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    SubjectRe: PATCH? process_32.c:__switch_to() calls __math_state_restore() before updating current_task
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    On Fri, 2010-02-05 at 04:44 -0800, Oleg Nesterov wrote:
    > Could you please explain me another issue? I know nothing about fpu
    > handling, I am reading this code trying to understand the unrelated
    > bug with the old kernel.
    >
    > In short: why restore_i387_xstate() does init_fpu() when !used_math(),
    > can't (or shouldn't) it merely do set_used_math() ?
    >
    > restore_i387_xstate:
    >
    > if (!used_math()) {
    > err = init_fpu(tsk);
    > if (err)
    > return err;
    > }
    >
    > note that buf != NULL. This means that used_math() was true when
    > get_sigframe() was called, otherwise buf == sigcontext->fpstate
    > would be NULL, right?

    No. When the signal is delivered sigcontext->fpstate can be null and
    when we return from the signal handler, user can update the
    sigcontext->fpstate pointer and the user expects the kernel to update
    the fpstate of the process accordingly.

    > So, the task must have the valid ->thread.xstate,

    Need not be.

    thanks,
    suresh



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