在 2013-8-28，下午10:50，Sebastian Huber <sebastian.hu...@embedded-brains.de> 写道：
> On 2013-08-28 16:26, Deng Hengyi wrote: >> >> 在 2013-8-28，下午9:19，Sebastian Huber <sebastian.hu...@embedded-brains.de> 写道： >> >>> 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. >>> >> Should this issue be a gcc bug which should be fixed by gcc? > > Provided my test code is ok and this is actually a GCC problem, I don't > expect this will be fixed soon. This potential bug is there for more than > one year and nobody noticed. Anyway this simple atomic flag is next to > useless for serious SMP stuff. > > See also http://en.wikipedia.org/wiki/Test-and-set, search for consensus > number. > > The generated PowerPC code for _SMP_barrier_Wait() which uses the unsigned > integer atomic operations in smptests/smpatomic08 is actually quite good. If the test-and-set ops is bad to be used we can consider to remove the atomic flag from the RTEMS API. Because now atomic flag only has atomic_flag_clear and atomic_flag_test_and_set two ops. > > >>> 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. >>> >> In the stdatomic.h and C11 standard it has defined a API >> atomic_flag_test_and_set_explicit, so it should support atomic test and set >> ops. If we encounter the error we should consider whether the gcc for this >> architecture is not supported or gcc will not support this ops. > > A test-and-set lock is only useful as a bad example: > > http://en.wikipedia.org/wiki/Test_and_Test-and-set > >>> 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 >>> Phone : +49 89 189 47 41-16 >>> Fax : +49 89 189 47 41-09 >>> E-Mail : sebastian.hu...@embedded-brains.de >>> PGP : Public key available on request. >>> >>> Diese Nachricht ist keine geschäftliche Mitteilung im Sinne des EHUG. >>> _______________________________________________ >>> rtems-devel mailing list >>> rtems-devel@rtems.org >>> http://www.rtems.org/mailman/listinfo/rtems-devel >> > > > -- > Sebastian Huber, embedded brains GmbH > > Address : Dornierstr. 4, D-82178 Puchheim, Germany > Phone : +49 89 189 47 41-16 > Fax : +49 89 189 47 41-09 > E-Mail : sebastian.hu...@embedded-brains.de > PGP : Public key available on request. > > Diese Nachricht ist keine geschäftliche Mitteilung im Sinne des EHUG. _______________________________________________ rtems-devel mailing list rtems-devel@rtems.org http://www.rtems.org/mailman/listinfo/rtems-devel
