Performing a pointer swap in a double-buffer multithread system

Question

When double-buffering data that's due to be shared between threads, I've used a system where one thread reads from one buffer, one thread reads from the other buffer and reads from the first buffer. The trouble is, how am I going to implement the pointer swap? Do I need to use a critical section? There's no Interlocked function available that will actually swap values. I can't have thread one reading from buffer one, then start reading from buffer two, in the middle of reading, that would be appcrash, even if the other thread didn't then begin writing to it.

I'm using native C++ on Windows in Visual Studio Ultimate 2010 RC.

Answer 1

Why can't you use InterlockedExchangePointer ?

edit: Ok, I get what you are saying now, IEP doesn't actually swap 2 live pointers with each other since it only takes a single value by reference.

Answer 2

Using critical sections is the accepted way of doing it. Just share a CRITICAL_SECTION object between all your threads and call EnterCriticalSection and LeaveCriticalSection on that object around your pointer manipulation/buffer reading/writing code. Try to finish your critical sections as soon as possible, leaving as much code outside the critical sections as possible.

Even if you use the double interlocked exchange trick, you still need a critical section or something to synchronize your threads, so might as well use it for this purpose too.

Answer 3

You have to build your own function to swap the pointers which uses a semaphore or critical section to control it. The same protection needs to be added to all users of pointers, since any code which reads a pointer which is in the midst of being modified is bad.

One way to manage this is to have all the pointer manipulation logic work under the protection of the lock.

Answer 4

See, I did originally design the threads so that they would be fully asynchronous and don't require any synchronizing in their regular operations. But, since I'm performing operations on a per-object basis in a thread pool, if a given object is unreadable because it's currently being synced, I can just do another while I'm waiting. In a sense, I can both wait and operate at the same time, since I have plenty of threads to go around.

Create two critical sections, one for each of the threads. While rendering, hold the render crit section. The other thread can still do what it likes to the other crit section though. Use TryEnterCriticalSection, and if it's held, then return false, and add the object in a list to be re-rendered later. This should allow us to keep rendering even if a given object is currently being updated. While updating, hold both crit sections. While doing game logic, hold the game logic crit section. If it's already held, that's no problem, because we have more threads than actual processors. So if this thread is blocked, then another thread will just use the CPU time and this doesn't need to be managed.

Answer 5

You haven't mentioned what your Windows platform limitations are, but if you don't need compatibility with older versions than Windows Server 2003, or Vista on the client side, you can use the InterlockedExchange64() function to exchange a 64 bit value. By packing two 32-bit pointers into a 64-bit pair structure, you can effectively swap two pointers.

There are the usual Interlocked* variantions on that; InterlockedExchangeAcquire64(), InterlockedCompareExchange64(), etc...

If you need to run on, say, XP, I'd go for a critical section. When the chance of contention is low, they perform quite well.

Answer 6

This sounds like a reader-writer-mutex type problem to me.

[ ... but I mostly do embedded development so this may make no sense for a Windows OS. Actually, in an embedded OS with a priority-based scheduler, you can do this without any synchronization mechanism at all, if you guarantee that the swap is atomic and only allow the lower-priority thread to swap the buffers. ]

Suppose you have two buffers, B1 and B2, and you have two threads, T1 and T2. It's OK if T1 is using B1 while T2 is using B2. By "using" I mean reading and/or writing the buffer. Then at some time, the buffers need to swap so that T1 is using B2 and T2 is using B1. The thing you have to be careful of is that the swap is done while neither thread is accessing its buffer.

Suppose you used just one simple mutex. T1 could acquire the mutex and use B1. If T2 wanted to use B2, it would have to wait for the mutex. When T1 completed, T2 would unblock and do its work with B2. If either thread (or some third-party thread) wanted to swap the buffers, it would also have to take the mutex. Thus, using just one mutex serializes access to the buffers -- not so good.

It might work better if you use a reader-writer mutex instead. T1 could acquire a read-lock on the mutex and use B1. T2 could also acquire a read-lock on the mutex and use B2. When one of those threads (or a third-party thread) decides it's time to swap the buffers, it would have to take a write-lock on the mutex. It won't be able to acquire the write-lock until there are no more read-locks. At that point, it can swap the buffer pointers, knowing that nobody is using either buffer because when there is a write-lock on the mutex, all attempts to read-lock will block.