The patch titled
local_t Documentation update 2
has been added to the -mm tree. Its filename is
local_t-documentation-update-2.patch
*** Remember to use Documentation/SubmitChecklist when testing your code ***
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 2
From: Mathieu Desnoyers <mathieu.desnoyers@xxxxxxxxxx>
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@xxxxxxxxxx>
Signed-off-by: Grant Grundler <grundler@xxxxxxxxxxxxxxxx>
Signed-off-by: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>
---
Documentation/local_ops.txt | 23 +++++++++++++++++++++++
1 files changed, 23 insertions(+)
diff -puN Documentation/local_ops.txt~local_t-documentation-update-2 Documentation/local_ops.txt
--- a/Documentation/local_ops.txt~local_t-documentation-update-2
+++ a/Documentation/local_ops.txt
@@ -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>
_
Patches currently in -mm which might be from mathieu.desnoyers@xxxxxxxxxx are
powerpc-include-pagemaph-in-asm-powerpc-tlbh.patch
git-mips.patch
fix-f_version-type-should-be-u64-instead-of-unsigned-long.patch
local_t-documentation-update-2.patch
local_t-update-documentation.patch
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