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]>

diff --git a/kernel/kthread.c b/kernel/kthread.c
index 1d9e2fdfd67ae880783bf355571afb8defa9eced..1c8964feeb013fcddeef1063faf85f1841ec83c1 100644 (file)
--- a/kernel/kthread.c
+++ b/kernel/kthread.c
@@ -1241,13 +1241,16 @@ void kthread_use_mm(struct mm_struct *mm)
        WARN_ON_ONCE(tsk->mm);
 
        task_lock(tsk);
+       /* Hold off tlb flush IPIs while switching mm's */
+       local_irq_disable();
        active_mm = tsk->active_mm;
        if (active_mm != mm) {
                mmgrab(mm);
                tsk->active_mm = mm;
        }
        tsk->mm = mm;
-       switch_mm(active_mm, mm, tsk);
+       switch_mm_irqs_off(active_mm, mm, tsk);
+       local_irq_enable();
        task_unlock(tsk);
 #ifdef finish_arch_post_lock_switch
        finish_arch_post_lock_switch();
@@ -1276,9 +1279,11 @@ void kthread_unuse_mm(struct mm_struct *mm)
 
        task_lock(tsk);
        sync_mm_rss(mm);
+       local_irq_disable();
        tsk->mm = NULL;
        /* active_mm is still 'mm' */
        enter_lazy_tlb(mm, tsk);
+       local_irq_enable();
        task_unlock(tsk);
 }
 EXPORT_SYMBOL_GPL(kthread_unuse_mm);