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    Subject[PATCH 3/5 v2] x86: Reset the debug_stack update counter
    From: Steven Rostedt <srostedt@redhat.com>

    When an NMI goes off and it sees that it preempted the debug stack,
    to keep the debug stack safe, it changes the IDT to point to one that
    does not modify the stack on breakpoint (to allow breakpoints in NMIs).

    But the variable that gets set to know to undo it on exit never gets
    cleared on exit. Thus every NMI will reset it on exit the first time
    it is done even if it does not need to be reset.

    [ Added H. Peter Anvin's suggestion to use this_cpu_read/write ]

    Cc: <stable@vger.kernel.org> # v3.3
    Signed-off-by: Steven Rostedt <rostedt@goodmis.org>
    ---
    arch/x86/kernel/nmi.c | 6 ++++--
    1 file changed, 4 insertions(+), 2 deletions(-)

    diff --git a/arch/x86/kernel/nmi.c b/arch/x86/kernel/nmi.c
    index 9087527..a0b2f84 100644
    --- a/arch/x86/kernel/nmi.c
    +++ b/arch/x86/kernel/nmi.c
    @@ -444,14 +444,16 @@ static inline void nmi_nesting_preprocess(struct pt_regs *regs)
    */
    if (unlikely(is_debug_stack(regs->sp))) {
    debug_stack_set_zero();
    - __get_cpu_var(update_debug_stack) = 1;
    + this_cpu_write(update_debug_stack, 1);
    }
    }

    static inline void nmi_nesting_postprocess(void)
    {
    - if (unlikely(__get_cpu_var(update_debug_stack)))
    + if (unlikely(this_cpu_read(update_debug_stack))) {
    debug_stack_reset();
    + this_cpu_write(update_debug_stack, 0);
    + }
    }
    #endif

    --
    1.7.10

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