Re: [PATCH v2 1/9] s390/uaccess: Add storage key checked cmpxchg access to user space

On Thu, Oct 20, 2022 at 03:40:56PM +0200, Nico Boehr wrote:

Quoting Janis Schoetterl-Glausch (2022-10-12 22:56:01)

...

•           "2:     lr      %[old_word],%[tmp]\n"
  

•           "3:     cs      %[tmp],%[new_word],%[aligned]\n"
  

•           "4:     jnl     5f\n"
  

•           /* We'll restore old_word before the cs, use reg for the diff */
  

•           "       xr      %[old_word],%[tmp]\n"
  

•           /* Apply diff assuming only bits outside target byte(s) changed */
  

•           "       xr      %[new_word],%[old_word]\n"
  

•           /* If prior assumption false we exit loop, so not an issue */
  

•           "       nr      %[old_word],%[mask]\n"
  

•           "       jz      2b\n"
  

So if the remainder changed but the actual value to exchange stays the same, we loop in the kernel. Does it maybe make sense to limit the number of iterations we spend retrying? I think while looping here the calling process can't be killed, can it?

Yes, the number of loops should be limited; quite similar what arm64 implemented with commit 03110a5cb216 ("arm64: futex: Bound number of LDXR/STXR loops in FUTEX_WAKE_OP").