From c79dac758dc59f7461a721aaf65301cde8a448bc Mon Sep 17 00:00:00 2001 From: "Paul E. McKenney" Date: Wed, 20 Apr 2016 09:22:54 -0700 Subject: [PATCH] documentation: Add RCU_NONIDLE() restrictions to requirements Signed-off-by: Paul E. McKenney --- .../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 e7e24b3e86e2..ece410f40436 100644 --- a/Documentation/RCU/Design/Requirements/Requirements.html +++ b/Documentation/RCU/Design/Requirements/Requirements.html @@ -2391,6 +2391,41 @@ and RCU_NONIDLE() on the other while inspecting idle-loop code. Steven Rostedt supplied _rcuidle event tracing, which is used quite heavily in the idle loop. +However, there are some restrictions on the code placed within +RCU_NONIDLE(): + +
    +
  1. Blocking is prohibited. + In practice, this is not a serious restriction given that idle + tasks are prohibited from blocking to begin with. +
  2. Although nesting RCU_NONIDLE() 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. +
  3. Any code path that enters RCU_NONIDLE() must sequence + out of that same RCU_NONIDLE(). + For example, the following is grossly illegal: + +
    + 
    + 1     RCU_NONIDLE({
+ 2       do_something();
+ 3       goto bad_idea;  /* BUG!!! */
+ 4       do_something_else();});
+ 5   bad_idea:
+
    +
    + +

    + It is just as illegal to transfer control into the middle of + RCU_NONIDLE()'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. +

It is similarly socially unacceptable to interrupt an