Will two overlapping mutexes lead to deadlock?

Question

Consider the following member functions. Will it lead to a deadlock?

void foo::insert(item_ptr item)
{
     lock_guard<mutex> lock(mu_);
     items_.insert(item);
} 

void foo::insert(vector<item_ptr> items)
{
      lock_guard<mutex> lock(mu_); // will this lead
                                                        // to deadlock?
      for(auto item:items)
            insert(item);
}

Can you please add more information? `foo:insert` is a syntax error. Which of the two are called in 2 separate threads, ... — Rakete1111, Sep 22 '16 at 15:26
Unless they both depend on each other, I wouldn't think so. Can you show us the full class? — kirbyfan64sos, Sep 22 '16 at 15:26
mutex is not re-entrant in conforming C++ library implementations. So yes. Use recursive_mutex instead. — Hans Passant, Sep 22 '16 at 15:28
@HansPassant Could use [`recursive_mutex`](http://en.cppreference.com/w/cpp/thread/recursive_mutex) though no? TBH I'm not sure why the vector part needs to lock since the single element overload does. — Borgleader, Sep 22 '16 at 15:30
If the mutex is nonrecursive, this [seems to be UB](http://stackoverflow.com/questions/11173532/why-is-locking-a-stdmutex-twice-undefined-behaviour). — Ami Tavory, Sep 22 '16 at 15:32
The two lock_guards are not overlapping. They would if the second insert called the first one, but this is not the case — Ionel POP, Sep 22 '16 at 16:12

score 1 · Accepted Answer · answered Sep 22 '16 at 16:31

If mu_ is a std::mutex, then you have a problem. A std::mutex does not support recursive locks and cannot be locked twice by the same thread. See: http://en.cppreference.com/w/cpp/thread/mutex

IF mu_ is a std::recursive_mutex, then you have no problem. A thread can lock a recursive mutex if it already has it locked. See http://en.cppreference.com/w/cpp/thread/recursive_mutex

Will two overlapping mutexes lead to deadlock?

1 Answers1