mm: fix kthread_use_mm() vs TLB invalidate

For SMP systems using IPI based TLB invalidation, looking at
current->active_mm is entirely reasonable.  This then presents the
following race condition:

  CPU0			CPU1

  flush_tlb_mm(mm)	use_mm(mm)
    <send-IPI>
			  tsk->active_mm = mm;
			  <IPI>
			    if (tsk->active_mm == mm)
			      // flush TLBs
			  </IPI>
			  switch_mm(old_mm,mm,tsk);

Where it is possible the IPI flushed the TLBs for @old_mm, not @mm,
because the IPI lands before we actually switched.

Avoid this by disabling IRQs across changing ->active_mm and
switch_mm().

Of the (SMP) architectures that have IPI based TLB invalidate:

  Alpha    - checks active_mm
  ARC      - ASID specific
  IA64     - checks active_mm
  MIPS     - ASID specific flush
  OpenRISC - shoots down world
  PARISC   - shoots down world
  SH       - ASID specific
  SPARC    - ASID specific
  x86      - N/A
  xtensa   - checks active_mm

So at the very least Alpha, IA64 and Xtensa are suspect.

On top of this, for scheduler consistency we need at least preemption
disabled across changing tsk->mm and doing switch_mm(), which is
currently provided by task_lock(), but that's not sufficient for
PREEMPT_RT.

[[email protected]: add comment]

Reported-by: Andy Lutomirski <[email protected]>
Signed-off-by: Peter Zijlstra (Intel) <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>
Cc: Nicholas Piggin <[email protected]>
Cc: Jens Axboe <[email protected]>
Cc: Kees Cook <[email protected]>
Cc: Jann Horn <[email protected]>
Cc: Will Deacon <[email protected]>
Cc: Christoph Hellwig <[email protected]>
Cc: Mathieu Desnoyers <[email protected]>
Cc: <[email protected]>
Link: http://lkml.kernel.org/r/[email protected]
Signed-off-by: Linus Torvalds <[email protected]>

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