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-rw-r--r--Documentation/local_ops.txt23
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diff --git a/Documentation/local_ops.txt b/Documentation/local_ops.txt
index 4269a1105b37..1a45f11e645e 100644
--- a/Documentation/local_ops.txt
+++ b/Documentation/local_ops.txt
@@ -68,6 +68,29 @@ typedef struct { atomic_long_t a; } local_t;
variable can be read when reading some _other_ cpu's variables.
+* 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+ synchronization is done by the writer CPU, an outdated copy of the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>