Implement shared futexes.

- Shared futex objects on shared mappings are represented by Mappable +
  offset, analogous to Linux's use of inode + offset. Add type
  futex.Key, and change the futex.Manager bucket API to use futex.Keys
  instead of addresses.

- Extend the futex.Checker interface to be able to return Keys for
  memory mappings. It returns Keys rather than just mappings because
  whether the address or the target of the mapping is used in the Key
  depends on whether the mapping is MAP_SHARED or MAP_PRIVATE; this
  matters because using mapping target for a futex on a MAP_PRIVATE
  mapping causes it to stop working across COW-breaking.

- futex.Manager.WaitComplete depends on atomic updates to
  futex.Waiter.addr to determine when it has locked the right bucket,
  which is much less straightforward for struct futex.Waiter.key. Switch
  to an atomically-accessed futex.Waiter.bucket pointer.

- futex.Manager.Wake now needs to take a futex.Checker to resolve
  addresses for shared futexes. CLONE_CHILD_CLEARTID requires the exit
  path to perform a shared futex wakeup (Linux:
  kernel/fork.c:mm_release() => sys_futex(tsk->clear_child_tid,
  FUTEX_WAKE, ...)). This is a problem because futexChecker is in the
  syscalls/linux package. Move it to kernel.

PiperOrigin-RevId: 216207039
Change-Id: I708d68e2d1f47e526d9afd95e7fed410c84afccf

0 files changed, 0 insertions, 0 deletions