Re: [PATCH 3/3] kref: Remove the memory barriers

[Ming Lei]

On Sun, Dec 11, 2011 at 8:47 PM, Peter Zijlstra <a.p.zijlstra@xxxxxxxxx> wrote:
> On Sun, 2011-12-11 at 10:22 +0800, Ming Lei wrote:
>> On Sun, Dec 11, 2011 at 3:49 AM, Peter Zijlstra <a.p.zijlstra@xxxxxxxxx> wrote:
>> > On Sat, 2011-12-10 at 23:57 +0800, Ming Lei wrote:
>> >
>> >> CPU0                  CPU1
>> >>
>> >> atomic_set(v)
>> >> smp_mb()
>> >>                               smp_mb()
>> >>                               atomic_dec_and_test(v)
>> >>
>> >> Without the barrier after atomic_set, CPU1 may see a stale
>> >> value of v first, then decrease it, so may miss a release operation.
>> >
>> > Your example is doubly broken. If there's concurrency possible with
>> > atomic_set() you've lost.
>>
>> kref_init is guaranteed to be run only one time __before__ executing
>> kref_get/kref_put.
>
> If used properly, yes. But in that case you still don't need the
> barrier. Whatever means you use to make the object visible to other CPUs
> will include a barrier.
>
>> > Lets change it to kref_get() aka atomic_inc():
>> >
>> >        CPU0            CPU1
>> >
>> >        atomic_inc()
>> >                        atomic_dec_and_test()
>> >
>> > and
>> >
>> >                        atomic_dec_and_test()
>> >        atomic_inc()
>> >
>> > For if the first is possible, then so is the second.
>>
>> Yes, both are reasonable.
>>
>> >
>> > This illustrates that no matter how many barriers you put in, you're
>> > still up shit creek without no paddle because the kref_put() can come in
>> > before you do the kref_get(), making the kref_get() the invalid
>> > operation.
>>
>> So one smp_mb__before_atomic_inc should be added before atomic_inc
>> to make sure that CPU0 can see the uptodate ref, right?
>
> No.
>
> Assume v == 1:
>
>        CPU0            CPU1
>
>                        atomic_dec_and_test(); /* --v == 0 */
>                                kfree()
>
>        smp_mb__before_atomic_inc()
>        atomic_inc(); <-- OOPS!
>
>
> You still got an access to already freed memory. There is no amount of
> memory barriers that will solve this problem.

Yes, kref is designed as so, and it is nothing to do with memory barrier,
and more like a logical issue.

The implicit rule about kref is that use should make sure
that kref can not be touched once it is released.


thanks,
-- 
Ming Lei
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