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    SubjectRe: [PATCH] Fix preemptible lazy mode bug
    Rusty Russell wrote:
    > On Tue, 2007-09-04 at 14:42 +0100, Jeremy Fitzhardinge wrote:
    >
    >> Rusty Russell wrote:
    >>
    >>> static inline void arch_flush_lazy_mmu_mode(void)
    >>> {
    >>> - PVOP_VCALL1(set_lazy_mode, PARAVIRT_LAZY_FLUSH);
    >>> + if (unlikely(__get_cpu_var(paravirt_lazy_mode) == PARAVIRT_LAZY_MMU))
    >>> + arch_leave_lazy_mmu_mode();
    >>> }
    >>>
    >>>
    >> This changes the semantics a bit; previously "flush" would flush
    >> anything pending but leave us in lazy mode. This just drops lazymode
    >> altogether?
    >>
    >> I guess if we assume that flushing is a rare event then its OK, but I
    >> think the name's a bit misleading. How does it differ from plain
    >> arch_leave_lazy_mmu_mode()?
    >>
    >
    > Whether it's likely or unlikely to be in lazy mode, basically. But
    > you're right, this should be folded, since we don't want to "leave" lazy
    > mode twice.
    >

    Hm, I think there's still a problem here. In the current code, you can
    legitimately flush lazy mode with preemption enabled (ie, there's no
    lazy mode currently active), but it's always a bug to enable/disable
    lazy mode with preemption enabled. Certainly enabling lazy mode with
    preemption enabled is always a bug, but you could make disable
    preempt-safe (and the bug checking should be in the common code).

    J
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