x86-64, fpu: Disable preemption when using TS_USEDFPU

x86-64, fpu: Disable preemption when using TS_USEDFPU

Consolidates code and fixes the below race for 64-bit.

commit 9fa2f37bfeb798728241cc4a19578ce6e4258f25
Author: torvalds <torvalds>
Date:   Tue Sep 2 07:37:25 2003 +0000

    Be a lot more careful about TS_USEDFPU and preemption

    We had some races where we testecd (or set) TS_USEDFPU together
    with sequences that depended on the setting (like clearing or
    setting the TS flag in %cr0) and we could be preempted in between,
    which screws up the FPU state, since preemption will itself change
    USEDFPU and the TS flag.

    This makes it a lot more explicit: the "internal" low-level FPU
    functions ("__xxxx_fpu()") all require preemption to be disabled,
    and the exported "real" functions will make sure that is the case.

    One case - in __switch_to() - was switched to the non-preempt-safe
    internal version, since the scheduler itself has already disabled
    preemption.

    BKrev: 3f5448b5WRiQuyzAlbajs3qoQjSobw

Signed-off-by: Brian Gerst <brgerst@gmail.com>
Acked-by: Pekka Enberg <penberg@kernel.org>
Cc: Suresh Siddha <suresh.b.siddha@intel.com>
LKML-Reference: <1283563039-3466-6-git-send-email-brgerst@gmail.com>
Signed-off-by: H. Peter Anvin <hpa@linux.intel.com>