by Steven Rostedt

[permalink] [raw]

Subject: Re: Regression :-) Re: [GIT PULL RESEND] x86/jumpmplabel changes for v3.12-rc1

On Wed, 11 Sep 2013 15:55:50 -0400
Konrad Rzeszutek Wilk <[email protected]> wrote:

> On Wed, Sep 11, 2013 at 03:14:52PM -0400, Steven Rostedt wrote:
> > On Wed, 11 Sep 2013 14:56:54 -0400
> > Konrad Rzeszutek Wilk <[email protected]> wrote:
> >
> >
> > > > I'm looking to NAK your patch because it is obvious that the jump label
> > > > code isn't doing what you expect it to be doing. And it wasn't until my
> > >
> > > Actually it is OK. They need to be enabled before the SMP code kicks in.
> > >
> > > > checks were in place for you to notice.
> > >
> > > Any suggestion on how to resolve the crash?
> > >
> > > The PV spinlock code is OK (I think, I need to think hard about this) until
> > > the spinlocks start being used by multiple CPUs. At that point the
> > > jump_lables have to be in place - otherwise you will end with a spinlock
> > > going in a slowpath (patched over) and an kicker not using the slowpath
> > > and never kicking the waiter. Which ends with a hanged system.
> >
> > Note, a simple early_initcall() could do the trick. SMP isn't set up
> > until much further in the boot process.
> >
> > >
> > > Or simple said - jump labels have to be setup before we boot
> > > the other CPUs.
> >
> > Right, and initcalls() can easily serve that purpose.
> >
> > >
> > > This would affect the KVM guests as well, I think if the slowpath
> > > waiter was blocking on the VCPU (which I think it is doing now, but
> > > not entirely sure?)
> > >
> > > P.S.
> > > I am out on vacation tomorrow for a week. Boris (CC-ed here) can help.
> >
> > Your patch isn't wrong per say, but I'm hesitant to apply it because it
> > the result is different depending on whether JUMP_LABEL is configured
> > or not. Using any jump_label() calls before jump_label_init() is
> > called, is entering a gray area, and I think it should be avoided.
> >
> > This patch should solve it for you:
>
> And also the pv_lock_ops need to be set before alternative_asm
> code is called :-) (Called from check_bugs()).
>
> Otherwise you end up with some code still using the native slowpath
> kicker/waiter while the modules might be using the Xen variant.
>
> I knew that I forgot to mention something ..
>
> With that in mind and your patch I made this one:
>
> diff --git a/arch/x86/xen/spinlock.c b/arch/x86/xen/spinlock.c
> index 253f63f..d90628d 100644
> --- a/arch/x86/xen/spinlock.c
> +++ b/arch/x86/xen/spinlock.c
> @@ -267,11 +267,18 @@ void __init xen_init_spinlocks(void)
> return;
> }
>
> - static_key_slow_inc(&paravirt_ticketlocks_enabled);
> -
> pv_lock_ops.lock_spinning = PV_CALLEE_SAVE(xen_lock_spinning);
> pv_lock_ops.unlock_kick = xen_unlock_kick;
> }
> +static __init int xen_init_spinlocks_jump(void)
> +{
> + if (!xen_pvspin)
> + return 0;
> +
> + static_key_slow_inc(&paravirt_ticketlocks_enabled);
> + return 0;
> +}
> +early_initcall(xen_init_spinlocks_jump);

Can you write up a nice change log for this (include our discussion)
and then send it as a formal patch.

If it works for you, I'll give it an ack, and we can have hpa pull it
in and send it off to Linus.

Thanks!

-- Steve

>
> static __init int xen_parse_nopvspin(char *arg)
> {
>
> which seem to work.