Atomic Operations and Required Architecture Support for RTEMS SMP

[Sebastian Huber]

Hello,

as part of a GSoC project we made the atomic operations API compatible with the 
C11 operations and use them for the implementation.  I added some tests today 
and now begin to think if this was really such a good idea:

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=58259

Surprisingly it seems that the atomic operations support is not shared by the C 
and C++ compiler parts in GCC.

I just realized that the C11 atomic_flag type has no load operation.  This 
makes it not suitable to implement a simple test and test and set lock.  So 
this type is useless for us.  The only special thing with atomic_flag is that 
it is guaranteed to be lock free.  This is the minimum requirement for atomic 
operations.

This leads to the following question: What is the RTEMS (!) minimum requirement 
for SMP support?

I think an architecture must support a compare and swap instruction or the load 
linked and store conditional model.  We should make it clear that a simple 
atomic swap is not enough to support SMP.

Under this requirement we can remove the atomic flag from the RTEMS API.

-- 
Sebastian Huber, embedded brains GmbH

Address : Dornierstr. 4, D-82178 Puchheim, Germany
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