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

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Date:	Thu, 18 Oct 2012 13:41:08 +0900
From:	Kamezawa Hiroyuki <kamezawa.hiroyu@...fujitsu.com>
To:	Linus Torvalds <torvalds@...ux-foundation.org>
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

(2012/10/18 13:14), Linus Torvalds wrote:
> 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.
>

you're right, I shouldn't send an ugly patch. I'm sorry.
V2 uses suggested style, I think.

Regards,
-Kame


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