linux-kernel - Re: [patch for-3.7 v2] mm, mempolicy: avoid taking mutex inside spinlock when reading numa

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Date:	Wed, 17 Oct 2012 21:14:38 -0700
From:	Linus Torvalds <torvalds@...ux-foundation.org>
To:	Kamezawa Hiroyuki <kamezawa.hiroyu@...fujitsu.com>
Cc:	David Rientjes <rientjes@...gle.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Dave Jones <davej@...hat.com>,
	KOSAKI Motohiro <kosaki.motohiro@...il.com>,
	bhutchings@...arflare.com,
	Konstantin Khlebnikov <khlebnikov@...nvz.org>,
	Naoya Horiguchi <n-horiguchi@...jp.nec.com>,
	Hugh Dickins <hughd@...gle.com>, linux-kernel@...r.kernel.org,
	linux-mm@...ck.org
Subject: Re: [patch for-3.7 v2] mm, mempolicy: avoid taking mutex inside
 spinlock when reading numa_maps

On Wed, Oct 17, 2012 at 9:06 PM, Kamezawa Hiroyuki
<kamezawa.hiroyu@...fujitsu.com> wrote:
>         if (vma && vma != priv->tail_vma) {
>                 struct mm_struct *mm = vma->vm_mm;
> +#ifdef CONFIG_NUMA
> +               task_lock(priv->task);
> +               __mpol_put(priv->task->mempolicy);
> +               task_unlock(priv->task);
> +#endif
>                 up_read(&mm->mmap_sem);
>                 mmput(mm);

Please don't put #ifdef's inside code. It makes things really ugly and
hard to read.

And that is *especially* true in this case, since there's a pattern to
all these things:

> +#ifdef CONFIG_NUMA
> +       task_lock(priv->task);
> +       mpol_get(priv->task->mempolicy);
> +       task_unlock(priv->task);
> +#endif

> +#ifdef CONFIG_NUMA
> +       task_lock(priv->task);
> +       __mpol_put(priv->task->mempolicy);
> +       task_unlock(priv->task);
> +#endif

it really sounds like what you want to do is to just abstract a
"numa_policy_get/put(priv)" operation.

So you could make it be something like

  #ifdef CONFIG_NUMA
  static inline numa_policy_get(struct proc_maps_private *priv)
  {
      task_lock(priv->task);
      mpol_get(priv->task->mempolicy);
      task_unlock(priv->task);
  }
  .. same for the "put" function ..
  #else
    #define numa_policy_get(priv) do { } while (0)
    #define numa_policy_put(priv) do { } while (0)
  #endif

and then you wouldn't have to have the #ifdef's in the middle of code,
and I think it will be more readable in general.

Sure, it is going to be a few more actual lines of patch, but there's
no duplicated code sequence, and the added lines are just the syntax
that makes it look better.

                Linus
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