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Message-ID: <4F2A9D56.3080102@gmail.com>
Date: Thu, 02 Feb 2012 22:27:34 +0800
From: Cong Wang <xiyou.wangcong@gmail.com>
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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
References: <1328079501-24746-1-git-send-email-xiyou.wangcong@gmail.com> <201202011527.35366.arnd@arndb.de> <4F2A90B4.1050709@gmail.com> <201202021344.19455.arnd@arndb.de>
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On 02/02/2012 09:44 PM, Arnd Bergmann wrote:
> On Thursday 02 February 2012, Cong Wang wrote:
>>> 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.
>
> Yes, it's one or the other: If you use the cmpxchg loop, you don't need a
> spinlock and vice versa.
>

The cmpxchg loop is for comparing and assigning to 'count', but still 
there is a printk() above that needs to read 'count'. Combining these 
two operations means we have to use a spinlock, correct? Because there 
is a chance that another process could change 'count' in between.

Thanks.