1.3 Unstructured Locks

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来自 Rice University 的课程

Concurrent Programming in Java

217 个评分

Rice University

Concurrent Programming in Java

217 个评分

课程 2（共 3 门，Specialization Parallel, Concurrent, and Distributed Programming in Java）

This course teaches learners (industry professionals and students) the fundamental concepts of concurrent programming in the context of Java 8. Concurrent programming enables developers to efficiently and correctly mediate the use of shared resources in parallel programs. By the end of this course, you will learn how to use basic concurrency constructs in Java such as threads, locks, critical sections, atomic variables, isolation, actors, optimistic concurrency and concurrent collections, as well as their theoretical foundations (e.g., progress guarantees, deadlock, livelock, starvation, linearizability). Why take this course? • It is important for you to be aware of the theoretical foundations of concurrency to avoid common but subtle programming errors. • Java 8 has modernized many of the concurrency constructs since the early days of threads and locks. • During the course, you will have online access to the instructor and mentors to get individualized answers to your questions posted on the forums. • Each of the four modules in the course includes an assigned mini-project that will provide you with the necessary hands-on experience to use the concepts learned in the course on your own, after the course ends. The desired learning outcomes of this course are as follows: • Concurrency theory: progress guarantees, deadlock, livelock, starvation, linearizability • Use of threads and structured/unstructured locks in Java • Atomic variables and isolation • Optimistic concurrency and concurrent collections in Java (e.g., concurrent queues, concurrent hashmaps) • Actor model in Java Mastery of these concepts will enable you to immediately apply them in the context of concurrent Java programs, and will also help you master other concurrent programming system that you may encounter in the future (e.g., POSIX threads, .NET threads).

从本节课中

Threads and Locks

In this module, we will learn about threads and locks, which have served as primitive building blocks for concurrent programming for over five decades. All computing platforms today include some form of support for threads and locks, and make them available for use by developers in a wide range of programming languages. We will learn how threads can be created, joined, and synchronized using structured (e.g., synchronized statements/methods) and unstructured (e.g., java.util.concurrent libraries) locks in Java. We will also learn about new classes of bugs that can arise when concurrent programs need to access shared resources. These bugs are referred to as violations of liveness/progress guarantees, and include deadlock, livelock, and starvation. We will conclude this module by studying different solutions to the classical "Dining Philosophers" problem, and use these solutions to illustrate instances of deadlock, livelock and starvation.

1.1 Threads5:11

1.2 Structured Locks7:49

1.3 Unstructured Locks7:32

1.4 Liveness7:43

1.5 Dining Philosophers8:05

与讲师见面

Vivek Sarkar
Professor
Department of Computer Science

Now that we've learned about structured locks,

let's look at a more general case of unstructured locks.

Consider a linked list example,

suppose we have a linked list with nodes N1, N2, N3, N4.

And we want a computation where multiple threads

can work on this linked list where they will first work with N1,

N2 then work with N2, N3.

And then work with N3, N4, and so on.

Now how do we set this up?

We know that we'll need to use locks and in the case of unstructured locks,

what we have to do is we have to allocate lock objects for each node.

So column L1, L2, L3.

You don't get these automatically as in the case of the synchonized construct and

structured locks, you have to allocate them.

But once you've allocated them you can perform explicit calls to lock.

You can say lock L1 and then you lock L2.

So at this point after you have locked L1 and L2,

you have access to both nodes N1 and N2 and the thread can work with N1, N2.

At this point it can unlock L1

because it no longer needs to work with node N1 and

lock N3, while acquiring the lock for L3.

It can work with N2 and N3, then it needs to unlock L2,

lock L4, and so on.

So this pattern is called hand-over-hand locking because

at any point a thread needs to lock two nodes, but

the nesting of the lock and unlock is very different from the synchronized case.

So for example if you see over here we have

L2 acquired and released over here with lock and unlock of L2.

And if we had filled this out further we would have had an unlock of L3 over here.

And the lock and unlock of L3 would appear in this interval.

What you notice about these two intervals in the sequence of code is they are not

nested.

With structured locks all the synchronization had to be nested.

With unstructured locks, we get to make explicit calls to lock and unlock, and

we can create these new general patterns like hand-over-hand locking.

There's another extension to unstructured locks which can be very helpful.

Sometimes the thread doesn't just want to wait indefinitely at a lock.

It may want to do something else if the lock is unavailable.

So, there's an option called tryLock, and if we said tryLock for

L1 we get a return value, let's say a Boolean called "success".

And then we could do something like, if success,

do all this code, else, do something else.

So this give's us even more flexibility because by calling tryLock

the threat has the option to do other pieces of work when a lock is unavailable.

Whereas in structured locking the thread is blocked

at the synchronized point waiting to acquire that lock.

And finally there is another refinement that you can get with unstructured locks,

and that is to differentiate readers and writers.

So there is something called a read-write lock, and I will use

a simple data structure over here, let's say that we have an array over here.

And we want to do two kinds of operations on the array, we want to do a search.

Maybe it's searching for some item in the array, and we want to do an update.

Maybe at index I, the element Y.

So over here, the array A sub I might be assigned Y.

And over here, we are searching for

different A sub Is to see whether they're equal to X.

Now, we cannot let multiple threads perform multiple search and

updates in parallel without synchronization.

So with what we've learned so far we would probably use a synchronized construct on

the array A or we would use unstructured locks with calls to lock and unlock.

But there's an observation over here.

This search operation is only performing read operations, this double equals

is only reading the value of A sub I whereas the update is performing a write.

So there's a further extension to unstructured locks where

you can actually say that you want, if we allocate for the array,

a read-write

lock L, this is the read-write lock that's protecting the array A.

Then we can say we want a read-lock on

L in the beginning, and then we can do an unlock on L when we're done.

And this would be the code in the search, when we're only reading the array.

And for update, we can do a write-lock

with L, the same lock L and do an unlock.

So what's the difference?

If you had a regular lock and unlock, only one thread would enter search at a time,

with a read-lock there's a special semantics that recognizes the fact

that multiple threads can read the same value without changing the outcome.

So multiple threads are allowed to acquire a read-lock but

only one thread is allowed to acquire a write-lock.

And if one thread has a write-lock no other thread can get a read-lock.

So the two modes are one or more threads holds a read-lock or

exactly one thread, or at most one thread holds a write-lock.

So now we see that with unstructured locks we go

a major step beyond structured locks.

And there may be certain kinds of concurrent programming where you need

the extra power of unstructured locks such as hand-over-hand locking or

the try-lock, where the thread wants to do some other operations or read-write locks.

So you can try out these primitives for

more sophisticated forms of concurrent programming.

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