rcu: move PREEMPT_RCU config option back under PREEMPT

The original preemptible-RCU patch put the choice between classic and
preemptible RCU into kernel/Kconfig.preempt, which resulted in build failures
on machines not supporting CONFIG_PREEMPT.  This choice was therefore moved to
init/Kconfig, which worked, but placed the choice between classic and
preemptible RCU at the top level, a very obtuse choice indeed.

This patch changes from the Kconfig "choice" mechanism to a pair of booleans,
only one of which (CONFIG_PREEMPT_RCU) is user-visible, and is located in
kernel/Kconfig.preempt, where one would expect it to be.  The other
(CONFIG_CLASSIC_RCU) is in init/Kconfig so that it is available to all
architectures, hopefully avoiding build breakage.  Thanks to Roman Zippel for
suggesting this approach.

Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
Cc: Ingo Molnar <mingo@elte.hu>
Acked-by: Steven Rostedt <rostedt@goodmis.org>
Cc: Dipankar Sarma <dipankar@in.ibm.com>
Cc: Josh Triplett <josh@freedesktop.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: Peter Zijlstra <a.p.zijlstra@chello.nl>
Cc: Roman Zippel <zippel@linux-m68k.org>
Cc: Sam Ravnborg <sam@ravnborg.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

2 files changed, 18 insertions, 31 deletions

diff --git a/init/Kconfig b/init/Kconfig
index 074ac97f55e3..a97924bc5b8d 100644
--- a/init/Kconfig
+++ b/init/Kconfig
@@ -865,38 +865,10 @@ source "block/Kconfig"
 config PREEMPT_NOTIFIERS
         bool
 
-choice
-        prompt "RCU implementation type:"
-        default CLASSIC_RCU
-        help
-          This allows you to choose either the classic RCU implementation
-          that is designed for best read-side performance on non-realtime
-          systems, or the preemptible RCU implementation for best latency
-          on realtime systems.  Note that some kernel preemption modes
-          will restrict your choice.
-
-          Select the default if you are unsure.
-
 config CLASSIC_RCU
-        bool "Classic RCU"
+        def_bool !PREEMPT_RCU
         help
           This option selects the classic RCU implementation that is
           designed for best read-side performance on non-realtime
-          systems.
-
-          Say Y if you are unsure.
-
-config PREEMPT_RCU
-        bool "Preemptible RCU"
-        depends on PREEMPT
-        help
-          This option reduces the latency of the kernel by making certain
-          RCU sections preemptible. Normally RCU code is non-preemptible, if
-          this option is selected then read-only RCU sections become
-          preemptible. This helps latency, but may expose bugs due to
-          now-naive assumptions about each RCU read-side critical section
-          remaining on a given CPU through its execution.
-
-          Say N if you are unsure.
-
-endchoice
+          systems.  Classic RCU is the default.  Note that the
+          PREEMPT_RCU symbol is used to select/deselect this option.
diff --git a/kernel/Kconfig.preempt b/kernel/Kconfig.preempt
index 0669b70fa6a3..9fdba03dc1fc 100644
--- a/kernel/Kconfig.preempt
+++ b/kernel/Kconfig.preempt
@@ -52,8 +52,23 @@ config PREEMPT
 
 endchoice
 
+config PREEMPT_RCU
+        bool "Preemptible RCU"
+        depends on PREEMPT
+        default n
+        help
+          This option reduces the latency of the kernel by making certain
+          RCU sections preemptible. Normally RCU code is non-preemptible, if
+          this option is selected then read-only RCU sections become
+          preemptible. This helps latency, but may expose bugs due to
+          now-naive assumptions about each RCU read-side critical section
+          remaining on a given CPU through its execution.
+
+          Say N if you are unsure.
+
 config RCU_TRACE
         bool "Enable tracing for RCU - currently stats in debugfs"
+        depends on PREEMPT_RCU
         select DEBUG_FS
         default y
         help