Message-ID: <20230320173751.GU2194297@hirez.programming.kicks-ass.net>
Date:   Mon, 20 Mar 2023 18:37:51 +0100
From:   Peter Zijlstra <peterz@...radead.org>
To:     Paul McKenney <paulmck@...nel.org>,
        Mark Rutland <mark.rutland@....com>,
        linux-kernel@...r.kernel.org
Cc:     frederic@...nel.org, quic_neeraju@...cinc.com,
        josh@...htriplett.org, rostedt@...dmis.org,
        mathieu.desnoyers@...icios.com, jiangshanlai@...il.com,
        joel@...lfernandes.org, rcu@...r.kernel.org
Subject: [PATCH] rcu: Remove RCU_NONIDLE()


Since there are now exactly _zero_ users of RCU_NONIDLE(), make it go
away before someone else decides to (ab)use it.

Signed-off-by: Peter Zijlstra (Intel) <peterz@...radead.org>
---
 .../RCU/Design/Requirements/Requirements.rst       | 36 +---------------------
 Documentation/RCU/whatisRCU.rst                    |  1 -
 include/linux/rcupdate.h                           | 25 ---------------
 3 files changed, 1 insertion(+), 61 deletions(-)

diff --git a/Documentation/RCU/Design/Requirements/Requirements.rst b/Documentation/RCU/Design/Requirements/Requirements.rst
index 49387d823619..77155b51d4c2 100644
--- a/Documentation/RCU/Design/Requirements/Requirements.rst
+++ b/Documentation/RCU/Design/Requirements/Requirements.rst
@@ -2071,41 +2071,7 @@ call.
 
 Because RCU avoids interrupting idle CPUs, it is illegal to execute an
 RCU read-side critical section on an idle CPU. (Kernels built with
-``CONFIG_PROVE_RCU=y`` will splat if you try it.) The RCU_NONIDLE()
-macro and ``_rcuidle`` event tracing is provided to work around this
-restriction. In addition, rcu_is_watching() may be used to test
-whether or not it is currently legal to run RCU read-side critical
-sections on this CPU. I learned of the need for diagnostics on the one
-hand 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():
-
-#. Blocking is prohibited. In practice, this is not a serious
-   restriction given that idle tasks are prohibited from blocking to
-   begin with.
-#. Although nesting RCU_NONIDLE() is permitted, 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.
-#. 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.
+``CONFIG_PROVE_RCU=y`` will splat if you try it.) 
 
 It is similarly socially unacceptable to interrupt an ``nohz_full`` CPU
 running in userspace. RCU must therefore track ``nohz_full`` userspace
diff --git a/Documentation/RCU/whatisRCU.rst b/Documentation/RCU/whatisRCU.rst
index 2c5563a91998..c3b1cbfa1530 100644
--- a/Documentation/RCU/whatisRCU.rst
+++ b/Documentation/RCU/whatisRCU.rst
@@ -1117,7 +1117,6 @@ in docbook.  Here is the list, by category.
 
 	RCU_LOCKDEP_WARN
 	rcu_sleep_check
-	RCU_NONIDLE
 
 All: Unchecked RCU-protected pointer access::
 
diff --git a/include/linux/rcupdate.h b/include/linux/rcupdate.h
index 094321c17e48..ddd42efc6224 100644
--- a/include/linux/rcupdate.h
+++ b/include/linux/rcupdate.h
@@ -156,31 +156,6 @@ static inline int rcu_nocb_cpu_deoffload(int cpu) { return 0; }
 static inline void rcu_nocb_flush_deferred_wakeup(void) { }
 #endif /* #else #ifdef CONFIG_RCU_NOCB_CPU */
 
-/**
- * RCU_NONIDLE - Indicate idle-loop code that needs RCU readers
- * @a: Code that RCU needs to pay attention to.
- *
- * RCU read-side critical sections are forbidden in the inner idle loop,
- * that is, between the ct_idle_enter() and the ct_idle_exit() -- RCU
- * will happily ignore any such read-side critical sections.  However,
- * things like powertop need tracepoints in the inner idle loop.
- *
- * This macro provides the way out:  RCU_NONIDLE(do_something_with_RCU())
- * will tell RCU that it needs to pay attention, invoke its argument
- * (in this example, calling the do_something_with_RCU() function),
- * and then tell RCU to go back to ignoring this CPU.  It is permissible
- * to nest RCU_NONIDLE() wrappers, but not indefinitely (but the limit is
- * on the order of a million or so, even on 32-bit systems).  It is
- * not legal to block within RCU_NONIDLE(), nor is it permissible to
- * transfer control either into or out of RCU_NONIDLE()'s statement.
- */
-#define RCU_NONIDLE(a) \
-	do { \
-		ct_irq_enter_irqson(); \
-		do { a; } while (0); \
-		ct_irq_exit_irqson(); \
-	} while (0)
-
 /*
  * Note a quasi-voluntary context switch for RCU-tasks's benefit.
  * This is a macro rather than an inline function to avoid #include hell.

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