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133	<article id="post-383" class="post-383 post type-post status-publish format-standard hentry category-c category-programming">
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136				<a href="http://barngoggles.com/mutex-diversion-from-atomics/" rel="bookmark">Mutex Diversion from Atomics</a>
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145			<p>I mentioned in the <a href="http://barngoggles.com/?p=341" target="_blank">memory re-ordering entry</a> that I did some disassembly of a mutex. I actually did two disassemblies, one with a lock guard and one without. Here&#8217;s what I found.</p>
146<p>I used this C++ code for the non-gaurded mutex, which successfully ensures that each thread has access to a shared resource without fear of another thread interfer

146ing</p>
147<pre class="EnlighterJSRAW" data-enlighter-language="null">...
148std::mutex mutex; //declared at global scope
149...
150mutex.lock(); //executed within a threaded function
151...
152mutex.unlock();
153...</pre>
154<p>The x86 for this is different in one major way from the x86 we saw in the primary post, and that&#8217;s because we are calling into the data segment here.</p>
155<p><a href="http://barngoggles.com/wp-content/uploads/2015/07/solveWithMutex_noLockGuard_disassembly1.png"><img class="alignnone size-full wp-image-399" src="http://barngoggles.com/wp-content/uploads/2015/07/solveWithMutex_noLockGuard_disassembly1.png" alt="solveWithMutex_noLockGuard_disassembly" width="715" height="394" srcset="http://barngoggles.com/wp-content/uploads/2015/07/solveWithMutex_noLockGuard_disassembly1-300x165.png 300w, http://barngoggles.com/wp-content/uploads/2015/07/solveWithMutex_noLockGuard_disassembly1-624x344.png 624w, http://barngoggles.com/wp-content/uploads/2015/07/solveWithMutex_noLockGuard_disassembly1.png 715w" sizes="(max-width: 715px) 100vw, 715px" /></a></p>
156<p>The code calls the _imp__Mtx_lock instruction then pops the last frame off the stack (that&#8217;s effectively what add esp, 4 does. There is actually a small <

156a href="http://stackoverflow.com/questions/5904018/pop-or-add-esp-4-what-is-the-difference" target="_blank">difference</a>.) and then checks if the return value is zero or not (error). If it&#8217;s zero then we jump down to the instructions we saw in the previous post (with a compiler reordering), otherwise we throw an exception. We see the same pattern of calling the function, checking the result and maybe throwing an exception in the __imp___Mtx_unlock section.</p>
157<p>If we look in the data segment we see the following for the mutex.</p>
158<p><a href="http://barngoggles.com/wp-content/uploads/2015/07/solveWithMutex_dataSegment_disassembly.png"><img class="alignnone size-full wp-image-396" src="http://barngoggles.com/wp-content/uploads/2015/07/solveWithMutex_dataSegment_disassembly.png" alt="solveWithMutex_dataSegment_disassembly" width="649" height="30" srcset="http://barngoggles.com/wp-content/uploads/2015/07/solveWithMutex_dataSegment_disassembly-300x14.png 300w, http://barngoggles.com/wp-content/uploads/2015/07/solveWithMutex_dataSegment_disassembly-624x29.png 624w, http://barngoggles.com/wp-content/uploads/2015/07/solveWithMutex_dataSegment_disassembly.png 649w" sizes="(max-width: 649px) 100vw, 649px" /></a></p>
159<p>It&#8217;s just a variable that&#8217;s cross referenced to our two threads, not terribly exciting. If we look at the extern segment we see both lock and unlock function calls. Just a refresher, extern is for declaring symbols which aren&#8217;t in the current assembly unit but are defined in some other unit that is going to be referenced by the current unit.</p>
160<p><a href="http://barngoggles.com/wp-content/uploads/2015/07/solveWithMutex_mutexExtern_disassembly.png"><img class="alignnone size-full wp-image-389" src="http://barngoggles.com/wp-content/uploads/2015/07/solveWithMutex_mutexExtern_disassembly.png" alt="solveWithMutex_mutexExtern_disassembly" width="591" height="48" srcset="http://barngoggles.com/wp-content/uploads/2015/07/solveWithMutex_mutexExtern_disassembly-300x24.png 300w, http://barngoggles.com/wp-content/uploads/2015/07/solveWithMutex_mutexExtern_disassembly.png 591w" sizes="(max-width: 591px) 100vw, 591px" /></a></p>
161<p>The mutex lock and unlock are kernel functions and this aside is meant to be a bit shorter than what it takes to do kernel disassembly. Let&#8217;s move on to the lock gaurded case. How do lock gaurds like this work.</p>
162<pre class="EnlighterJSRAW" data-enlighter-language="null">...
163std::mutex mutex;
164...
165std::lock_guard&lt;std::mutex&gt; lock(mutex);
166...</pre>
167<p>Notice that the above code doesn&#8217;t contain an unlock statement. This would be an error if we were just using the mutex&#8217;s lock method because the mutex would never unlock. Looking at the disassembly below we see that the mutex does unlock when the function finishes.</p>
168<p><a href="http://barngoggles.com/wp-content/uploads/2015/07/solveWithMutex_disassembly.png"><img class="alignnone size-full wp-image-388" src="http://barngoggles.com/wp-content/uploads/2015/07/solveWithMutex_disassembly.png" alt="solveWithMutex_disassembly" width="554" height="377" srcset="http://barngoggles.com/wp-content/uploads/2015/07/solveWithMutex_disassembly-300x204.png 300w, http://barngoggles.com/wp-content/uploads/2015/07/solveWithMutex_disassembly.png 554w" sizes="(max-width: 554px) 100vw, 554px" /></a></p>
169<p>I had half expected that the lock gaurd would do something fancy but it doesn&#8217;t seem be much more than insurance for unlocking the mutex. Yep, that&#8217;s the same code we saw above in the manual lock and unlock cases. If you&#8217;d like to explore the exciting world of kernel mutexs, take a look at <a href="https://github.com/torvalds/linux/blob/de182468d1bb726198abaab315820542425270b7/kernel/locking/mutex.c" target="_blank">this C code for the linux mutex</a>.</p>
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173			This entry was posted in <a href="http://barngoggles.com/category/programming/c/" rel="category tag">C++</a>, <a href="http://barngoggles.com/category/programming/" rel="category tag">Programming</a> on <a href="http://barngoggles.com/mutex-diversion-from-atomics/" title="8:02 am" rel="bookmark"><time class="entry-date" datetime="2015-07-17T08:02:09+00:00">17 July, 2015</time></a><span class="by-author"> by <span class="author vcard"><a class="url fn n" href="http://barngoggles.com/author/barngoggles/" title="View all posts by barngoggles" rel="author">barngoggles</a></span></span>.						
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