linux-kernel - [PATCH v2 09/21] rcu: Update Requirements.rst for RCU Tasks Trace

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Message-Id: <20251001144832.631770-9-paulmck@kernel.org>
Date: Wed,  1 Oct 2025 07:48:20 -0700
From: "Paul E. McKenney" <paulmck@...nel.org>
To: rcu@...r.kernel.org
Cc: linux-kernel@...r.kernel.org,
	kernel-team@...a.com,
	rostedt@...dmis.org,
	"Paul E. McKenney" <paulmck@...nel.org>,
	Andrii Nakryiko <andrii@...nel.org>,
	Alexei Starovoitov <ast@...nel.org>,
	Peter Zijlstra <peterz@...radead.org>,
	bpf@...r.kernel.org
Subject: [PATCH v2 09/21] rcu: Update Requirements.rst for RCU Tasks Trace

This commit updates the documentation to declare that RCU Tasks Trace
is implemented as a thin wrapper around SRCU-fast.

Signed-off-by: Paul E. McKenney <paulmck@...nel.org>
Cc: Andrii Nakryiko <andrii@...nel.org>
Cc: Alexei Starovoitov <ast@...nel.org>
Cc: Peter Zijlstra <peterz@...radead.org>
Cc: <bpf@...r.kernel.org>
---
 .../RCU/Design/Requirements/Requirements.rst         | 12 ++++++------
 1 file changed, 6 insertions(+), 6 deletions(-)

diff --git a/Documentation/RCU/Design/Requirements/Requirements.rst b/Documentation/RCU/Design/Requirements/Requirements.rst
index f24b3c0b9b0dc6..4a116d7a564edc 100644
--- a/Documentation/RCU/Design/Requirements/Requirements.rst
+++ b/Documentation/RCU/Design/Requirements/Requirements.rst
@@ -2779,12 +2779,12 @@ Tasks Trace RCU
 ~~~~~~~~~~~~~~~
 
 Some forms of tracing need to sleep in readers, but cannot tolerate
-SRCU's read-side overhead, which includes a full memory barrier in both
-srcu_read_lock() and srcu_read_unlock().  This need is handled by a
-Tasks Trace RCU that uses scheduler locking and IPIs to synchronize with
-readers.  Real-time systems that cannot tolerate IPIs may build their
-kernels with ``CONFIG_TASKS_TRACE_RCU_READ_MB=y``, which avoids the IPIs at
-the expense of adding full memory barriers to the read-side primitives.
+SRCU's read-side overhead, which includes a full memory barrier in
+both srcu_read_lock() and srcu_read_unlock().  This need is handled by
+a Tasks Trace RCU API implemented as thin wrappers around SRCU-fast,
+which avoids the read-side memory barriers, at least for architectures
+that apply noinstr to kernel entry/exit code (or that build with
+``CONFIG_TASKS_TRACE_RCU_NO_MB=y``.
 
 The tasks-trace-RCU API is also reasonably compact,
 consisting of rcu_read_lock_trace(), rcu_read_unlock_trace(),
-- 
2.40.1