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From: Linus Torvalds <torvalds@linux-foundation.org>
Date: Wed, 17 Oct 2012 21:14:38 -0700
Message-ID: <CA+55aFy9uYifEQ20-tQXvFCYSqb2VG74XhM6ZofHJW_VuqWdZQ@mail.gmail.com>
Subject: Re: [patch for-3.7 v2] mm, mempolicy: avoid taking mutex inside
 spinlock when reading numa_maps
To: Kamezawa Hiroyuki <kamezawa.hiroyu@jp.fujitsu.com>
Cc: David Rientjes <rientjes@google.com>,
        Andrew Morton <akpm@linux-foundation.org>,
        Dave Jones <davej@redhat.com>,
        KOSAKI Motohiro <kosaki.motohiro@gmail.com>, bhutchings@solarflare.com,
        Konstantin Khlebnikov <khlebnikov@openvz.org>,
        Naoya Horiguchi <n-horiguchi@ah.jp.nec.com>,
        Hugh Dickins <hughd@google.com>, linux-kernel@vger.kernel.org,
        linux-mm@kvack.org
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Sender: linux-kernel-owner@vger.kernel.org
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On Wed, Oct 17, 2012 at 9:06 PM, Kamezawa Hiroyuki
<kamezawa.hiroyu@jp.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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