Messages in this thread | | | Date | Thu, 02 Feb 2012 22:27:34 +0800 | From | Cong Wang <> | Subject | Re: [PATCH 1/2] lkdtm: use atomic_t to replace count_lock |
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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.
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