`compare_exchange` and spinlock implementation

[onur-ozkan]

compare_exchange is quite heavy operation which should never spin in a loop over and over like here here on multi-threaded systems. Since it tries to have exclusive access to the desired memory location, the running threads have to try getting that access over and over. Which is obviously not an efficent way to do so.

There are many things can be done on this implementation to make it more efficent. Like instead of trying compare_exchange it can be replaced with the compare_exchange_weak function (this performs little bit better on some platforms and there is no problem with using it if it's running under the loop). But still, it should not be executed over and over since we don't want to make our threads wasting too much power for exclusive access.

An efficent way to use compare_exchange_weak(dummy code):

while x.compare_exchange_weak(true(current value), false(new value), ...).is_err() {
    while x.load() == false {} // Checking if the current value isn't true, so no need to execute compare_exchange_weak over and over
}

ToDos:

• compare_exchange usages should be checked and optimized.
• spinlock implementation needs to be optimized.

cc @artemii235 @sergeyboyko0791 @shamardy

[onur-ozkan]

PS: For x86 family(and probably some arm families specially modern ones if I am not mistaken) compare_exchange has single atomic instruction which reduces the overhead. But still heavier than just loading the value.