RTEMS Chain API and SMP

[Sebastian Huber]

Hello,

I am glad that we had no RTEMS 4.11 release yet.  I have to revisit the 
RTEMS chain API.

This solution was a bad mistake as it runs out now:

http://git.rtems.org/rtems/commit/cpukit/sapi/include/rtems/chain.h?id=1215fd4d9426a59d568560e9a485628560363133

In order to support profiling of SMP locks and provide a future 
compatible SMP locks API I have to add an SMP lock destroy function.  
Since the patch adds an SMP lock to each chain control we would have to 
add a rtems_chain_destroy() function as well.  This is complicates the 
chain usage dramatically.  So I propose to revert the patch above.  A 
global SMP lock for all chains should be use to implement the protected 
chain operations.

Advantages:

* The RTEMS chain API is now identical on SMP and non-SMP configurations.

* The size of the chain control is reduced and is then equal to the 
Score chains.

* The protected chain operations work correctly on SMP.

Disadvantage:

* Applications using many different chains and the protected operations 
may notice lock contention.

I think the chain control size drop is a huge benefit (RTEMS chain 
controls are 66% larger than the Score chain controls).  The only 
disadvantage is not really a problem since these applications can use 
specific interrupt locks and unprotected chain operations to avoid this 
issue.

-- 
Sebastian Huber, embedded brains GmbH

Address : Dornierstr. 4, D-82178 Puchheim, Germany
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E-Mail  : sebastian.huber at embedded-brains.de
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