Is incr/decr atomic in multithread mode?

[dormando]

Steve Chu wrote:
> I'm sorry that my expression may confuse you. I try to make it more clear.
> My requirement is that clients to incr/decr a value must be atomic,
> that get from slabber and
> add 1 must be an atomic operation according to clients(php clients or
> libmemcached etc.). If two clients incr a value 1,  the value should
> be 3, not that B client overwrite the value, and the  final value is
> still 2. This operation in Non-thread is atmoic, because they are not
> in concurrent environment, and only one event is processed one time.
> 
> But now I found in multithread version it is not atomic, if put the
> item_get in add_delta, this may be done, but too many locks.
> 
> I wonder if you understand my meaning, anyway,
> 

I understand your meaning. It is actually atomic, if two threads hit 
that code at the same time, the final value will be 3, it will not be two.

The value in the slabber is updated *inside* of add_delta, so there's no 
way it could end up being 2 if two threads incr it from 1.

-Dormando