linux-kernel - Re: [PATCH] md: avoid potential long delay under pers

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Message-ID: <20140926103620.7c397832@notabene.brown>
Date:	Fri, 26 Sep 2014 10:36:20 +1000
From:	NeilBrown <neilb@...e.de>
To:	Chao Yu <chao2.yu@...sung.com>
Cc:	linux-raid@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] md: avoid potential long delay under pers_lock

On Thu, 25 Sep 2014 15:28:34 +0800 Chao Yu <chao2.yu@...sung.com> wrote:

> printk may cause long time lapse if value of printk_delay in sysctl is
> configured large by user. If register_md_personality takes long time to print in
> spinlock pers_lock, we may encounter high CPU usage rate when there are other
> pers_lock competitors who may be blocked to spin.
> We can avoid this condition by moving printk out of coverage of pers_lock
> spinlock.
> 
> Signed-off-by: Chao Yu <chao2.yu@...sung.com>
> ---
>  drivers/md/md.c | 3 ++-
>  1 file changed, 2 insertions(+), 1 deletion(-)
> 
> diff --git a/drivers/md/md.c b/drivers/md/md.c
> index e02de05..5fcf215 100644
> --- a/drivers/md/md.c
> +++ b/drivers/md/md.c
> @@ -7200,9 +7200,10 @@ static const struct file_operations md_seq_fops = {
>  
>  int register_md_personality(struct md_personality *p)
>  {
> +	printk(KERN_INFO "md: %s personality registered for level %d\n",
> +						p->name, p->level);
>  	spin_lock(&pers_lock);
>  	list_add_tail(&p->list, &pers_list);
> -	printk(KERN_INFO "md: %s personality registered for level %d\n", p->name, p->level);
>  	spin_unlock(&pers_lock);
>  	return 0;
>  }

I'm not sure I see the pressing need for this - have you noticed actual
problems?
However it seems to make sense so I've applied it.

Thanks,
NeilBrown

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