[PATCH] local_t Documentation update 2

[Mathieu Desnoyers]

local_t Documentation update 2

Grant Grundler was asking for more detail about correct usage of local atomic
operations and suggested adding the resulting summary to local_ops.txt.

"Please add a bit more detail. If DaveM is correct (he normally is), then
there must be limits on how the local_t can be used in the kernel process
and interrupt contexts. I'd like those rules spelled out very clearly
since it's easy to get wrong and tracking down such a bug is quite painful."

It applies on top of 2.6.23-rc3-mm1.

Signed-off-by: Mathieu Desnoyers <mathieu.desnoyers@xxxxxxxxxx>
CC: Grant Grundler <grundler@xxxxxxxxxxxxxxxx>
---
 Documentation/local_ops.txt |   23 +++++++++++++++++++++++
 1 file changed, 23 insertions(+)

Index: linux-2.6-lttng/Documentation/local_ops.txt
===================================================================
--- linux-2.6-lttng.orig/Documentation/local_ops.txt	2007-08-29 08:09:34.000000000 -0400
+++ linux-2.6-lttng/Documentation/local_ops.txt	2007-08-29 08:15:37.000000000 -0400
@@ -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+  synchronization is done by the writer CPU, an outdated copy of the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>

-- 
Mathieu Desnoyers
Computer Engineering Ph.D. Student, Ecole Polytechnique de Montreal
OpenPGP key fingerprint: 8CD5 52C3 8E3C 4140 715F  BA06 3F25 A8FE 3BAE 9A68
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