linux-kernel - Re: [PATCH] Fix preemptible lazy mode bug

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Date:	Wed, 05 Sep 2007 10:05:09 -0700
From:	Zachary Amsden <zach@...are.com>
To:	Rusty Russell <rusty@...tcorp.com.au>
Cc:	Jeremy Fitzhardinge <jeremy@...p.org>,
	Linus Torvalds <torvalds@...l.org>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	Andrew Morton <akpm@...l.org>,
	Chris Wright <chrisw@...s-sol.org>, stable@...nel.org,
	Virtualization Mailing List <virtualization@...ts.osdl.org>,
	Andi Kleen <ak@...e.de>,
	Anthony Liguori <anthony@...emonkey.ws>
Subject: Re: [PATCH] Fix preemptible lazy mode bug

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@...tcorp.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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