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    SubjectRe: [PATCH 2.5.73] Signal stack fixes #1 introduce PF_SS_ACTIVE

    On Fri, 4 Jul 2003, Jörn Engel wrote:
    >
    > This is the generic part of the signal stack fixes, it simply
    > introduces a new PF-flag that indicates whether we are using the
    > signal stack right now or not.

    My reason for disliking this patch is that it adds user-space information
    to the kernel - in a place where user space cannot get at it.

    In particular, any traditional cooperative user-space threading package
    wants to switch its own stack around, and they all do it by just changing
    %esp directly. The whole point of such threading is that it's _fast_,
    since it doesn't need any kernel support (and since it's cooperative, you
    can avoid locking).

    The old "optimization" that you didn't like was not an optimization at
    all: it got the case of user space changing stacks _right_, while still
    allowing yet another stack for signal handling and exiting the signal by
    hand.

    Does anybody do that? I don't know. But it was done the way it was done on
    purpose.

    Linus

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