[PATCH 1/2] score: Add _Per_CPU_Get_snapshot()

[Gedare Bloom]

On Fri, Apr 11, 2014 at 2:00 PM, Sebastian Huber
<sebastian.huber at embedded-brains.de> wrote:
> On 04/11/2014 07:29 PM, Gedare Bloom wrote:
>>
>> On Fri, Apr 11, 2014 at 11:15 AM, Sebastian Huber
>> <sebastian.huber at embedded-brains.de>  wrote:
>>>
>>> >---
>>> >  cpukit/score/include/rtems/score/percpu.h |    8 +++++---
>>> >  1 files changed, 5 insertions(+), 3 deletions(-)
>>> >
>>> >diff --git a/cpukit/score/include/rtems/score/percpu.h
>>> > b/cpukit/score/include/rtems/score/percpu.h
>>> >index 75ff3e2..92a6e8a 100644
>>> >--- a/cpukit/score/include/rtems/score/percpu.h
>>> >+++ b/cpukit/score/include/rtems/score/percpu.h
>>> >@@ -424,11 +424,13 @@ extern Per_CPU_Control_envelope
>>> > _Per_CPU_Information[] CPU_STRUCTURE_ALIGNMENT;
>>> >    _ISR_Enable( isr_cookie )
>>> >  #endif
>>> >
>>> >+#define _Per_CPU_Get_snapshot() \
>>> >+  ( &_Per_CPU_Information[ _SMP_Get_current_processor() ].per_cpu )
>>> >+
>>
>> I think of a snapshot as a copy of some data that might change in the
>> future. Since this gets the reference without making even a
>> shallow-copy, I don't know what is being "snapshotted" or what
>> guarantees if any are made about the snapshot.
>>
>
> If you read the processor index of the current processor in a context which
> allows thread dispatching, then you may already run on another processor
> right after the read instruction.  There are very few cases in which this
> makes sense (here we can use _Per_CPU_Get_snapshot()).  All other places
> must use _Per_CPU_Get() so that we can add checks for RTEMS_DEBUG.
>
> Do you have a better name?  I will add the comment above to the source code.
>
I don't have a better name. The explanation helps. In case the
processor changes, then it makes sense to call it a snapshot since it
can reflect stale state.

> --
> Sebastian Huber, embedded brains GmbH
>
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>