Re: [PATCH v2 10/10] zram: use atomic64_xxx() to replace zram_stat64_xxx()

[Jiang Liu <liuj97@gmail.com>]

On Thu 06 Jun 2013 11:07:13 PM CST, Jerome Marchand wrote:
> On 06/06/2013 04:36 PM, Jiang Liu wrote:
>> On Thu 06 Jun 2013 05:37:19 PM CST, Jerome Marchand wrote:
>>> On 06/05/2013 06:21 PM, Jiang Liu wrote:
>>>> On Wed 05 Jun 2013 08:02:12 PM CST, Jerome Marchand wrote:
>>>>> On 06/04/2013 06:06 PM, Jiang Liu wrote:
>>>>>> Use atomic64_xxx() to replace open-coded zram_stat64_xxx().
>>>>>> Some architectures have native support of atomic64 operations,
>>>>>> so we can get rid of the spin_lock() in zram_stat64_xxx().
>>>>>> On the other hand, for platforms use generic version of atomic64
>>>>>> implement, it may cause an extra save/restore of the interrupt
>>>>>> flag.  So it's a tradeoff.
>>>>>>
>>>>>> Signed-off-by: Jiang Liu <jiang.liu@huawei.com>
>>>>>
>>>>> Before optimizing stats, I'd like to make sure that they're correct.
>>>>> What makes 64 bits fields so different that they need atomicity while
>>>>> 32 bits wouldn't? Actually all of them save compr_size only increase,
>>>>> which would make a race less critical than for 32 bits fields that all
>>>>> can go up and down (if a decrement overwrites a increment, the counter
>>>>> can wrap around zero).
>>>>>
>>>>> Jerome
>>>>>
>>>> Hi Jerome,
>>>>           I'm not sure about the design decision, but I could give a
>>>> guess here.
>>>> 1) All 32-bit counters are only modified by
>>>> zram_bvec_write()/zram_page_free()
>>>> and is/should be protected by down_write(&zram->lock).
>>>
>>> Good point!
>>>
>>>> 2) __zram_make_request() modifies some 64-bit counters without
>>>> protection.
>>>> 3) zram_bvec_write() modifies some 64-bit counters and it's protected
>>>> with
>>>>      down_read(&zram->lock).
>>>
>>> I assume you mean down_write().
>> Actually I mean "zram_bvec_read()" instead of "zram_bvec_write()".
>
> Indeed, failed_reads is updated there.
>
>> Read side is protected by down_read(&zram->lock).
>
> which does not prevent concurrent read access. The counter isn't
> protected by zram_lock here.
Hi Jerome,
     Yeah, it's true. The down_read(&zram->lock) can't protect 
modification
to stat counters because there may be multiple readers. So zram uses
zram_stat_inc() here.
Regards!
Gerry

>
> Jerome
>
>> Regards!
>> Gerry
>>
>>>
>>>> 4) It's always safe for sysfs handler to read 32bit counters.
>>>> 5) It's unsafe for sysfs handler to read 64bit counters on 32bit
>>>> platforms.
>>>
>>> I was unaware of that.
>>>
>>>>
>>>> So it does work with current design, but very hard to understand.
>>>> Suggest to use atomic_t for 32bit counters too for maintainability,
>>>> though may be a little slower.
>>>> Any suggestion?
>>>
>>> If atomic counter aren't necessary, no need to use them, but a comment
>>> in zram_stats definition would be nice. Could you add one in your next
>>> version of this patch?
>> Sure!
>>
>>>
>>> Thanks
>>> Jerome
>>>
>>>> Regards!
>>>> Gerry
>>>>
>>>
>>
>>
>