Is there a C++ design pattern that implements a mechanism or mutex that controls the amount of time a thread can own a locked resource?

Question

I am looking for a way to guarantee that any time a thread locks a specific resource, it is forced to release that resource after a specific period of time (if it has not already released it). Envision a connection where you need to limit the amount of time any specific thread can own that connection for.

I envision this is how it could be used:

{
    std::lock_guard<std::TimeLimitedMutex> lock(this->myTimeLimitedMutex, timeout);
    try {
        // perform some operation with the resource that myTimeLimitedMutex guards. 
    }
    catch (MutexTimeoutException ex) {
        // perform cleanup
    }
}

I see that there is a timed_mutex that lets the program timeout if a lock cannot be acquired. I need the timeout to occur after the lock is acquired.

There are already some situations where you get a resource that can be taken away unexpectedly. For instance, a tcp sockets -- once a socket connection is made, code on each side needs to handle the case where the other side drops the connection.

I am looking for a pattern that handle types of resources that normally time out on their own, but when they don't, they need to be reset. This does not have to handle every type of resource.

Answer 1

This can't work, and it will never work. In other words, this can never be made. It goes against all concept of ownership and atomic transactions. Because when thread acquires the lock and implements two transactions in a row, it expects them to become atomically visible to outside word. In this scenario, it would be very possible that the transaction will be torn - first part of it will be performed, but the second will be not.

What's worse is that since the lock will be forcefully removed, the part-executed transaction will become visible to outside word, before the interrupted thread has any chance to roll-back.

This idea goes contrary to all school of multi-threaded thinking.

Answer 2

I support SergeyAs answer. Releasing a locked mutex after a timeout is a bad idea and cannot work. Mutex stands for mutual exclusion and this is a rock-hard contract which cannot be violated.

But you can do what you want:

Problem: You want to guarantee that your threads do not hold the mutex longer than a certain time T.

Solution: Never lock the mutex for longer than time T. Instead write your code so that the mutex is locked only for the absolutely necessary operations. It is always possible to give such a time T (modulo the uncertainties and limits given my a multitasking and multiuser operating system of course).

To achieve that (examples):

• Never do file I/O inside a locked section.
• Never call a system call while a mutex is locked.
• Avoid sorting a list while a mutex is locked (*).
• Avoid doing a slow operation on each element of a list while a mutex is locked (*).
• Avoid memory allocation/deallocation while a mutex is locked (*).

There are exceptions to these rules, but the general guideline is:

• Make your code slightly less optimal (e.g. do some redundant copying inside the critical section) to make the critical section as short as possible. This is good multithreading programming.

(*) These are just examples for operations where it is tempting to lock the entire list, do the operations and then unlock the list. Instead it is advisable to just take a local copy of the list and clear the original list while the mutex is locked, ideally by using the swap() operation offered by most STL containers. And then do the slow operation on the local copy outside of the critical section. This is not always possible but always worth considering. Sorting has square complexity in the worst case and usually needs random access to the entire list. It is useful to sort (a copy of) the list outside of the critical section and later check whether elements need to be added or removed. Memory allocations also have quite some complexity behind them, so massive memory allocations/deallocations should be avoided.

Answer 3

You can't do that with only C++.

If you are using a Posix system, it can be done. You'll have to trigger a SIGALARM signal that's only unmasked for the thread that'll timeout. In the signal handler, you'll have to set a flag and use longjmp to return to the thread code. In the thread code, on the setjmp position, you can only be called if the signal was triggered, thus you can throw the Timeout exception.

Please see this answer for how to do that.

Also, on linux, it seems you can directly throw from the signal handler (so no longjmp/setjmp here).

BTW, if I were you, I would code the opposite. Think about it: You want to tell a thread "hey, you're taking too long, so let's throw away all the (long) work you've done so far so I can make progress". Ideally, you should have your long thread be more cooperative, doing something like "I've done A of a ABCD task, let's release the mutex so other can progress on A. Then let's check if I can take it again to do B and so on." You probably want to be more fine grained (have more mutex on smaller objects, but make sure you're locking in the same order) or use RW locks (so that other threads can use the objects if you're not modifying them), etc...

Answer 4

Such an approach cannot be enforced because the holder of the mutex needs the opportunity to clean up anything which is left in an invalid state part way through the transaction. This can take an unknown arbitrary amount of time.

The typical approach is to release the lock when doing long tasks, and re-aquire it as needed. You have to manage this yourself as everyone will have a slightly different approach.

The only situation I know of where this sort of thing is accepted practice is at the kernel level, especially with respect to microcontrollers (which either have no kernel, or are all kernel, depending on who you ask). You can set an interrupt which modifies the call stack, so that when it is triggered it unwinds the particular operations you are interested in.

Answer 5

"Condition" variables can have timeouts. This allows you to wait until a thread voluntarily releases a resource (with notify_one() or notify_all()), but the wait itself will timeout after a specified fixed amount of time.

Examples in the Boost documentation for "conditions" might make this more clear.

If you want to force a release, you have to write the code which will force it though. This could be dangerous. The code written in C++ can be doing some pretty close-to-the-metal stuff. The resource could be accessing real hardware and it could be waiting on it to finish something. It may not be physically possible to end whatever the program is stuck on.

However, if it is possible, then you can handle it in the thread in which the wait() times out.