Dr. Dobb's is part of the Informa Tech Division of Informa PLC

This site is operated by a business or businesses owned by Informa PLC and all copyright resides with them. Informa PLC's registered office is 5 Howick Place, London SW1P 1WG. Registered in England and Wales. Number 8860726.


Channels ▼
RSS

C/C++

Lock-Free Code: A False Sense of Security


Ordering Problems in Produce

Second, reads and writes of a lock-free variable must occur in an expected order, which is nearly always the exact order they appear in the program source code. But compilers, processors, and caches love to optimize reads and writes, and will helpfully reorder, invent, and remove memory reads and writes unless you prevent it from happening. The right prevention happens implicitly when you use mutex locks or ordered atomic variables (C++0x std::atomic, Java/.NET volatile); you can also do it explicitly, but with considerably more effort, using ordered API calls (e.g., Win32 InterlockedExchange) or memory fences/barriers (e.g., Linux mb). Trying to write lock-free code without using any of these tools can't possibly work.

Consider again this code from Produce, and ignore that the assignment iTail isn't atomic as we look for other problems:


list.push_back(t);	// A: add the new item
iTail = list.end();	// B: publish it

This is a classic publication race because lines A and B can be (partly or entirely) reordered. For example, let's say that some of the writes to the T object's members are delayed until after the write to iTail, which publishes that the new object is available; then the consumer thread can see a partly assigned T object.

What is the minimum necessary fix? We might be tempted to write a memory barrier between the two lines:


// Is this change enough?
list.push_back(t);	// A: add the new item
<font color="#FF0000">mb();		// full fence</font>
iTail = list.end();	// B: publish it

Before reading on, think about it and see if you're convinced that this is (or isn't) right.

Have you thought about it? As a starter, here's one issue: Although list.end is probably unlikely to perform writes, it's possible that it might, and those are side effects that need to be complete before we publish iTail. The general issue is that you can't make assumptions about the side effects of library functions you call, and you have to make sure they're fully performed before you publish the new state. So a slightly improved version might try to store the result of list.end into a local unshared variable and assign it after the barrier:


// Better, but is it enough?
list.push_back(t);
<font color="#FF0000">tmp</font> = list.end();
<font color="#FF0000">mb();		// full fence</font>
iTail <font color="#FF0000">= tmp;</font>

Unfortunately, this still isn't enough. Besides the fact that assigning to iTail isn't atomic and that we still have a race on iTail in general, compilers and processors can also invent writes to iTail that break this code. Let's consider write invention in the context of another problem area: Consume.


Related Reading


More Insights






Currently we allow the following HTML tags in comments:

Single tags

These tags can be used alone and don't need an ending tag.

<br> Defines a single line break

<hr> Defines a horizontal line

Matching tags

These require an ending tag - e.g. <i>italic text</i>

<a> Defines an anchor

<b> Defines bold text

<big> Defines big text

<blockquote> Defines a long quotation

<caption> Defines a table caption

<cite> Defines a citation

<code> Defines computer code text

<em> Defines emphasized text

<fieldset> Defines a border around elements in a form

<h1> This is heading 1

<h2> This is heading 2

<h3> This is heading 3

<h4> This is heading 4

<h5> This is heading 5

<h6> This is heading 6

<i> Defines italic text

<p> Defines a paragraph

<pre> Defines preformatted text

<q> Defines a short quotation

<samp> Defines sample computer code text

<small> Defines small text

<span> Defines a section in a document

<s> Defines strikethrough text

<strike> Defines strikethrough text

<strong> Defines strong text

<sub> Defines subscripted text

<sup> Defines superscripted text

<u> Defines underlined text

Dr. Dobb's encourages readers to engage in spirited, healthy debate, including taking us to task. However, Dr. Dobb's moderates all comments posted to our site, and reserves the right to modify or remove any content that it determines to be derogatory, offensive, inflammatory, vulgar, irrelevant/off-topic, racist or obvious marketing or spam. Dr. Dobb's further reserves the right to disable the profile of any commenter participating in said activities.

 
Disqus Tips To upload an avatar photo, first complete your Disqus profile. | View the list of supported HTML tags you can use to style comments. | Please read our commenting policy.