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    SubjectRe: question about save_xstate_sig() - WHY DOES THIS WORK?
    On 01/29/2015 03:45 PM, Oleg Nesterov wrote:
    > On 01/27, Rik van Riel wrote:
    >>
    >> On 01/27/2015 02:40 PM, Oleg Nesterov wrote:
    >>>>>
    >>>>> - Is unlazy_fpu()->__save_init_fpu() safe wrt
    >>>>> __kernel_fpu_begin() from irq?
    >>
    >> It looks like it should be safe, as long as __save_init_fpu()
    >> knows that the task no longer has the FPU after __kernel_fpu_end(),
    >> so it does not try to save the kernel FPU state to the user's
    >> task->thread.fpu.state->xstate
    >
    > Not sure this is enough, but...
    >
    >> The caveat here is that __kernel_fpu_begin()/__kernel_fpu_end()
    >> needs to be kept from running during unlazy_fpu().
    >
    > Yes,
    >
    >> This means interrupted_kernel_fpu_idle and/or irq_fpu_usable
    >> need to check whether preemption is disabled, and lock out
    >> __kernel_fpu_begin() when preemption is disabled.
    >
    > But we already have kernel_fpu_disable/enable. unlazy_cpu() can use
    > it to avoid the race ?

    I suspect this will be fine, if __kernel_fpu_end()
    from IRQ context always restores the FPU context,
    or calls stts, so things like save_init_fpu() will
    either continue where they left off, or trap and
    then continue where they left off.

    __kernel_fpu_end() from process context can be
    lazier, something I can work on in my next version
    of the "defer FPU loading to kernel -> user space
    boundary" patch series.

    >> I can certainly merge unlazy_fpu() and save_init_fpu() into the
    >> same function, but I am not sure whether or not it should call
    >> __thread_fpu_end() - it looks like that would be desirable in some
    >> cases, but not in others...
    >
    > I _think_ that we never actually want __thread_fpu_end(), although it
    > doesn't really hurt if !eager. Probably ulazy/save should do
    >
    > if (!__save_init_fpu())
    > __thread_fpu_end();

    There is at least one case where we want __thread_fpu_end(),
    and that is xstateregs_set. I got this by moving the
    __thread_fpu_end() call from save_init_fpu() into init_fpu().

    I am not sure about __math_error. I suspect we may need
    __thread_fpu_end() in there so the math state can be
    re-initialized if the task catches SIGFPE and continues.

    On the other hand, I do not see code in there that
    actually does that at the moment...

    Let me send my RFC patch to clean up & merge
    unlazy_fpu and save_init_fpu() in the next email.




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