[PATCH] local_t Documentation update 2

Mathieu Desnoyers
2007-10-31T01:58:09Z

local_t Documentation update 2

(this patch seems to have fallen off the grid, but is still providing
useful information. It applies to 2.6.23-mm1.)

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."

Signed-off-by: Mathieu Desnoyers <mathieu.desnoyers@polymtl.ca>
Signed-off-by: Grant Grundler <grundler@parisc-linux.org>
---
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 2007-09-04 11:53:23.000000000 -0400
+++ linux-2.6-lttng/Documentation/local_ops.txt 2007-09-04 12:19:31.000000000 -0400
@@ -68,6 +68,29 @@ typedef struct { atomic_long_t a; } loca
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+ synchronization is done by the writer CPU, an outdated copy of the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>
--
Mathieu Desnoyers
Computer Engineering Ph.D. Student, Ecole Polytechnique de Montreal
OpenPGP key fingerprint: 8CD5 52C3 8E3C 4140 715F BA06 3F25 A8FE 3BAE 9A68