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 22:27:34 +0800 [thread overview]
Message-ID: <4F2A9D56.3080102@gmail.com> (raw)
In-Reply-To: <201202021344.19455.arnd@arndb.de>
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.
next prev parent reply other threads:[~2012-02-02 14:27 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 ` [PATCH 1/2] lkdtm: use atomic_t to replace count_lock Arnd Bergmann
2012-02-02 13:33 ` Cong Wang
2012-02-02 13:44 ` Arnd Bergmann
2012-02-02 14:27 ` Cong Wang [this message]
2012-02-02 14:55 ` Arnd Bergmann
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