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    SubjectRe: [PATCH 1/1] x86/fpu: math_state_restore() should not blindly disable irqs
    On 03/06, Ingo Molnar wrote:
    >
    > * Oleg Nesterov <oleg@redhat.com> wrote:
    >
    > > On 03/06, Ingo Molnar wrote:
    > > >
    > > > * Oleg Nesterov <oleg@redhat.com> wrote:
    > > >
    > > > > [...] The patch above looks "obviously safe", but perhaps I am
    > > > > paranoid too much...
    > > >
    > > > IMHO your hack above isn't really acceptable, even for a backport.
    > > > So lets test the patch below (assuming it's the right thing to do)
    > > > and move forward?
    > >
    > > OK, but please note that this patch is not beckportable. If you think
    > > that -stable doesn't need this fix, then I agree.
    > >
    > > If the caller is do_device_not_available(), then we can not enable
    > > irqs before __thread_fpu_begin() + restore_fpu_checking().
    > >
    > > 1. Preemption in between can destroy ->fpu.state initialized by
    > > fpu_finit(), __switch_to() will save the live (wrong) FPU state
    > > again.
    > >
    > > 2. kernel_fpu_begin() from irq right after __thread_fpu_begin() is
    > > not nice too. It will do __save_init_fpu() and this overwrites
    > > ->fpu.state too.
    > >
    > > Starting from v4.0 it does kernel_fpu_disable(), but the older kernels
    > > do not.
    > >
    > > Ingo, this code is really horrible and fragile. We need to cleanup it
    > > step-by-step, imho.
    >
    > How about the patch from David Vrabel? That seems to solve the
    > irq-disable problem too, right?

    I wasn't cc'ed, I guess you mean

    [PATCHv4] x86, fpu: remove the logic of non-eager fpu mem allocation at the first usage
    http://marc.info/?l=linux-kernel&m=142564237705311&w=2

    Not sure I understand it correctly after the first quick look, but

    1. It conflicts with the recent changes in tip/x86/fpu

    2. fpu_ini() initializes current->thread.fpu.state. This looks unneeded,
    the kernel threads no longer have FPU context and do not abuse CPU.

    3. I can be easily wrong, but it looks buggy... Note that
    arch_dup_task_struct() doesn't allocate child->fpu.state if
    !tsk_used_math(parent).

    Add David...

    No, I do not think this patch is a good idea. Perhaps I am wrong, but I
    think we need other changes. And they should start from init_fpu().

    Oleg.



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