Re: [PATCH] x86/mm: Don't reenter flush_tlb_func_common()

[zhong jiang]

On 2017/6/19 12:48, Andy Lutomirski wrote:
> It was historically possible to have two concurrent TLB flushes
> targeting the same CPU: one initiated locally and one initiated
> remotely.  This can now cause an OOPS in leave_mm() at
> arch/x86/mm/tlb.c:47:
>
>         if (this_cpu_read(cpu_tlbstate.state) == TLBSTATE_OK)
>                 BUG();
>
> with this call trace:
>  flush_tlb_func_local arch/x86/mm/tlb.c:239 [inline]
>  flush_tlb_mm_range+0x26d/0x370 arch/x86/mm/tlb.c:317
>
> Without reentrancy, this OOPS is impossible: leave_mm() is only
> called if we're not in TLBSTATE_OK, but then we're unexpectedly
> in TLBSTATE_OK in leave_mm().
>
> This can be caused by flush_tlb_func_remote() happening between
> the two checks and calling leave_mm(), resulting in two consecutive
> leave_mm() calls on the same CPU with no intervening switch_mm()
> calls.
>
> We never saw this OOPS before because the old leave_mm()
> implementation didn't put us back in TLBSTATE_OK, so the assertion
> didn't fire.
  HI, Andy

  Today, I see same OOPS in linux 3.4 stable. It prove that it indeed has fired.
   but It is rarely to appear.  I review the code. I found the a  issue.
  when current->mm is NULL,  leave_mm will be called. but  it maybe in
  TLBSTATE_OK,  eg: unuse_mm call after task->mm = NULL , but before enter_lazy_tlb.

   therefore,  it will fire. is it right?

  Thanks
  zhongjiang
> Nadav noticed the reentrancy issue in a different context, but
> neither of us realized that it caused a problem yet.
>
> Cc: Nadav Amit <nadav.amit@xxxxxxxxx>
> Cc: Dave Hansen <dave.hansen@xxxxxxxxx>
> Reported-by: "Levin, Alexander (Sasha Levin)" <alexander.levin@xxxxxxxxxxx>
> Fixes: 3d28ebceaffa ("x86/mm: Rework lazy TLB to track the actual loaded mm")
> Signed-off-by: Andy Lutomirski <luto@xxxxxxxxxx>
> ---
>  arch/x86/mm/tlb.c | 15 +++++++++++++--
>  1 file changed, 13 insertions(+), 2 deletions(-)
>
> diff --git a/arch/x86/mm/tlb.c b/arch/x86/mm/tlb.c
> index 2a5e851f2035..f06239c6919f 100644
> --- a/arch/x86/mm/tlb.c
> +++ b/arch/x86/mm/tlb.c
> @@ -208,6 +208,9 @@ void switch_mm_irqs_off(struct mm_struct *prev, struct mm_struct *next,
>  static void flush_tlb_func_common(const struct flush_tlb_info *f,
>  				  bool local, enum tlb_flush_reason reason)
>  {
> +	/* This code cannot presently handle being reentered. */
> +	VM_WARN_ON(!irqs_disabled());
> +
>  	if (this_cpu_read(cpu_tlbstate.state) != TLBSTATE_OK) {
>  		leave_mm(smp_processor_id());
>  		return;
> @@ -313,8 +316,12 @@ void flush_tlb_mm_range(struct mm_struct *mm, unsigned long start,
>  		info.end = TLB_FLUSH_ALL;
>  	}
>  
> -	if (mm == this_cpu_read(cpu_tlbstate.loaded_mm))
> +	if (mm == this_cpu_read(cpu_tlbstate.loaded_mm)) {
> +		local_irq_disable();
>  		flush_tlb_func_local(&info, TLB_LOCAL_MM_SHOOTDOWN);
> +		local_irq_enable();
> +	}
> +
>  	if (cpumask_any_but(mm_cpumask(mm), cpu) < nr_cpu_ids)
>  		flush_tlb_others(mm_cpumask(mm), &info);
>  	put_cpu();
> @@ -370,8 +377,12 @@ void arch_tlbbatch_flush(struct arch_tlbflush_unmap_batch *batch)
>  
>  	int cpu = get_cpu();
>  
> -	if (cpumask_test_cpu(cpu, &batch->cpumask))
> +	if (cpumask_test_cpu(cpu, &batch->cpumask)) {
> +		local_irq_disable();
>  		flush_tlb_func_local(&info, TLB_LOCAL_SHOOTDOWN);
> +		local_irq_enable();
> +	}
> +
>  	if (cpumask_any_but(&batch->cpumask, cpu) < nr_cpu_ids)
>  		flush_tlb_others(&batch->cpumask, &info);
>  	cpumask_clear(&batch->cpumask);