Thread synchronization mechanisms (Locks)

To share it with you and making sure I won’t forget, I present you with (some of) the different thread synchronization mechanisms known to mankind. Most of the mechanisms are used to lock operations or data, some aren’t. You know, those nifty little things that ought to help you with thread hell.

The semaphore is the cournerstone of locking – often other locks are implemented using one or more semaphores. A semaphore is a simple integer. The initial value of this integer is the amount of threads that can attain (aka. lock, down, enter, pass …) the semaphore before it will block incoming threads – a blocked thread will have to wait until a thread that has attained the semaphore releases (aka. unlocks, ups, leaves …) the semaphore.

The implementation of a semaphore is an atomic integer on most systems. When a thread wants to attain the semaphore it will check whether the value is greater than 0. If it is it will decrement the value and the semaphore has been attained. If the value is 0 it means that there are already as much threads that have attained the lock as the semaphore allows – and the thread will wait a while and try again. If a thread releases the semaphore it will increment the value.

You can compare a semaphore with a dining table in a mess hall at which only a certain amount of people can dine. People will either have to wait for someone to finish and stand up, or they’ll have to look for some other place.

The mutual exclusion lock is the lock you’ll be most familiar with and is often just referred to as a lock, because of its popularity. A mutex is either locked or unlocked. Only one thread can hold the lock – all others will have to wait for that other thread to release the mutex. The mutex is often used to assure mutual exclusive access to a certain operation or piece of data.

A mutex is simply just implemented as a semphore with a starting value of 1. The first thread that would attain the mutex would decrement the value to 0 and block all other incoming threads – until it releases the lock.

You can compare a mutex with the door lock on your restroom, it will help you make sure that you got some privacy.

An event will block all incoming threads, until it is signalled to release a thread or all of them at the same time. Events are quite useful when dealing with situations when you want to queue threads before they are allowed to pass. Events aren’t locks that you lock and after that have to unlock. A thread just waits for an event, and doesn’t have to ‘unlock’ it afterwards.

An event is usually also implemented using a semaphore. This time the default value of the semaphore would be 0. If a thread wants to pass the event it’ll try to decrement the semaphore`s value, unless it is 0 in which case it’ll wait, just as you would normally lock a semaphore. If the event is signalled to release a thread it will increment the semaphore`s value – one of the waiting threads will notice that the value isn’t 0 anymore, decrement it to 0 again and pass. If the event is signalled to release all waiting threads it’ll keep on incrementing the value until all threads have passed. Usually there is a mutex alongside the semaphore to make sure that a thread can’t slip in, while all threads are being released, and be released to soon.

You can compare an event with a (chaotic) queue in front of a fun park attraction. People have to wait until the operator of the attraction tells one of them to continue, or tells all of them to continue at once. (the attraction could be nearly empty with enough room for all of them).

A gate is a flavour of an event. It blocks incoming threads just like an event, and lets all of them through if signalled. The difference it that it won’t close again – once signalled (‘opened’) it’ll allow all threads through until it is explicitly closed. It also can’t let threads through one by one.

A gate is usually implemented using a semaphore, with the initial value of 0 so it will block incoming threads. Each incoming thread tries to attain the semaphore as is done too with the event – but when a semaphore lock is attained it’ll directly release the semaphore so it’ll be available for another thread waiting to pass too.

You can compare a gate with a … gate. Or a draw bridge if you like. If it’s closed none can pass. If it’s opened everyone can pass. Very straightforward.

A barrier blocks all incoming thread until there are a certain number of threads. In that case the barrier will release all those threads, but will remain to block incoming threads (until they have reached the specified count together too, etc). A barrier is useful to make sure that a set of threads all start something at the same time and maybe even more important aren’t doing what they did before.

A barrier is usually implemented as a custom event, that will have a waiter-count-field (and a mutex to synchronize access to it), which will auto release all threads when the waiter count has reached the specified number.

You can compare a barrier with watter drops from a tap. The drop won’t fall until there is a specific amount of water that causes it to fall.

So far for the thread synchronization mechanisms today. Tomorrow I’ll maybe talk about the hassles of implementing them yourself.

Some credit goes to Kaja, who told me about all these locks in the first place :-).