Re: [PATCH bpf] Fold smp_mb__before_atomic() into atomic_set_release()

[Hou Tao <houtao@huaweicloud.com>]

Hi Paul,

On 10/19/2023 12:54 PM, Paul E. McKenney wrote:
> On Thu, Oct 19, 2023 at 09:07:07AM +0800, Hou Tao wrote:
>> Hi Paul,
>>
>> On 10/19/2023 6:28 AM, Paul E. McKenney wrote:
>>> bpf: Fold smp_mb__before_atomic() into atomic_set_release()
>>>
>>> The bpf_user_ringbuf_drain() BPF_CALL function uses an atomic_set()
>>> immediately preceded by smp_mb__before_atomic() so as to order storing
>>> of ring-buffer consumer and producer positions prior to the atomic_set()
>>> call's clearing of the ->busy flag, as follows:
>>>
>>>         smp_mb__before_atomic();
>>>         atomic_set(&rb->busy, 0);
>>>
>>> Although this works given current architectures and implementations, and
>>> given that this only needs to order prior writes against a later write.
>>> However, it does so by accident because the smp_mb__before_atomic()
>>> is only guaranteed to work with read-modify-write atomic operations,
>>> and not at all with things like atomic_set() and atomic_read().
>>>
>>> Note especially that smp_mb__before_atomic() will not, repeat *not*,
>>> order the prior write to "a" before the subsequent non-read-modify-write
>>> atomic read from "b", even on strongly ordered systems such as x86:
>>>
>>>         WRITE_ONCE(a, 1);
>>>         smp_mb__before_atomic();
>>>         r1 = atomic_read(&b);
>> The reason is smp_mb__before_atomic() is defined as noop and
>> atomic_read() in x86-64 is just READ_ONCE(), right ?
> The real reason is that smp_mb__before_atomic() is not defined to do
> anything unless followed by an atomic read-modify-write operation,
> and atomic_read(), atomic_64read(), atomic_set(), and so on are not
> read-modify-write operations.

I see. Thanks for explanation. It seems I did not read
Documentation/atomic_t.txt carefully, it said:

    The barriers:

    smp_mb__{before,after}_atomic()

    only apply to the RMW atomic ops and can be used to augment/upgrade the
    ordering inherent to the op.

>
> As you point out, one implementation consequence of this is that
> smp_mb__before_atomic() is nothingness on x86.
>
>> And it seems that I also used smp_mb__before_atomic() in a wrong way for
>> patch [1]. The memory order in the posted patch is
>>
>> process X                                    process Y
>>     atomic64_dec_and_test(&map->usercnt)
>>     READ_ONCE(timer->timer)
>>                                             timer->time = t
> The above two lines are supposed to be accessing the same field, correct?
> If so, process Y's store really should be WRITE_ONCE().

Yes. These two processes are accessing the same field (namely
timer->timer). Is WRITE_ONCE(xx) still necessary when the write of
timer->time in process Y is protected by a spin-lock ?


>
>>                                             // it won't work
>>                                             smp_mb__before_atomic()
>>                                             atomic64_read(&map->usercnt)
>>
>> For the problem, it seems I need to replace smp_mb__before_atomic() by
>> smp_mb() to fix the memory order, right ?
> Yes, because smp_mb() will order the prior store against that later load.

Thanks. Will fix the patch.

Regards,
Hou
>
> 							Thanx, Paul
>
>> Regards,
>> Hou
>>
>> [1]:
>> https://lore.kernel.org/bpf/20231017125717.241101-2-houtao@huaweicloud.com/
>>                                                                 
>>
>>> Therefore, replace the smp_mb__before_atomic() and atomic_set() with
>>> atomic_set_release() as follows:
>>>
>>>         atomic_set_release(&rb->busy, 0);
>>>
>>> This is no slower (and sometimes is faster) than the original, and also
>>> provides a formal guarantee of ordering that the original lacks.
>>>
>>> Signed-off-by: Paul E. McKenney <paulmck@kernel.org>
>>> Acked-by: David Vernet <void@manifault.com>
>>> Cc: Andrii Nakryiko <andrii@kernel.org>
>>> Cc: Alexei Starovoitov <ast@kernel.org>
>>> Cc: Daniel Borkmann <daniel@iogearbox.net>
>>> Cc: Martin KaFai Lau <martin.lau@linux.dev>
>>> Cc: Song Liu <song@kernel.org>
>>> Cc: Yonghong Song <yonghong.song@linux.dev>
>>> Cc: John Fastabend <john.fastabend@gmail.com>
>>> Cc: KP Singh <kpsingh@kernel.org>
>>> Cc: Stanislav Fomichev <sdf@google.com>
>>> Cc: Hao Luo <haoluo@google.com>
>>> Cc: Jiri Olsa <jolsa@kernel.org>
>>> Cc: <bpf@vger.kernel.org>
>>>
>>> diff --git a/kernel/bpf/ringbuf.c b/kernel/bpf/ringbuf.c
>>> index f045fde632e5..0ee653a936ea 100644
>>> --- a/kernel/bpf/ringbuf.c
>>> +++ b/kernel/bpf/ringbuf.c
>>> @@ -770,8 +770,7 @@ BPF_CALL_4(bpf_user_ringbuf_drain, struct bpf_map *, map,
>>>  	/* Prevent the clearing of the busy-bit from being reordered before the
>>>  	 * storing of any rb consumer or producer positions.
>>>  	 */
>>> -	smp_mb__before_atomic();
>>> -	atomic_set(&rb->busy, 0);
>>> +	atomic_set_release(&rb->busy, 0);
>>>  
>>>  	if (flags & BPF_RB_FORCE_WAKEUP)
>>>  		irq_work_queue(&rb->work);
>>>
>>> .