rcu: Mark accesses to ->rcu_read_lock_nesting

KCSAN flags accesses to ->rcu_read_lock_nesting as data races, but
in the past, the overhead of marked accesses was excessive.  However,
that was long ago, and much has changed since then, both in terms of
hardware and of compilers.  Here is data taken on an eight-core laptop
using Intel(R) Core(TM) i9-10885H CPU @ 2.40GHz with a kernel built
using gcc version 9.3.0, with all data in nanoseconds.

Unmarked accesses (status quo), measured by three refscale runs:

	Minimum reader duration:  3.286  2.851  3.395
	Median reader duration:   3.698  3.531  3.4695
	Maximum reader duration:  4.481  5.215  5.157

Marked accesses, also measured by three refscale runs:

	Minimum reader duration:  3.501  3.677  3.580
	Median reader duration:   4.053  3.723  3.895
	Maximum reader duration:  7.307  4.999  5.511

This focused microbenhmark shows only sub-nanosecond differences which
are unlikely to be visible at the system level.  This commit therefore
marks data-racing accesses to ->rcu_read_lock_nesting.

Signed-off-by: Paul E. McKenney <paulmck@kernel.org>

1 files changed, 6 insertions, 3 deletions

diff --git a/kernel/rcu/tree_plugin.h b/kernel/rcu/tree_plugin.h
index de1dc3bb7f70..83a702a4e296 100644
--- a/kernel/rcu/tree_plugin.h
+++ b/kernel/rcu/tree_plugin.h
@@ -405,17 +405,20 @@ static int rcu_preempt_blocked_readers_cgp(struct rcu_node *rnp)
 
 static void rcu_preempt_read_enter(void)
 {
-	current->rcu_read_lock_nesting++;
+	WRITE_ONCE(current->rcu_read_lock_nesting, READ_ONCE(current->rcu_read_lock_nesting) + 1);
 }
 
 static int rcu_preempt_read_exit(void)
 {
-	return --current->rcu_read_lock_nesting;
+	int ret = READ_ONCE(current->rcu_read_lock_nesting) - 1;
+
+	WRITE_ONCE(current->rcu_read_lock_nesting, ret);
+	return ret;
 }
 
 static void rcu_preempt_depth_set(int val)
 {
-	current->rcu_read_lock_nesting = val;
+	WRITE_ONCE(current->rcu_read_lock_nesting, val);
 }
 
 /*