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

From: Arnd Bergmann
Date: Wed Feb 01 2012 - 10:27:38 EST


On Wednesday 01 February 2012, Cong Wang wrote:
> static void lkdtm_handler(void)
> {
> - unsigned long flags;
> -
> - spin_lock_irqsave(&count_lock, flags);
> - count--;
> printk(KERN_INFO "lkdtm: Crash point %s of type %s hit, trigger in %d rounds\n",
> - cp_name_to_str(cpoint), cp_type_to_str(cptype), count);
> + cp_name_to_str(cpoint), cp_type_to_str(cptype), atomic_dec_return(&count));
>
> - if (count == 0) {
> + if (!atomic_cmpxchg(&count, 0, cpoint_count))
> lkdtm_do_action(cptype);
> - count = cpoint_count;
> - }
> - spin_unlock_irqrestore(&count_lock, flags);
> }

This use is not atomic, you could have two threads doing atomic_dec_return
at the same time, and after that the value will be -1 so the atomic_cmpxchg
does not trigger.

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?

Arnd
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