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

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Date:	Thu, 2 Feb 2012 14:55:38 +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 Thursday 02 February 2012, Cong Wang wrote:
> 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);

		    ^^^^^^^
I guess I meant "new = cmpxchg(...)" here, sorry.

> >>>        } 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.

No, you can just print the value of "old" in the above example,
which was atomically read.

	Arnd
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