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POfine-grained locking queue in c++
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  1. POfine-grained locking queue in c++
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    copied!<p>Here's a fine-grained locking queue introduced by Anthony Williams in chapter 6.2.3 C++ Concurrency in Action.</p> <pre><code>/* pop only need lock head_mutex and a small section of tail_mutex,push only need tail_mutex mutex.maximum container concurrency. */ template&lt;typename T&gt; class threadsafe_queue { private: struct node { std::shared_ptr&lt;T&gt; data; std::unique_ptr&lt;node&gt; next; } std::mutex head_mutex; //when change the head lock it. std::unique_ptr&lt;node&gt; head; std::mutex tail_mutex; //when change the tail lock it. node* tail; std::condition_variable data_cond; node* get_tail() { std::lock_guard&lt;std::mutex&gt; tail_lock(tail_mutex); return tail; } public: /* create a dummy node */ threadsafe_queue(): head(new node),tail(head.get()) {} std::shared_ptr&lt;T&gt; wait_and_pop() { std::unique_lock&lt;std::mutex&gt; head_lock; data_cond.wait(head_lock,[&amp;]{return head.get()!=get_tail();}); //#1 std::unique_ptr&lt;node&gt; old_head=std::move(head); head=std::move(old_head-&gt;next); return old_head; } void push(T new_value) { std::shared_ptr&lt;T&gt; new_data( std::make_shared&lt;T&gt;(std::move(new_value))); std::unique_ptr&lt;node&gt; p(new node); { std::lock_guard&lt;std::mutex&gt; tail_lock(tail_mutex); tail-&gt;data=new_data; node* const new_tail=p.get(); tail-&gt;next=std::move(p); tail=new_tail; } data_cond.notify_one(); } } </code></pre> <p>Here's the situation: There are two threads (<code>thread1</code> and <code>thread2</code>). <code>thread1</code> is doing a <code>wait_and_pop</code> and <code>thread2</code> is doing a <code>push</code>. The queue is empty.</p> <p><code>thread1</code> is in #2, had already checked <code>head.get()!=get_tail()</code> before <code>data_cond.wait()</code>. At this time its CPU period had run out. <code>thread2</code> begins.</p> <p><code>thread2</code> finished the <code>push</code> function and did <code>data_cond.notify_one()</code>. <code>thread1</code> begins again.</p> <p>Now <code>thread1</code> begins <code>data_cond.wait()</code>, but it waits forever.</p> <p>Would this situation possibly happen ?If so, how to get this container fixed ?</p>
 

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