Re: atomic use is not atomic?

[Lars-Peter Clausen <lars@metafoo.de>]

On 6/8/21 3:14 PM, Andy Shevchenko wrote:
> On Tue, Jun 8, 2021 at 3:44 PM Lars-Peter Clausen <lars@metafoo.de> wrote:
>> On 6/8/21 2:35 PM, Andy Shevchenko wrote:
>>> Hi!
>>>
>>> Can anybody explain this code [1] in terms of atomicity?
>>>
>>>     if (!atomic_read(&trig->use_count)) {
>>>       atomic_set(&trig->use_count, CONFIG_IIO_CONSUMERS_PER_TRIGGER);
>>>       ...
>>>     }
>>>
>>> AFAIU how atomics are supposed to work the above doesn't do anything
>>> atomically. I.o.w. use_count may be simple int with the same effect.
>>>
>> The operations here do not require atomic access, it is just operating
>> on a atomic type. The atomic access is the atomic_dec_and_test() in
>> iio_trigger_notify_done().
> So, between atomic_read and atomic_set somebody can call
> atomic_dec_and_test(), for example. Is it a problem to actually lose
> the value?
> Why are atomic types being used here when there is no atomicity guaranteed?

We know that if the value is zero that there are no more active 
consumers. Then we set the value to the number of consumers, and then 
launch the consumers, which will decrement the counter once they are done.