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    SubjectRe: [PATCH -v5][RFC]: mutex: implement adaptive spinning

    * Linus Torvalds <> wrote:

    > On Wed, 7 Jan 2009, Matthew Wilcox wrote:
    > >
    > > I appreciate this is sample code, but using __get_user() on
    > > non-userspace pointers messes up architectures which have separate
    > > user/kernel spaces (eg the old 4G/4G split for x86-32). Do we have an
    > > appropriate function for kernel space pointers? Is this a good reason
    > > to add one?
    > Yes, you're right.
    > We could do the whole "oldfs = get_fs(); set_fs(KERNEL_DS); ..
    > set_fs(oldfs);" crud, but it would probably be better to just add an
    > architected accessor. Especially since it's going to generally just be a
    > #define get_kernel_careful(val,p) __get_user(val,p)
    > for most architectures.
    > We've needed that before (and yes, we've simply mis-used __get_user() on
    > x86 before rather than add it).

    for the oldfs stuff we already have probe_kernel_read(). OTOH, that
    involves pagefault_disable() which is an atomic op, so
    __get_user_careful() should be much more lightweight - and we already know
    that the memory range at least _used to_ be a valid kernel address.

    (Theoretical race: with memory hotplug that kernel pointer address could
    have gotten unmapped and we could get device memory there - with
    side-effects if accessed. Wont happen in practice.)


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