From: Cong Wang <xiyou.wangcong@gmail.com>
To: Arnd Bergmann <arnd@arndb.de>
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: Thu, 02 Feb 2012 21:33:40 +0800 [thread overview]
Message-ID: <4F2A90B4.1050709@gmail.com> (raw)
In-Reply-To: <201202011527.35366.arnd@arndb.de>
On 02/01/2012 11:27 PM, Arnd Bergmann wrote:
> 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.
Yeah, simply combining two atomic operations is not atomic. :-/
>
> 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?
If we still need spinlock, I think we don't need to bother atomic_t at all.
next prev parent reply other threads:[~2012-02-02 13:34 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 ` [PATCH 1/2] lkdtm: use atomic_t to replace count_lock Arnd Bergmann
2012-02-02 13:33 ` Cong Wang [this message]
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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