srcu: Add SRCU-fast readers

This commit adds srcu_read_{,un}lock_fast(), which is similar
to srcu_read_{,un}lock_lite(), but avoids the array-indexing and
pointer-following overhead.  On a microbenchmark featuring tight
loops around empty readers, this results in about a 20% speedup
compared to RCU Tasks Trace on my x86 laptop.

Please note that SRCU-fast has drawbacks compared to RCU Tasks
Trace, including:

o	Lack of CPU stall warnings.
o	SRCU-fast readers permitted only where rcu_is_watching().
o	A pointer-sized return value from srcu_read_lock_fast() must
	be passed to the corresponding srcu_read_unlock_fast().
o	In the absence of readers, a synchronize_srcu() having _fast()
	readers will incur the latency of at least two normal RCU grace
	periods.
o	RCU Tasks Trace priority boosting could be easily added.
	Boosting SRCU readers is more difficult.

SRCU-fast also has a drawback compared to SRCU-lite, namely that the
return value from srcu_read_lock_fast()-fast is a 64-bit pointer and
that from srcu_read_lock_lite() is only a 32-bit int.

[ paulmck: Apply feedback from Akira Yokosawa. ]

Signed-off-by: Paul E. McKenney <paulmck@kernel.org>
Cc: Alexei Starovoitov <ast@kernel.org>
Cc: Andrii Nakryiko <andrii@kernel.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Kent Overstreet <kent.overstreet@linux.dev>
Cc: <bpf@vger.kernel.org>
Signed-off-by: Boqun Feng <boqun.feng@gmail.com>

0 files changed, 0 insertions, 0 deletions