Commit-ID: c79dac758dc59f7461a721aaf65301cde8a448bc
Gitweb: http://git.kernel.org/tip/c79dac758dc59f7461a721aaf65301cde8a448bc
Author: Paul E. McKenney <paulmck@xxxxxxxxxxxxxxxxxx>
AuthorDate: Wed, 20 Apr 2016 09:22:54 -0700
Committer: Paul E. McKenney <paulmck@xxxxxxxxxxxxxxxxxx>
CommitDate: Tue, 14 Jun 2016 16:00:59 -0700
documentation: Add RCU_NONIDLE() restrictions to requirements
Signed-off-by: Paul E. McKenney <paulmck@xxxxxxxxxxxxxxxxxx>
---
.../RCU/Design/Requirements/Requirements.html | 35 ++++++++++++++++++++++
1 file changed, 35 insertions(+)
diff --git a/Documentation/RCU/Design/Requirements/Requirements.html b/Documentation/RCU/Design/Requirements/Requirements.html
index e7e24b3..ece410f 100644
--- a/Documentation/RCU/Design/Requirements/Requirements.html
+++ b/Documentation/RCU/Design/Requirements/Requirements.html
@@ -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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