Move _RET_IP_ and _THIS_IP_ to include/linux/kernel.h

[pandora-kernel.git] / Documentation / local_ops.txt

diff --git a/Documentation/local_ops.txt b/Documentation/local_ops.txt
index b0aca07..4269a11 100644 (file)
--- a/Documentation/local_ops.txt
+++ b/Documentation/local_ops.txt
@@ -27,7 +27,7 @@ CPU which owns the data. Therefore, care must taken to make sure that only one
 CPU writes to the local_t data. This is done by using per cpu data and making
 sure that we modify it from within a preemption safe context. It is however
 permitted to read local_t data from any CPU : it will then appear to be written
-out of order wrt other memory writes on the owner CPU.
+out of order wrt other memory writes by the owner CPU.
 
 
 * Implementation for a given architecture
@@ -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+  synchronization is done by the writer CPU, an outdated copy of the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>