.. _Lock_Pathologies:
Lock Pathologies
================
Locks can introduce performance and correctness problems. If you are new
to locking, here are some of the problems to avoid:
.. container:: section
.. rubric:: Deadlock
:class: sectiontitle
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:
- There is a cycle of threads
- Each thread holds at least one lock on a mutex, and is waiting on
a mutex for which the *next* thread in the cycle already has a
lock.
- No thread is willing to give up its lock.
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:
- 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.
- 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.
- Use atomic operations instead of locks.
.. container:: section
.. rubric:: Convoying
:class: sectiontitle
Another common problem with locks is *convoying*. 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.
To minimize convoying, try to hold the lock as briefly as possible.
Precompute whatever you can before acquiring the lock.
To avoid convoying, use atomic operations instead of locks where
possible.
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