Ok, time to dive into the kernel and find out what "execmem" exactly means:

 /*
  * We are making executable an anonymous mapping or a
  * private file mapping that will also be writable.
  * This has an additional check.
  */

Basically, this appears to be a mmap with PROT_EXEC and PROT_WRITE.

Now lets see if I can find where this is used in pthreads, and if I can just remove either PROT_EXEC or PROT_WRITE there...

Okay, most likely, this is the offending line:

nptl/allocatestack.c
static int allocate_stack (...
	mem = mmap (NULL, size, PROT_READ | PROT_WRITE | PROT_EXEC,
		MAP_PRIVATE | MAP_ANONYMOUS | ARCH_MAP_FLAGS, -1, 0);

So pthreads stacks are executeable memory by default... but disabling that sounds risky to me... :-(

Thanks to whoever fixed (read: enhanced) the radeon driver or Scorched 3D.

With visible water the game is even more fun. ;-)