tree 6402f7cb2a0916711fc27fab89c4568b0a4c8653
parent 2be57f732889277b07ccddd205ef0616c8c1941f
author Paul E. McKenney <paulmck@kernel.org> 1621542950 -0700
committer Paul E. McKenney <paulmck@kernel.org> 1628282508 -0700

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>