1 files changed, 23 insertions, 0 deletions

diff --git a/Documentation/local_ops.txt b/Documentation/local_ops.txt
index b0aca0705d1e..a134a563db32 100644
--- a/Documentation/local_ops.txt
+++ b/Documentation/local_ops.txt
@@ -45,6 +45,29 @@ long fails. The definition looks like :
 typedef struct { atomic_long_t a; } local_t;
 
 
+* Rules to follow when using local atomic operations
+
+- Variables touched by local ops must be per cpu variables.
+- _Only_ the CPU owner of these variables must write to them.
+- This CPU can use local ops from any context (process, irq, softirq, nmi, ...)
+  to update its local_t variables.
+- Preemption (or interrupts) must be disabled when using local ops in
+  process context to   make sure the process won't be migrated to a
+  different CPU between getting the per-cpu variable and doing the
+  actual local op.
+- When using local ops in interrupt context, no special care must be
+  taken on a mainline kernel, since they will run on the local CPU with
+  preemption already disabled. I suggest, however, to explicitly
+  disable preemption anyway to make sure it will still work correctly on
+  -rt kernels.
+- Reading the local cpu variable will provide the current copy of the
+  variable.
+- Reads of these variables can be done from any CPU, because updates to
+  "long", aligned, variables are always atomic. Since no memory
+  synchronization is done by the writer CPU, an outdated copy of the
+  variable can be read when reading some _other_ cpu's variables.
+
+
 * How to use local atomic operations
 
 #include <linux/percpu.h>