* Rich Felker: > I just spent a number of hours helping someone track down a bug that > looks like it's some kind of futex_cmpxchg_enabled detection error on > powerpc64 (still not sure of the root cause; set_robust_list producing > -ENOSYS), and a while back I hit the same problem on sh2 due to lack > of EFAULT on nommu, leading to commit 72cc564f16ca. I think the test > (introduced way back in commit a0c1e9073ef7) is fundamentally buggy; > if anything, it should be checking for !=-ENOSYS, not ==-EFAULT. > Presumably it could also fail to produce -EFAULT if mmap_min_addr is 0 > and page 0 is mapped (a bad idea, but maybe someone does it...). And > of course other nommu archs are possibly still broken. Maybe it was related to this (“Kernel 4.15 lost set_robust_list support on POWER 9”): <https://lists.ozlabs.org/pipermail/linuxppc-dev/2018-February/168570.html> The Kconfig change you suggest was explicitly rejected as the fix. I believe the expected userspace interface is that you probe support with set_robust_list first, and then start using the relevant futex interfaces only if that call succeeded. If you do that, most parts of a typical system will work as expected even if the kernel support is not there, which is a bit surprising. It definitely makes the root cause harder to spot.
