linux-kernel - Re: [PATCH 1/2][resend] pid: tighten pidmap spinlock critical section by removing kfree()

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Message-Id: <20091204143802.17f3fded.akpm@linux-foundation.org>
Date:	Fri, 4 Dec 2009 14:38:02 -0800
From:	Andrew Morton <akpm@...ux-foundation.org>
To:	André Goddard Rosa <andre.goddard@...il.com>
Cc:	"linux list" <linux-kernel@...r.kernel.org>,
	Pekka Enberg <penberg@...helsinki.fi>
Subject: Re: [PATCH 1/2][resend] pid: tighten pidmap spinlock critical
 section by removing kfree()

On Thu,  3 Dec 2009 12:53:37 -0200
Andr__ Goddard Rosa <andre.goddard@...il.com> wrote:

> Avoid calling kfree() under pidmap spinlock, calling it afterwards.
> 
> Normally kfree() is fast, but sometimes it can be slow, so avoid
> calling it under the spinlock if we can do it.
> 
> Signed-off-by: Andr__ Goddard Rosa <andre.goddard@...il.com>
> cc: Pekka Enberg <penberg@...helsinki.fi>
> ---
>  kernel/pid.c |    7 ++++---
>  1 files changed, 4 insertions(+), 3 deletions(-)
> 
> diff --git a/kernel/pid.c b/kernel/pid.c
> index d3f722d..55fd590 100644
> --- a/kernel/pid.c
> +++ b/kernel/pid.c
> @@ -141,11 +141,12 @@ static int alloc_pidmap(struct pid_namespace *pid_ns)
>  			 * installing it:
>  			 */
>  			spin_lock_irq(&pidmap_lock);
> -			if (map->page)
> -				kfree(page);
> -			else
> +			if (!map->page) {
>  				map->page = page;
> +				page = NULL;
> +			}
>  			spin_unlock_irq(&pidmap_lock);
> +			kfree(page);
>  			if (unlikely(!map->page))
>  				break;
>  		}

um, OK, but the chances of that kfree() actually being executed are
very small.

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