On Mon, Jun 19, 2017 at 6:33 AM, zhong jiang <zhongjiang@xxxxxxxxxx> wrote:
> 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?
Is there a code path that does this?
Also, the IPI handler on 3.4 looks like this:
if (f->flush_mm == percpu_read(cpu_tlbstate.active_mm)) {
if (percpu_read(cpu_tlbstate.state) == TLBSTATE_OK) {
if (f->flush_va == TLB_FLUSH_ALL)
local_flush_tlb();
else
__flush_tlb_one(f->flush_va);
} else
leave_mm(cpu);
}
but leave_mm() checks the same condition (cpu_tlbstate.state, not
current->mm). How is the BUG triggering?
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