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SubjectRe: [PATCH] Fix preemptible lazy mode bug
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On Thu, 2007-09-06 at 02:33 +1000, 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.
>
> ===
> Hoist per-cpu lazy_mode variable up into common code.
>
> VMI, Xen and lguest all track paravirt_lazy_mode individually, meaning
> we hand a "magic" value for set_lazy_mode() to say "flush if currently
> active".
>
> We can simplify the logic by hoisting this variable into common code.
>
> Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>
>
> ---
> arch/i386/kernel/vmi.c | 16 ++++------------
> arch/i386/xen/enlighten.c | 15 +++------------
> arch/i386/xen/multicalls.h | 2 +-
> arch/i386/xen/xen-ops.h | 7 -------
> drivers/lguest/lguest.c | 25 ++++++-------------------
> include/asm-i386/paravirt.h | 29 ++++++++++++++++-------------
> 6 files changed, 30 insertions(+), 64 deletions(-)
>
> diff -r 072a0b3924fb arch/i386/kernel/vmi.c
> --- a/arch/i386/kernel/vmi.c Thu Aug 30 04:47:43 2007 +1000
> +++ b/arch/i386/kernel/vmi.c Wed Sep 05 04:06:20 2007 +1000
> @@ -554,22 +554,14 @@ vmi_startup_ipi_hook(int phys_apicid, un
>
> static void vmi_set_lazy_mode(enum paravirt_lazy_mode mode)
> {
> - static DEFINE_PER_CPU(enum paravirt_lazy_mode, lazy_mode);
> -
> if (!vmi_ops.set_lazy_mode)
> return;
>
> /* Modes should never nest or overlap */
> - BUG_ON(__get_cpu_var(lazy_mode) && !(mode == PARAVIRT_LAZY_NONE ||
> - mode == PARAVIRT_LAZY_FLUSH));
> -
> - if (mode == PARAVIRT_LAZY_FLUSH) {
> - vmi_ops.set_lazy_mode(0);
> - vmi_ops.set_lazy_mode(__get_cpu_var(lazy_mode));
> - } else {
> - vmi_ops.set_lazy_mode(mode);
> - __get_cpu_var(lazy_mode) = mode;
> - }
> + BUG_ON(get_lazy_mode() && !(mode == PARAVIRT_LAZY_NONE |
> + | mode == PARAVIRT_LAZY_FLUSH));

That's a pretty strange line break.



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