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From: Arnd Bergmann <arnd@arndb.de>
To: Cong Wang <xiyou.wangcong@gmail.com>
Cc: linux-kernel@vger.kernel.org,
	Andrew Morton <akpm@linux-foundation.org>,
	Prarit Bhargava <prarit@redhat.com>,
	"Greg Kroah-Hartman" <greg@kroah.com>,
	Dave Young <dyoung@redhat.com>
Subject: Re: [PATCH 1/2] lkdtm: use atomic_t to replace count_lock
Date: Wed, 1 Feb 2012 15:27:35 +0000	[thread overview]
Message-ID: <201202011527.35366.arnd@arndb.de> (raw)
In-Reply-To: <1328079501-24746-1-git-send-email-xiyou.wangcong@gmail.com>

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

  parent reply	other threads:[~2012-02-01 15:27 UTC|newest]

Thread overview: 7+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2012-02-01  6:58 [PATCH 1/2] lkdtm: use atomic_t to replace count_lock Cong Wang
2012-02-01  6:58 ` [PATCH 2/2] lkdtm: avoid calling sleeping functions in interrupt context Cong Wang
2012-02-01 15:27 ` Arnd Bergmann [this message]
2012-02-02 13:33   ` [PATCH 1/2] lkdtm: use atomic_t to replace count_lock Cong Wang
2012-02-02 13:44     ` Arnd Bergmann
2012-02-02 14:27       ` Cong Wang
2012-02-02 14:55         ` Arnd Bergmann

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