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    Subject[PATCH 1/4] x86/fpu: document user_fpu_begin()
    Currently user_fpu_begin() has a single caller and it is not clear that
    why do we actually need it, and why we should not worry about preemption
    right after preempt_enable().

    Signed-off-by: Oleg Nesterov <oleg@redhat.com>
    ---
    arch/x86/include/asm/fpu-internal.h | 4 +++-
    1 files changed, 3 insertions(+), 1 deletions(-)

    diff --git a/arch/x86/include/asm/fpu-internal.h b/arch/x86/include/asm/fpu-internal.h
    index 4bec98f..c615ae9 100644
    --- a/arch/x86/include/asm/fpu-internal.h
    +++ b/arch/x86/include/asm/fpu-internal.h
    @@ -464,7 +464,9 @@ static inline int restore_xstate_sig(void __user *buf, int ia32_frame)
    * Need to be preemption-safe.
    *
    * NOTE! user_fpu_begin() must be used only immediately before restoring
    - * it. This function does not do any save/restore on their own.
    + * it. This function does not do any save/restore on its own. In a lazy
    + * fpu mode this is just optimization to avoid a dna fault, the task can
    + * lose FPU right after preempt_enable().
    */
    static inline void user_fpu_begin(void)
    {
    --
    1.5.5.1



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