[PATCH v7 0/6] Provide RCU changes needed to switch __DECLARE_TRACE() to SRCU for -rt

["Paul E. McKenney" <paulmck@kernel.org>]

Hello!

This is version 7 of a patch series creating a new notrace variant of
SRCU-fast for eventual use by the __DECLARE_TRACE() macro in place of the
current preemption disabling for PREEMPT_RT kernels.  This change will
enable preemption of BPF programs attached to tracepoints in PREEMPT_RT
kernels, as is required for runtime use of BPF in real-time systems.
Non-PREEMPT_RT kernels will continue to have such BPF programs be
non-preemptible.

The patches are as follows:

1.	Move rcu_is_watching() checks to srcu_read_{,un}lock_fast().

2.	Add srcu_read_lock_fast_notrace() and
	srcu_read_unlock_fast_notrace().

3.	Add guards for notrace variants of SRCU-fast readers.

4.	Document __srcu_read_{,un}lock_fast() implicit RCU readers.

5.	Document srcu_flip() memory-barrier D relation to SRCU-fast.

6.	Remove preempt_disable/enable() in srcu_gp_start_if_needed(),
	courtesy of Zqiang.

Changes since v6:

o	Deferred the application of SRCU-fast to __DECLARE_TRACE()
	due to bugs located by the good services of kernel test robot.
	Although the current version is looking good, it needs more
	test time.

o	Added patch 6.

Changes since v5:

o	Forward-port to v6.17-rc1.

Changes since v4:

o	Remove the redundant "notrace" tags per Boqun Feng feedback.

o	Disable preemption for perf and ftrace per Steven Rostedt
	feedback, which addresses the kernel test robot reports
	provoked by earlier versions.

	https://lore.kernel.org/all/45397494-544e-41c0-bf48-c66d213fce05@paulmck-laptop/

Changes since v3:

o	Add "notrace" per Joel, Steven, and Matthew feedback.

o	Upgrade explanatory comments and add new ones per Joel feedback.

	https://lore.kernel.org/all/20250721162433.10454-1-paulmck@kernel.org/

Changes since v2:

o	Posting standalone as opposed to a reply.

	https://lore.kernel.org/all/3cecf6c9-b2ee-4f34-9d1b-ca4cfb8e56a7@paulmck-laptop/

Changes since RFC version:

o	RFC patch 6/4 has been pulled into the shared RCU tree:
	e88c632a8698 ("srcu: Add guards for SRCU-fast readers")

o	RFC patch 5/4 (which removed the now-unnecessary special boot-time
	avoidance of SRCU) has been folded into patch 4/4 shown above,
	as suggested by Steven Rostedt.

	https://lore.kernel.org/all/bb20a575-235b-499e-aa1d-70fe9e2c7617@paulmck-laptop/

						Thanx, Paul

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

 b/include/linux/srcu.h     |    4 +++
 b/include/linux/srcutree.h |    2 -
 b/kernel/rcu/srcutiny.c    |    4 ---
 b/kernel/rcu/srcutree.c    |   10 +++++++++
 include/linux/srcu.h       |   30 ++++++++++++++++++++++++++++
 include/linux/srcutree.h   |   47 ++++++++++++++++++++++++++++-----------------
 6 files changed, 75 insertions(+), 22 deletions(-)