[PATCH RFC tip/core/rcu] Make SRCU be once again optional

["Paul E. McKenney" <paulmck@linux.vnet.ibm.com>]

Hello, Nicolas!

Saw the TTY write up LWN and figured I should send this your way.
It should be worth about 2K compared to current -next, which gave
up the 2K compared to v4.10.  So really getting things back to where
they were.

My current plan is to push this into v4.13.

							Thanx, Paul

------------------------------------------------------------------------

commit e01ef0529ed548c1b30206058c2b5eecbbc07998
Author: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
Date:   Fri Apr 28 13:53:04 2017 -0700

    srcu: Make SRCU be once again optional
    
    Commit d160a727c40e ("srcu: Make SRCU be built by default") in response
    to build errors, which were caused by code that included srcu.h
    despite !SRCU.  However, srcutiny.o is almost 2K of code, which is not
    insignificant for those attempting to run the Linux kernel on IoT devices.
    This commit therefore makes SRCU be once again optional, and adjusts
    srcu.h to allow error-free inclusion in !SRCU kernel builds.
    
    Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
    Cc: Nicolas Pitre <nicolas.pitre@linaro.org>

diff --git a/include/linux/srcu.h b/include/linux/srcu.h
index 167ad8831aaf..c0143fe2e39d 100644
--- a/include/linux/srcu.h
+++ b/include/linux/srcu.h
@@ -62,7 +62,7 @@ int init_srcu_struct(struct srcu_struct *sp);
 #include <linux/srcutree.h>
 #elif defined(CONFIG_CLASSIC_SRCU)
 #include <linux/srcuclassic.h>
-#else
+#elif defined(CONFIG_SRCU)
 #error "Unknown SRCU implementation specified to kernel configuration"
 #endif
 
diff --git a/init/Kconfig b/init/Kconfig
index 42a346b0df43..fe72c12e06a5 100644
--- a/init/Kconfig
+++ b/init/Kconfig
@@ -521,7 +521,6 @@ config RCU_EXPERT
 
 config SRCU
 	bool
-	default y
 	help
 	  This option selects the sleepable version of RCU. This version
 	  permits arbitrary sleeping or blocking within RCU read-side critical