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Simplify Code by Encapsulating Locks

Wednesday, 15 June 2011

Over on the Future Chips blog, Aater Suleman argues that while(1) can make parallel code better. Whilst I agree that the code using while(1) is simpler than the original in terms of analysing the lock patterns, it achieves this by testing the logical condition inside the while and using break. This is additional, unnecessary, complexity.

What is wanted instead is a way of encapsulating the locking, so that the loop logic is simple, and yet the lock logic is also clear.

Here is the original code from the blog post:

  //do any actual work in the iteration - Thanks to Caleb for this comment


The implication here is that check_condition() must be called with the lock held, but the lock need not be held for the actual iteration work. The code thus acquires and releases the mutex in two places, which is unnecessary duplication, and a potential source of errors — if the loop exits early then the lock may be released twice, for example.

Rather than moving the condition check into the loop to avoid this duplication, a better solution is to move the lock acquisition and release into the condition check:

bool atomic_check_condition()
  bool result=check_condition();
  return result;

  //do any actual work in the iteration - Thanks to Caleb for this comment

This gives us the best of both worlds: the lock is now held only across the check_condition() call, but the logic of the while loop is still clear.

If you're programming in C++, then the C++0x library allows us to make atomic_check_condition() even simpler by using lock_guard as in the code below, but extracting the function is always an improvement.

bool atomic_check_condition()
  std::lock_guard<mutex_type> guard(lock);
  return check_condition();

Posted by Anthony Williams
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Hi Anthony,

It is also possible to use for instead. Actually for is more powerful, so you can do something like that.

for (std::unique_lock<std::mutex> lk(m, std::defer_lock); lk.lock(), check_condition(); lk.unlock()) { //do any actual work in the iteration }
by TA at 09:54:44 on Wednesday, 15 June 2011

@TA: Your for statement has different semantics: the lock is only released at the end of each iteration, not immediately after checking the condition.

Even if the semantics were the same, I would still prefer encapsulating the lock in atomic_check_condition() --- it's clearer all round.

by Anthony Williams at 10:02:59 on Wednesday, 15 June 2011

Yes, that's right. lk.unlock() should be moved to the body, however making a function allows code reuse, so in most of cases I think that is more preferable solution.

by TA at 10:11:52 on Wednesday, 15 June 2011

I must be listening to Herb Sutter too much... This looks to me another place where lambdas can be used (assuming <code>atomic_check_condition</code> isn't used anywhere else.) <pre> while(([]() -> bool { std::lock_guard&lt;mutex_type&gt; guard(lock); return check_condition(); })()){ //do any actual work in the iteration } </pre>

by Motti at 07:39:33 on Thursday, 16 June 2011

@Motti: Yes, you could use a lambda, but I think a separate function would be clearer --- the name clearly specifies what it is doing, and reduces the clutter. On the other hand, a named lambda might be useful:

auto atomic_check_condition=[&](){std::lock_guard<mutex_type> guard(lock); return check_condition();};

while(atomic_check_condition()) { ... }

by Anthony Williams at 07:43:42 on Thursday, 16 June 2011

On one project I wrote a custom locking smart pointer whose operator->() returned a helper class whose constructor locked and destructor unlocked before and after its own operator->() was called. This meant that I could write code like this:

while (! server->stopped) { .... }

where server is an instance of said locking smart pointer. The code has the advantage of being clear and you cannot forget to lock or unlock (presuming that you can't access the underlying data directly). I was using the Poco libraries so this code uses a Poco::Mutex. The CONTENTION() macro is non-empty for testing purposes: it tries to lock the mutex and records in a file whether it succeeded or not along with __FILE__ and __LINE__. This allows me to see if the mutex is a contention hotspot or not (it wasn't, as I suspected).

namespace utils { template <typename T> class LockingPtr { T * ptr; Poco::Mutex mutex; public: class LockingPtrHelper { LockingPtr * parent; public: T * operator->() { return parent->ptr; }

explicit LockingPtrHelper(LockingPtr * p) : parent(p) { CONTENTION(parent->mutex).lock(); }

~LockingPtrHelper() { parent->mutex.unlock(); } };

friend class LockingPtrHelper; explicit LockingPtr(T * p = 0) : ptr(p) {} ~LockingPtr() { delete ptr; }

// Stop copying explicit LockingPtr(const LockingPtr & other); LockingPtr operator=(const LockingPtr & rhs);

LockingPtrHelper operator->() { return LockingPtrHelper(this); }

/// Swap pointers over. Use addresses of mutexes to enforce /// a global ordering on locking to avoid deadlocks. void swap(LockingPtr & other) { if (&mutex < &other.mutex) { CONTENTION(mutex).lock(); CONTENTION(other.mutex).lock(); } else { CONTENTION(other.mutex).lock(); CONTENTION(mutex).lock(); } std::swap(ptr, other.ptr);

if (&mutex < &other.mutex) { mutex.unlock(); other.mutex.unlock(); } else { other.mutex.unlock(); mutex.unlock(); } } }; }
by Hubert Matthews at 23:19:57 on Thursday, 16 June 2011

@Hubert: Yes, wrapping the mutex lock in a type like this is the next logical step after encapsulating it in a function.

An alternative to a locking pointer is to wrap the mutex and data together, so all accesses to the data lock the mutex. See my article at DDJ: http://drdobbs.com/cpp/225200269

by Anthony Williams at 07:13:01 on Friday, 17 June 2011

This is the usual process followed by all to have the gems and gold for the game that is so interesting to play.

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