linux-kernel - Re: [PATCH v4 2/2] locking/pvqspinlock, hv: Enable PV qspinlock for Hyper-V

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Date:   Wed, 10 Oct 2018 07:50:26 +0200
From:   Ingo Molnar <mingo@...nel.org>
To:     Yi Sun <yi.y.sun@...ux.intel.com>
Cc:     linux-kernel@...r.kernel.org, x86@...nel.org, tglx@...utronix.de,
        jgross@...e.com, chao.p.peng@...el.com, chao.gao@...el.com,
        isaku.yamahata@...el.com, michael.h.kelley@...rosoft.com,
        tianyu.lan@...rosoft.com, "K. Y. Srinivasan" <kys@...rosoft.com>,
        Haiyang Zhang <haiyangz@...rosoft.com>,
        Stephen Hemminger <sthemmin@...rosoft.com>
Subject: Re: [PATCH v4 2/2] locking/pvqspinlock, hv: Enable PV qspinlock for
 Hyper-V


* Yi Sun <yi.y.sun@...ux.intel.com> wrote:

> On 18-10-09 12:54:27, Ingo Molnar wrote:
> > 
> > * Yi Sun <yi.y.sun@...ux.intel.com> wrote:
> > 
> > > Follow PV spinlock mechanism to implement the callback functions
> > > to allow the CPU idling and kicking operations on Hyper-V.
> > 
> > > +#if defined(CONFIG_SMP)
> > > +	smp_ops.smp_prepare_boot_cpu = hv_smp_prepare_boot_cpu;
> > > +#endif
> > 
> > What's wrong with using the common pattern of:
> > 
> > 	#ifdef CONFIG_SMP
> > 
> > ?
> 
> There is no difference between "#ifdef" and "#if defined". I just copied
> it from "hv_smp_prepare_boot_cpu()". Do you need me submit a fix patch
> after this set is merged?

It's equivalent code, my point was that '#ifdef CONFIG_XYZ' is the shorter, more canonical 
pattern we use in the kernel most of the time, it's shorter, easier to read. We only use 
defined() for longer preprocessor directive conditions, and it's a pattern for "there's 
something more complex here than a simple CONFIG_XYZ dependency".

Anyway, Thomas fixed it up in the latest iteration.

Thanks,

	Ingo