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<h1 class="topictitle1">Lock Pathologies</h1>
<div>
<p>Locks can introduce performance and correctness problems. If you are new
to locking, here are some of the problems to avoid:
</p>
<div class="section"><h2 class="sectiontitle">Deadlock</h2>
<p>Deadlock happens when threads are trying to acquire more than one
lock, and each holds some of the locks the other threads need to proceed. More
precisely, deadlock happens when:
</p>
<ul type="disc">
<li>
<p>There is a cycle of threads
</p>
</li>
<li>
<p>Each thread holds at least one lock on a mutex, and is waiting on
a mutex for which the
<em>next</em> thread in the cycle already has a lock.
</p>
</li>
<li>
<p>No thread is willing to give up its lock.
</p>
</li>
</ul>
<p>Think of classic gridlock at an intersection – each car has "acquired"
part of the road, but needs to "acquire" the road under another car to get
through. Two common ways to avoid deadlock are:
</p>
<ul type="disc">
<li>
<p>Avoid needing to hold two locks at the same time. Break your
program into small actions in which each can be accomplished while holding a
single lock.
</p>
</li>
<li>
<p>Always acquire locks in the same order. For example, if you have
"outer container" and "inner container" mutexes, and need to acquire a lock on
one of each, you could always acquire the "outer sanctum" one first. Another
example is "acquire locks in alphabetical order" in a situation where the locks
have names. Or if the locks are unnamed, acquire locks in order of the mutex’s
numerical addresses.
</p>
</li>
<li>
<p>Use atomic operations instead of locks.
</p>
</li>
</ul>
</div>
<div class="section"><h2 class="sectiontitle">Convoying</h2>
<p>Another common problem with locks is
<em>convoying</em>. Convoying occurs when the operating system interrupts
a thread that is holding a lock. All other threads must wait until the
interrupted thread resumes and releases the lock. Fair mutexes can make the
situation even worse, because if a waiting thread is interrupted, all the
threads behind it must wait for it to resume.
</p>
<p>To minimize convoying, try to hold the lock as briefly as possible.
Precompute whatever you can before acquiring the lock.
</p>
<p>To avoid convoying, use atomic operations instead of locks where
possible.
</p>
</div>
</div>
<div class="familylinks">
<div class="parentlink"><strong>Parent topic:</strong> <a href="../tbb_userguide/Mutual_Exclusion.htm">Mutual Exclusion</a></div>
</div>
<div class="See Also">
<h2>See Also</h2>
<div class="linklist">
<div><a href="Atomic_Operations.htm">Atomic Operations
</a></div></div>
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