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

From: Cong Wang
Date: Thu Feb 02 2012 - 09:27:50 EST

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.

To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to majordomo@xxxxxxxxxxxxxxx
More majordomo info at
Please read the FAQ at