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