[PATCH 0/6] Switch __DECLARE_TRACE() to new notrace variant of SRCU-fast

[Paul E. McKenney]

Hello!

This is version 4 of a patch series creating a new notrace variant of
SRCU-fast and introducing it to the __DECLARE_TRACE() in place of the
current preemption disabling.  This change enable preemption of BPF
programs attached to tracepoints, as is required for runtime use of BPF
in real-time systems.

This triggers continues to trigger a kernel test robot report of a
"using smp_processor_id() in preemptible" splat.  I looked for issues
with explicit preemption disabling, and, not finding any, will next turn
my attention to accesses to per-CPU variables.  Any and all insights
are welcome.

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.	Guard __DECLARE_TRACE() use of __DO_TRACE_CALL() with SRCU-fast.

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

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

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@xxxxxxxxxx/

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/include/linux/tracepoint.h |    6 +++--
 b/kernel/rcu/srcutree.c      |   10 +++++++++
 b/kernel/tracepoint.c        |   21 ++++++++++++++++++-
 include/linux/srcu.h         |   35 ++++++++++++++++++++++++++++++--
 include/linux/srcutree.h     |   47 +++++++++++++++++++++++++++----------------
 7 files changed, 101 insertions(+), 24 deletions(-)