linux-kernel - Re: [PATCH] pid: tighten pidmap

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Message-ID: <84144f020911220117p5c4720e0g58587b97efdbb46b@mail.gmail.com>
Date:	Sun, 22 Nov 2009 11:17:45 +0200
From:	Pekka Enberg <penberg@...helsinki.fi>
To:	André Goddard Rosa <andre.goddard@...il.com>
Cc:	Andrew Morton <akpm@...ux-foundation.org>,
	Catalin Marinas <catalin.marinas@....com>,
	Oleg Nesterov <oleg@...hat.com>, Jiri Kosina <jkosina@...e.cz>,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH] pid: tighten pidmap_lock critical section

Hi Andre,

On Sat, Nov 21, 2009 at 8:04 AM, André Goddard Rosa
<andre.goddard@...il.com> wrote:
> Avoid calling kfree() under pidmap_lock and doing unnecessary work.
> It doesn't change behavior.
>
> It decreases code size by 16 bytes on my gcc 4.4.1 on Core 2:
>   text    data     bss     dec     hex filename
>   4314    2216       8    6538    198a kernel/pid.o-BEFORE
>   4298    2216       8    6522    197a kernel/pid.o-AFTER
>
> Signed-off-by: André Goddard Rosa <andre.goddard@...il.com>

This patch is doing a lot more than the changelog above says it does.
What exactly is the purpose of this patch? What's the upside?

> ---
>  kernel/pid.c |   16 ++++++++--------
>  1 files changed, 8 insertions(+), 8 deletions(-)
>
> diff --git a/kernel/pid.c b/kernel/pid.c
> index d3f722d..ec06912 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);

OK, maybe. The upside seem rather small and the resulting code is IMHO
slightly less readable.

>                        if (unlikely(!map->page))
>                                break;
>                }
> @@ -225,11 +226,11 @@ static void delayed_put_pid(struct rcu_head *rhp)
>  void free_pid(struct pid *pid)
>  {
>        /* We can be called with write_lock_irq(&tasklist_lock) held */
> -       int i;
> +       int i = 0;
>        unsigned long flags;
>
>        spin_lock_irqsave(&pidmap_lock, flags);
> -       for (i = 0; i <= pid->level; i++)
> +       for ( ; i <= pid->level; i++)
>                hlist_del_rcu(&pid->numbers[i].pid_chain);
>        spin_unlock_irqrestore(&pidmap_lock, flags);

This has nothing to do with kfree(). AFAICT, it just obfuscates the
code as the initial assignment to zero is lost in the noise anyway.

> @@ -268,12 +269,11 @@ struct pid *alloc_pid(struct pid_namespace *ns)
>        for (type = 0; type < PIDTYPE_MAX; ++type)
>                INIT_HLIST_HEAD(&pid->tasks[type]);
>
> +       upid = pid->numbers + ns->level;
>        spin_lock_irq(&pidmap_lock);
> -       for (i = ns->level; i >= 0; i--) {
> -               upid = &pid->numbers[i];
> +       for ( ; upid >= pid->numbers; --upid)
>                hlist_add_head_rcu(&upid->pid_chain,
>                                &pid_hash[pid_hashfn(upid->nr, upid->ns)]);
> -       }
>        spin_unlock_irq(&pidmap_lock);

Again, this has nothing to do with kfree(). I suspect this is where
most of the 16 byte text savings come from. I'm not convinced it's
worth the hit in readability, though.

                        Pekka
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