Commit:     e1265205c0ee3919c3f2c750662630154c8faab2
Parent:     bf2cdef30667d0d3d09c6934c95d1fb87c43345a
Author:     Mathieu Desnoyers <[EMAIL PROTECTED]>
AuthorDate: Mon Nov 5 14:50:54 2007 -0800
Committer:  Linus Torvalds <[EMAIL PROTECTED]>
CommitDate: Mon Nov 5 15:12:32 2007 -0800

    local_t Documentation update
    Grant Grundler was asking for more detail about correct usage of local
    atomic operations and suggested adding the resulting summary to
    "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 <[EMAIL PROTECTED]>
    Signed-off-by: Grant Grundler <[EMAIL PROTECTED]>
    Signed-off-by: Andrew Morton <[EMAIL PROTECTED]>
    Signed-off-by: Linus Torvalds <[EMAIL PROTECTED]>
 Documentation/local_ops.txt |   23 +++++++++++++++++++++++
 1 files changed, 23 insertions(+), 0 deletions(-)

diff --git a/Documentation/local_ops.txt b/Documentation/local_ops.txt
index 4269a11..1a45f11 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+  synchronization is done by the writer CPU, an outdated copy of the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>
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