Note -latomic needed when using -mcx16

The ABA example relies on 16-byte atomic compare-and-swap operations,
which require the `-mcx16` compiler flag on x86-64 to enable the
corresponding instructions.

However, due to current limitations in GCC, 16-byte atomic operations
(e.g., `__atomic_load_16` and `__atomic_compare_exchange_16`) are
implemented via calls to `libatomic` on many platforms. This results
in linker errors unless `-latomic` is explicitly added.

To avoid confusion and ensure portability, this patch adds a footnote
to the LaTeX document explaining that `-latomic` must be linked when
using `-mcx16`. For more details, see:
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=104688

Author: weiso131

concurrency-primer.tex

@@ -959,7 +959,7 @@ \subsection{ABA problem}
 On x86-64 processors, for atomic instructions that load or store more than a CPU word size, it needs additional hardware support.
 You can use \monobox{grep cx16 /proc/cpuinfo} to check if the processor supports 16-byte compare-and-swap.
 For hardware that does not support the desired size, software implementations which may have locks involve are used instead, as mentioned in \secref{atomictype}.
-Back to the example, ABA problem in the following code is fixed by using an version number that increments each time a job is added to the empty queue. On x86-64, add a compiler flag \monobox{-mcx16} to enable 16-byte compare-and-swap in \monobox{worker} function.
+Back to the example, ABA problem in the following code is fixed by using an version number that increments each time a job is added to the empty queue. On x86-64, add a compiler flag \monobox{-mcx16} to enable 16-byte compare-and-swap in \monobox{worker} function.\footnote{When using \texttt{-mcx16}, programs relying on 16-byte atomic operations (e.g., using \texttt{\_\_int128}) may require linking with \texttt{-latomic}, as these operations are implemented via function calls to \texttt{libatomic} on some systems. See \url{https://gcc.gnu.org/bugzilla/show_bug.cgi?id=104688}.}
 
 \inputminted{c}{./examples/rmw_example_aba.c}