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<h1 class="topictitle1">Atomic Operations</h1>
<div>
<p>You can avoid mutual exclusion using atomic operations. When a thread
performs an atomic operation, the other threads see it as happening
instantaneously. The advantage of atomic operations is that they are relatively
quick compared to locks, and do not suffer from deadlock and convoying. The
disadvantage is that they only do a limited set of operations, and often these
are not enough to synthesize more complicated operations efficiently. But
nonetheless you should not pass up an opportunity to use an atomic operation in
place of mutual exclusion. Class
<samp class="codeph">atomic<<var>T</var>></samp> implements atomic
operations with C++ style.
</p>
<p>A classic use of atomic operations is for thread-safe reference
counting. Suppose x is a reference count of type
<samp class="codeph">int</samp>, and the program needs to take some action when the
reference count becomes zero. In single-threaded code, you could use a plain
<samp class="codeph">int</samp> for x, and write
<samp class="codeph">--x; if(x==0) action().</samp> But this method might fail for
multithreaded code, because two threads might interleave their operations as
shown in the following table, where t<sub>a</sub> and t<sub>b</sub> represent
machine registers, and time progresses downwards:
</p>
<div class="tablenoborder"><a name="tbl12"><!-- --></a><table cellpadding="4" summary="" id="tbl12" width="100%" frame="border" border="1" cellspacing="0" rules="all"><caption><span class="tablecap">Interleaving of Machine Instructions</span></caption>
<thead align="left">
<tr>
<th class="cellrowborder" valign="top" width="53.92156862745098%" id="d126840e63">
<p>Thread A
</p>
</th>
<th class="cellrowborder" valign="top" width="46.07843137254902%" id="d126840e69">
<p>Thread B
</p>
</th>
</tr>
</thead>
<tbody>
<tr>
<td class="cellrowborder" valign="top" width="53.92156862745098%" headers="d126840e63 ">
<pre>t<sub>a</sub> = x</pre>
</td>
<td class="cellrowborder" valign="top" width="46.07843137254902%" headers="d126840e69 ">
<pre> </pre>
</td>
</tr>
<tr>
<td class="cellrowborder" valign="top" width="53.92156862745098%" headers="d126840e63 ">
<pre> </pre>
</td>
<td class="cellrowborder" valign="top" width="46.07843137254902%" headers="d126840e69 ">
<pre>t<sub>b</sub> = x</pre>
</td>
</tr>
<tr>
<td class="cellrowborder" valign="top" width="53.92156862745098%" headers="d126840e63 ">
<pre>x = t<sub>a</sub> -<sub> </sub>1</pre>
</td>
<td class="cellrowborder" valign="top" width="46.07843137254902%" headers="d126840e69 ">
<pre> </pre>
</td>
</tr>
<tr>
<td class="cellrowborder" valign="top" width="53.92156862745098%" headers="d126840e63 ">
<pre> </pre>
</td>
<td class="cellrowborder" valign="top" width="46.07843137254902%" headers="d126840e69 ">
<pre>x = t<sub>b</sub> –<sub> </sub>1</pre>
</td>
</tr>
<tr>
<td class="cellrowborder" valign="top" width="53.92156862745098%" headers="d126840e63 ">
<pre>if( x==0 )</pre>
</td>
<td class="cellrowborder" valign="top" width="46.07843137254902%" headers="d126840e69 ">
<pre> </pre>
</td>
</tr>
<tr>
<td class="cellrowborder" valign="top" width="53.92156862745098%" headers="d126840e63 ">
<pre> </pre>
</td>
<td class="cellrowborder" valign="top" width="46.07843137254902%" headers="d126840e69 ">
<pre>if( x==0 )</pre>
</td>
</tr>
</tbody>
</table>
</div>
<p>Though the code intended for
<samp class="codeph">x</samp> to be decremented twice, it ends up with only one less
than its original value. Also, another problem results because the test of
<var>x</var> is separate from the decrement: If
<var>x</var> starts out as two, and both threads decrement
<var>x</var> before either thread evaluates the
<samp class="codeph">if</samp> condition,
<em>both</em> threads would call
<samp class="codeph">action()</samp>. To correct this problem, you need to ensure that
only one thread at a time does the decrement
<em>and</em> ensure that the value checked by the "if" is the result of the
decrement. You can do this by introducing a mutex, but it is much faster and
simpler to declare
<var>x</var> as
<samp class="codeph">atomic<int></samp> and write
"<samp class="codeph">if(--<var>x</var>==0) action()</samp>". The method
<samp class="codeph">atomic<int>::operator--</samp> acts atomically; no other
thread can interfere.
</p>
<p><samp class="codeph">atomic<<var>T</var>></samp> supports atomic
operations on type
<var>T</var>, which must be an integral, enumeration, or pointer
type. There are five fundamental operations supported, with additional
interfaces in the form of overloaded operators for syntactic convenience. For
example,
<samp class="codeph">++</samp>,
<samp class="codeph">--</samp>,
<samp class="codeph">-=</samp>, and
<samp class="codeph">+=</samp> operations on
<samp class="codeph">atomic<<var>T</var>></samp> are all forms of the
fundamental operation
<em>fetch-and-add</em>. The following are the five fundamental operations on
a variable
<var>x</var> of type
<samp class="codeph">atomic<<var>T</var>></samp>.
