The sole remaining usage of CLONE_IDLETASK is to determine whether pid
  allocation should be performed in copy_process().  This patch eliminates
  that last branch on CLONE_IDLETASK in the normal process creation path,
  removes the masking of CLONE_IDLETASK from clone_flags as it's now ignored
  under all circumstances, and furthermore eliminates the symbol
  CLONE_IDLETASK entirely.

From: William Lee Irwin III <wli@holomorphy.com>

  Fix the fork-idle consolidation.  During that consolidation, the generic
  code was made to pass a pointer to on-stack pt_regs that had been memset()
  to 0.  ia64, however, requires a NULL pt_regs pointer argument and
  dispatches on that in its copy_thread() function to do SMP
  trampoline-specific RSE -related setup.  Passing pointers to zeroed pt_regs
  resulted in SMP wakeup -time deadlocks and exceptions.
Signed-off-by: Andrew Morton <akpm@osdl.org>
Signed-off-by: Linus Torvalds <torvalds@osdl.org>