New way for syncing with kernel headers

[Florian Weimer <fweimer at redhat dot com>]

Would this be an acceptable way to obtain definitions from the Linux 
headers in places where namespace cleanliness is not a must?

/* Obtain the definitions of the MEMBARRIER_CMD_* constants.  */

#include <linux/version.h>
#if LINUX_VERSION_CODE >= KERNEL_VERSION(4, 14, 0)
# include <linux/membarrier.h>
#else

/* Definitions from Linux 4.14 follow.  */

enum membarrier_cmd
{
  MEMBARRIER_CMD_QUERY = 0,
  MEMBARRIER_CMD_SHARED = 1,
  MEMBARRIER_CMD_PRIVATE_EXPEDITED = 8,
  MEMBARRIER_CMD_REGISTER_PRIVATE_EXPEDITED = 16,
};

#endif


The idea is to have a built-in fallback definition in case the installed 
kernel headers are too old.  But for newer kernels, the UAPI header is 
used, to avoid duplicate definitions and a need for maintaining our own 
copy continuously.

The inline copy of the definitions reflects what is needed for building 
glibc itself (including tests) and the manual.  But if the end user 
installs newer kernel headers, their definitions are immediately 
available for use with glibc-based applications.  Eventually, we can 
remove the inline copy once the minimum kernel version for building 
moves past the baseline requirement.

Header include ordering does not matter because <linux/membarrier.h> has 
its own include guard.

Has this already been tried?  Do you see any downsides?

Thanks,
Florian