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    SubjectRe: [PATCH -v5][RFC]: mutex: implement adaptive spinning
    On Wed, 7 Jan 2009 22:32:22 +0100
    Ingo Molnar <mingo@elte.hu> wrote:

    > > 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

    tisn't. pagefault_disable() is just preempt_count()+=1;barrier() ?

    Am suspecting that you guys might be over-optimising this
    contended-path-were-going-to-spin-anyway code?


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