[tip: core/rcu] rcu: Add Kconfig option for strict RCU grace periods

["tip-bot2 for Paul E. McKenney" <tip-bot2@linutronix.de>]

The following commit has been merged into the core/rcu branch of tip:

Commit-ID:     8cbd0e38a9f2de38e8991c5c1c6f9024b2731d17
Gitweb:        https://git.kernel.org/tip/8cbd0e38a9f2de38e8991c5c1c6f9024b2731d17
Author:        Paul E. McKenney <paulmck@kernel.org>
AuthorDate:    Wed, 05 Aug 2020 15:51:20 -07:00
Committer:     Paul E. McKenney <paulmck@kernel.org>
CommitterDate: Mon, 24 Aug 2020 18:40:23 -07:00

rcu: Add Kconfig option for strict RCU grace periods

People running automated tests have asked for a way to make RCU minimize
grace-period duration in order to increase the probability of KASAN
detecting a pointer being improperly leaked from an RCU read-side critical
section, for example, like this:

	rcu_read_lock();
	p = rcu_dereference(gp);
	do_something_with(p); // OK
	rcu_read_unlock();
	do_something_else_with(p); // BUG!!!

The rcupdate.rcu_expedited boot parameter is a start in this direction,
given that it makes calls to synchronize_rcu() instead invoke the faster
(and more wasteful) synchronize_rcu_expedited().  However, this does
nothing to shorten RCU grace periods that are instead initiated by
call_rcu(), and RCU pointer-leak bugs can involve call_rcu() just as
surely as they can synchronize_rcu().

This commit therefore adds a RCU_STRICT_GRACE_PERIOD Kconfig option
that will be used to shorten normal (non-expedited) RCU grace periods.
This commit also dumps out a message when this option is in effect.
Later commits will actually shorten grace periods.

Reported-by Jann Horn <jannh@google.com>
Signed-off-by: Paul E. McKenney <paulmck@kernel.org>
---
 kernel/rcu/Kconfig.debug | 15 +++++++++++++++
 kernel/rcu/tree_plugin.h |  2 ++
 2 files changed, 17 insertions(+)

diff --git a/kernel/rcu/Kconfig.debug b/kernel/rcu/Kconfig.debug
index 3cf6132..cab5a4b 100644
--- a/kernel/rcu/Kconfig.debug
+++ b/kernel/rcu/Kconfig.debug
@@ -114,4 +114,19 @@ config RCU_EQS_DEBUG
 	  Say N here if you need ultimate kernel/user switch latencies
 	  Say Y if you are unsure
 
+config RCU_STRICT_GRACE_PERIOD
+	bool "Provide debug RCU implementation with short grace periods"
+	depends on DEBUG_KERNEL && RCU_EXPERT
+	default n
+	select PREEMPT_COUNT if PREEMPT=n
+	help
+	  Select this option to build an RCU variant that is strict about
+	  grace periods, making them as short as it can.  This limits
+	  scalability, destroys real-time response, degrades battery
+	  lifetime and kills performance.  Don't try this on large
+	  machines, as in systems with more than about 10 or 20 CPUs.
+	  But in conjunction with tools like KASAN, it can be helpful
+	  when looking for certain types of RCU usage bugs, for example,
+	  too-short RCU read-side critical sections.
+
 endmenu # "RCU Debugging"
diff --git a/kernel/rcu/tree_plugin.h b/kernel/rcu/tree_plugin.h
index 982fc5b..44cf77d 100644
--- a/kernel/rcu/tree_plugin.h
+++ b/kernel/rcu/tree_plugin.h
@@ -36,6 +36,8 @@ static void __init rcu_bootup_announce_oddness(void)
 		pr_info("\tRCU dyntick-idle grace-period acceleration is enabled.\n");
 	if (IS_ENABLED(CONFIG_PROVE_RCU))
 		pr_info("\tRCU lockdep checking is enabled.\n");
+	if (IS_ENABLED(CONFIG_RCU_STRICT_GRACE_PERIOD))
+		pr_info("\tRCU strict (and thus non-scalable) grace periods enabled.\n");
 	if (RCU_NUM_LVLS >= 4)
 		pr_info("\tFour(or more)-level hierarchy is enabled.\n");
 	if (RCU_FANOUT_LEAF != 16)