Messages in this thread |  | | | Date | Thu, 02 Feb 2012 21:33:40 +0800 | | From | Cong Wang <> | | Subject | Re: [PATCH 1/2] lkdtm: use atomic_t to replace count_lock |
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On 02/01/2012 11:27 PM, Arnd Bergmann wrote: > 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.
Yeah, simply combining two atomic operations is not atomic. :-/
> > 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.
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