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    SubjectRe: [PATCH 2/2] futex, x86/mce: Avoid double machine checks
    On Fri, Jan 08, 2021 at 02:22:51PM -0800, Tony Luck wrote:
    > futex_wait_setup() first tries to read the user value with page faults
    > disabled (because it holds a lock, and so cannot sleep). If that read
    > fails it drops the lock and tries again.
    >
    > But there are now two reasons why the user space read can fail. Either:
    > 1) legacy case of a page fault, in which case it is reasonable to retry
    > 2) machine check on the user address, bad idea to re-read
    >
    > Add some infrastructure to differentiate these cases.

    > --- a/kernel/futex.c
    > +++ b/kernel/futex.c
    > @@ -2658,6 +2658,9 @@ static int futex_wait_setup(u32 __user *uaddr, u32 val, unsigned int flags,
    > if (ret) {
    > queue_unlock(*hb);
    >
    > + if (arch_memory_failure(uaddr))
    > + return ret;
    > +
    > ret = get_user(uval, uaddr);
    > if (ret)
    > return ret;


    I think this is horrid; why can't we have it return something different
    then -EFAULT instead?

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