[PATCH rcu 2/7] docs: Improve discussion of this_cpu_ptr(), add raw_cpu_ptr()

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

Most of the this_cpu_*() operations may be used in preemptible code,
but not this_cpu_ptr(), and for good reasons.  Therefore, better explain
the reasons and call out raw_cpu_ptr() as an alternative in certain very
special cases.

Signed-off-by: Paul E. McKenney <paulmck@kernel.org>
Cc: Jonathan Corbet <corbet@lwn.net>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Mark Rutland <mark.rutland@arm.com>
Cc: Boqun Feng <boqun.feng@gmail.com>
Cc: <linux-doc@vger.kernel.org>
---
 Documentation/core-api/this_cpu_ops.rst | 22 ++++++++++++++++------
 1 file changed, 16 insertions(+), 6 deletions(-)

diff --git a/Documentation/core-api/this_cpu_ops.rst b/Documentation/core-api/this_cpu_ops.rst
index 91acbcf30e9bd..11e3e48731553 100644
--- a/Documentation/core-api/this_cpu_ops.rst
+++ b/Documentation/core-api/this_cpu_ops.rst
@@ -138,12 +138,22 @@ get_cpu/put_cpu sequence requires. No processor number is
 available. Instead, the offset of the local per cpu area is simply
 added to the per cpu offset.
 
-Note that this operation is usually used in a code segment when
-preemption has been disabled. The pointer is then used to
-access local per cpu data in a critical section. When preemption
-is re-enabled this pointer is usually no longer useful since it may
-no longer point to per cpu data of the current processor.
-
+Note that this operation can only be used in code segments where
+smp_processor_id() may be used, for example, where preemption has been
+disabled. The pointer is then used to access local per cpu data in a
+critical section. When preemption is re-enabled this pointer is usually
+no longer useful since it may no longer point to per cpu data of the
+current processor.
+
+The special cases where it makes sense do obtain a per-CPU pointer in
+preemptible code are addressed by raw_cpu_ptr(), but please note that such
+use cases need to handle cases where two different CPUs are accessing
+the same per cpu variable, which might well be that of a third CPU.
+These use cases are typically performance optimizations.  For example,
+SRCU implements a pair of counters as a pair of per-CPU variables,
+and rcu_read_lock_nmisafe() uses raw_cpu_ptr() to get a pointer to some
+CPU's counter, and uses atomic_inc_long() to handle migration between
+the raw_cpu_ptr() and the atomic_inc_long().
 
 Per cpu variables and offsets
 -----------------------------
-- 
2.40.1