linux-kernel - Re: [PATCH 1/2] lkdtm: use atomic_t to replace count

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Message-Id: <201202011527.35366.arnd@arndb.de>
Date:	Wed, 1 Feb 2012 15:27:35 +0000
From:	Arnd Bergmann <arnd@...db.de>
To:	Cong Wang <xiyou.wangcong@...il.com>
Cc:	linux-kernel@...r.kernel.org,
	Andrew Morton <akpm@...ux-foundation.org>,
	Prarit Bhargava <prarit@...hat.com>,
	"Greg Kroah-Hartman" <greg@...ah.com>,
	Dave Young <dyoung@...hat.com>
Subject: Re: [PATCH 1/2] lkdtm: use atomic_t to replace count_lock

On Wednesday 01 February 2012, Cong Wang wrote:
>  static void lkdtm_handler(void)
>  {
> -       unsigned long flags;
> -
> -       spin_lock_irqsave(&count_lock, flags);
> -       count--;
>         printk(KERN_INFO "lkdtm: Crash point %s of type %s hit, trigger in %d rounds\n",
> -                       cp_name_to_str(cpoint), cp_type_to_str(cptype), count);
> +                       cp_name_to_str(cpoint), cp_type_to_str(cptype), atomic_dec_return(&count));
>  
> -       if (count == 0) {
> +       if (!atomic_cmpxchg(&count, 0, cpoint_count))
>                 lkdtm_do_action(cptype);
> -               count = cpoint_count;
> -       }
> -       spin_unlock_irqrestore(&count_lock, flags);
>  }

This use is not atomic, you could have two threads doing atomic_dec_return
at the same time, and after that the value will be -1 so the atomic_cmpxchg
does not trigger.

In order to have an atomic here, you have to use a loop around
atomic_cmpxchg, like


	int old, new;
	old = atomic_read(&count);
	do {
		new = old ? old - 1 : cpoint_count;
		old = cmpxchg(&count, old, new);
	} while (old != new);

I suppose you could also just keep the spinlock and move lkdtm_do_action()
outside of it?

	Arnd
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