On Wed, Aug 29, 2007 at 08:19:53AM -0400, Mathieu Desnoyers wrote:
> 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 <[email protected]>
> CC: Grant Grundler <[email protected]>
Signed-off-by: Grant Grundler <[email protected]>
thanks!
grant
> ---
> 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 + synchronization is done by the writer CPU, an outdated copy of the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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