Re: [PATCH] pid: tighten pidmap_lock critical section
From: Pekka Enberg
Date: Sun Nov 22 2009 - 04:17:54 EST
Hi Andre,
On Sat, Nov 21, 2009 at 8:04 AM, André Goddard Rosa
<andre.goddard@xxxxxxxxx> 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@xxxxxxxxx>
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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