Re: [PATCH 1/2] lkdtm: use atomic_t to replace count_lock

[Cong Wang]

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.
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