C11 had finally brought a standardized interface for dealing with fine-grained atomic operations for lock-less concurrent programming. The _Atomic support is one of the big improvements in C11 over C99 in allowing for better multi-threading support for uninterruptive object access. Code for _Atomic in GCC has been baking for a while in a c11-atomic branch, and some atomic operations support landed in GCC 4.7, but now it appears _Atomic is ready for mainline.
Myers wrote in a new mailing list message, "This patch, relative to trunk and based on work done on the C11-atomic branch, adds support for C11 _Atomic. It is intended to include all the required language support." While the language support is there, still to be firmed up is the stdatomic.h header file for mainline. Another shortcoming is that the _Atomic support is disabled for GCC's Objective-C and OpenMP support right now.
For those interested in more details on the proposed GCC _Atomic support for C11, see the aforelinked mailing list message. More details as well can be found via the GCC Wiki. The current GCC C11 status can be found via this Wiki page. The C11 _Atomic support should be hopefully merged soon thus making it a feature in GCC 4.9.