@@ -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 to obtain a per-CPU pointer in
+preemptible code are addressed by raw_cpu_ptr(), but 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
-----------------------------
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(-)