:
    > Max Krasnyanskiy <maxk@qualcomm.com> writes:
    > > static struct module *load_module(void __user *umod,
    > > unsigned long len,
    > > const char __user *uargs)
    > > {
    > > ...
    > >
    > > /* Now sew it into the lists so we can get lockdep and oops
    > > * info during argument parsing. Noone should access us, since
    > > * strong_try_module_get() will fail. */
    > > stop_machine_run(__link_module, mod, NR_CPUS);
    > > ...
    > > }
    >
    > Wow you found some really bad code. I bet it wouldn't be that
    > difficult to fix the code to allow oops safe list insertion
    > without using the big stop machine overkill hammer.

    Yes it's overkill and it's becoming more noticable with larger machines.
    (Also, stop_machine is heavier than it should be, Kathy is working on fixing
    that though).

    Most obvious is to add __ "lockless, but I'm in trouble anyway" accessors, but
    last I looked they'd have to be plumbed through a fair bit of code.

    From a quick reading of the code, a lockless add is possible, but it's fragile
    if one of the readers does something more complex in future.

    Thanks,
    Rusty.



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