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.
Ultimately from an API perspective, the functionality that depends on futex_cmpxchg_enabled is non-optional, and the current approach of treating it as something that can be disabled via detection at runtime is fragile and wrong. If there are no archs that support SMP but don't provide their own asm/futex.h (as opposed to the asm-generic one that does -ENOSYS on SMP), the detection code should just be removed, and the SMP case in asm-generic/futex.h should be made into #error. If there are archs that support SMP but don't provide their own working asm/futex.h, then asm-generic/futex.h's SMP case should be enhanced to perform a stop-the-world IPI and then do the same thing as the non-SMP case (disable preemption[/interrupts?], perform the cmpxchg non-atomically). Thoughts? Would a patch to do this be acceptable? Rich
