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@kernel.org>
Cc: Andrii Nakryiko <andrii@kernel.org>
Cc: Alexei Starovoitov <ast@kernel.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: bpf@vger.kernel.org
Reviewed-by: Joel Fernandes <joelagnelf@nvidia.com>
Signed-off-by: Boqun Feng <boqun.feng@gmail.com>

diff --git a/Documentation/RCU/Design/Requirements/Requirements.rst b/Documentation/RCU/Design/Requirements/Requirements.rst
index ba417a08b93dfb2304b9e7cf37d1d5df371caba4..b5cdbba3ec2e710cfd1722ea05d5d90a2d8f46d8 100644 (file)
--- a/Documentation/RCU/Design/Requirements/Requirements.rst
+++ b/Documentation/RCU/Design/Requirements/Requirements.rst
@@ -2780,12 +2780,12 @@ 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(),