Commit c79dac75 authored by Paul E. McKenney's avatar Paul E. McKenney

documentation: Add RCU_NONIDLE() restrictions to requirements

Signed-off-by: default avatarPaul E. McKenney <paulmck@linux.vnet.ibm.com>
parent db4855b5
......@@ -2391,6 +2391,41 @@ and <tt>RCU_NONIDLE()</tt> on the other while inspecting
idle-loop code.
Steven Rostedt supplied <tt>_rcuidle</tt> event tracing,
which is used quite heavily in the idle loop.
However, there are some restrictions on the code placed within
<tt>RCU_NONIDLE()</tt>:
<ol>
<li> Blocking is prohibited.
In practice, this is not a serious restriction given that idle
tasks are prohibited from blocking to begin with.
<li> Although nesting <tt>RCU_NONIDLE()</tt> is permited, they cannot
nest indefinitely deeply.
However, given that they can be nested on the order of a million
deep, even on 32-bit systems, this should not be a serious
restriction.
This nesting limit would probably be reached long after the
compiler OOMed or the stack overflowed.
<li> Any code path that enters <tt>RCU_NONIDLE()</tt> must sequence
out of that same <tt>RCU_NONIDLE()</tt>.
For example, the following is grossly illegal:
<blockquote>
<pre>
1 RCU_NONIDLE({
2 do_something();
3 goto bad_idea; /* BUG!!! */
4 do_something_else();});
5 bad_idea:
</pre>
</blockquote>
<p>
It is just as illegal to transfer control into the middle of
<tt>RCU_NONIDLE()</tt>'s argument.
Yes, in theory, you could transfer in as long as you also
transferred out, but in practice you could also expect to get sharply
worded review comments.
</ol>
<p>
It is similarly socially unacceptable to interrupt an
......
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