</p>
<div class="tablenoborder"><a name="tbl13"><!-- --></a><table cellpadding="4" summary="" id="tbl13" width="100%" frame="border" border="1" cellspacing="0" rules="all"><caption><span class="tablecap">Fundamental Operations on a Variable x of Type atomic<T></span></caption>
<tbody>
<tr>
<td class="cellrowborder" valign="top" width="30%">
<p><samp class="codeph">=
<var>x</var></samp>
</p>
</td>
<td class="cellrowborder" valign="top" width="70%">
<p>read the value of
<var>x</var>
</p>
</td>
</tr>
<tr>
<td class="cellrowborder" valign="top" width="30%">
<p><samp class="codeph"><var>x</var>=</samp>
</p>
</td>
<td class="cellrowborder" valign="top" width="70%">
<p>write the value of
<var>x</var>, and return it
</p>
</td>
</tr>
<tr>
<td class="cellrowborder" valign="top" width="30%">
<p><samp class="codeph"><var>x</var>.fetch_and_store(y)</samp>
</p>
</td>
<td class="cellrowborder" valign="top" width="70%">
<p>do
<samp class="codeph"><var>x</var>=<var>y</var></samp> and
return the old value of
<var>x</var>
</p>
</td>
</tr>
<tr>
<td class="cellrowborder" valign="top" width="30%">
<p><samp class="codeph"><var>x</var>.fetch_and_add(<var>y</var>)</samp>
</p>
</td>
<td class="cellrowborder" valign="top" width="70%">
<p>do
<samp class="codeph"><var>x</var>+=<var>y</var></samp> and
return the old value of
<var>x</var>
</p>
</td>
</tr>
<tr>
<td class="cellrowborder" valign="top" width="30%">
<p><samp class="codeph"><var>x</var>.compare_and_swap(<var>y</var>,<var>z</var>)</samp>
</p>
</td>
<td class="cellrowborder" valign="top" width="70%">
<p>if
<var>x</var> equals
<var>z</var>, then do
<samp class="codeph"><var>x</var>=<var>y</var></samp>. In
either case, return old value of
<var>x</var>.
</p>
</td>
</tr>
</tbody>
</table>
</div>
<p>Because these operations happen atomically, they can be used safely
without mutual exclusion. Consider the following example:
</p>
<pre>atomic<unsigned> counter;
unsigned GetUniqueInteger() {
return counter.fetch_and_add(1);
}</pre>
<p>The routine
<samp class="codeph">GetUniqueInteger</samp> returns a different integer each time it
is called, until the counter wraps around. This is true no matter how many
threads call
<samp class="codeph">GetUniqueInteger</samp> simultaneously.
</p>
<p>The operation
<samp class="codeph">compare_and_swap</samp> is a fundamental operation to many
non-blocking algorithms. A problem with mutual exclusion is that if a thread
holding a lock is suspended, all other threads are blocked until the holding
thread resumes. Non-blocking algorithms avoid this problem by using atomic
operations instead of locking. They are generally complicated and require
sophisticated analysis to verify. However, the following idiom is
straightforward and worth knowing. It updates a shared variable
<samp class="codeph">globalx</samp> in a way that is somehow based on its old value:
</p>
<pre>atomic<int> globalx;
int UpdateX() { // Update x and return old value of x.
do {
// Read globalX
oldx = globalx;
// Compute new value
newx = ...expression involving oldx....
// Store new value if another thread has not changed globalX.
} while( globalx.compare_and_swap(newx,oldx)!=oldx );
return oldx;
}</pre>
<p>Worse, some threads iterate the loop until no other thread interferes.
Typically, if the update takes only a few instructions, the idiom is faster
than the corresponding mutual-exclusion solution.
</p>
<div class="Note"><h3 class="NoteTipHead">
Caution</h3>
<p>If the following sequence thwarts your intent, then the update idiom is
inappropriate:
</p>
<ol>
<li>
<p>A thread reads a value
<var>A</var> from
<samp class="codeph">globalx</samp>
</p>
</li>
<li>
<p>Other threads change
<samp class="codeph">globalx</samp> from
<var>A</var> to
<var>B</var> to
<var>A</var>
</p>
</li>
<li>
<p>The thread in step 1 does its
<samp class="codeph">compare_and_swap</samp>, reading
<var>A</var> and thus not detecting the intervening change to
<var>B</var>.
</p>
</li>
</ol>
</div>
<p>The problem is called the
<em>ABA</em>
<em>problem</em>. It is frequently a problem in designing non-blocking
algorithms for linked data structures. See the Internet for more information.
</p>
</div>
<div class="familylinks">
<div class="parentlink"><strong>Parent topic:</strong> <a href="../tbb_userguide/title.htm">Intel® Threading Building Blocks (Intel® TBB) User Guide</a></div>
</div>
<div>
<ul class="ullinks">
<li class="ulchildlink"><a href="../tbb_userguide/Why_atomic_T_Has_No_Constructors.htm">Why atomic<T> Has No Constructors in C++03 mode</a><br>
</li>
<li class="ulchildlink"><a href="../tbb_userguide/Memory_Consistency.htm">Memory Consistency</a><br>
</li>
</ul>
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