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    SubjectRe: [PATCH 2/2] Futex non-page-pinning fix
    Date
    In message <20030902065144.GC7619@mail.jlokier.co.uk> you write:
    > What happens after this sequence:
    >
    > 1. process A forks, making process B
    > 2. B does FUTEX_FD, or splits into threads and one does FUTEX_WAIT,
    > on a private page that has not been written to since the fork
    > 3. A does FUTEX_WAIT on the same address
    > 3. The page is swapped out
    > 4. B does FUTEX_WAKE at the same address
    >
    > Won't the futex be hashed on the swap entry at step 4, so that
    > both processes are woken, yet only the waiter in B should be woken?

    Part of step (4) is to swap the page back in (see __pin_page).

    > Related: could COW sharing after fork() explain the spurious wakeups I
    > saw mentioned earlier in the thread?

    In case others are sharing this misconception: there *are* no spurious
    wakeups. But if they were to happen, the current code doesn't handle
    them correctly, unlike every other primitive I know of in the kernel,
    which is why I fixed it while tidying the code.

    I don't know of a rule which says "thou shalt not wake a random thread
    in the kernel": for all I know wierd things like CPU hotplug or
    software suspend may do this in the future.

    Hope that clarifies,
    Rusty.
    --
    Anyone who quotes me in their sig is an idiot. -- Rusty Russell.
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