The patch titled
local_t : Documentation - update
has been added to the -mm tree. Its filename is
local_t-documentation-fix.patch
See http://www.zip.com.au/~akpm/linux/patches/stuff/added-to-mm.txt to find
out what to do about this
------------------------------------------------------
Subject: local_t : Documentation - update
From: Mathieu Desnoyers <mathieu.desnoyers@xxxxxxxxxx>
Signed-off-by: Andrew Morton <akpm@xxxxxxxx>
---
Documentation/local_ops.txt | 15 +++++++++++++++
1 files changed, 15 insertions(+)
diff -puN Documentation/local_ops.txt~local_t-documentation-fix Documentation/local_ops.txt
--- a/Documentation/local_ops.txt~local_t-documentation-fix
+++ a/Documentation/local_ops.txt
@@ -22,6 +22,13 @@ require disabling interrupts to protect
coherent counters in NMI handlers. It is especially useful for tracing purposes
and for various performance monitoring counters.
+Local atomic operations only guarantee variable modification atomicity wrt the
+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.
+
* Implementation for a given architecture
@@ -31,6 +38,12 @@ i386 and x86_64) and any SMP sychronizat
not have a different behavior between SMP and UP, including asm-generic/local.h
in your archtecture's local.h is sufficient.
+The local_t type is defined as an opaque signed long by embedding an
+atomic_long_t inside a structure. This is made so a cast from this type to a
+long fails. The definition looks like :
+
+typedef struct { atomic_long_t a; } local_t;
+
* How to use local atomic operations
@@ -42,6 +55,8 @@ static DEFINE_PER_CPU(local_t, counters)
* Counting
+Counting is done on all the bits of a signed long.
+
In preemptible context, use get_cpu_var() and put_cpu_var() around local atomic
operations : it makes sure that preemption is disabled around write access to
the per cpu variable. For instance :
_
Patches currently in -mm which might be from mathieu.desnoyers@xxxxxxxxxx are
relay-add-cpu-hotplug-support.patch
local_t-documentation.patch
local_t-documentation-fix.patch
-
To unsubscribe from this list: send the line "unsubscribe mm-commits" in
the body of a message to majordomo@xxxxxxxxxxxxxxx
More majordomo info at http://vger.kernel.org/majordomo-info.html
