Re: [PATCH v2 10/10] zram: use atomic64_xxx() to replace zram_stat64_xxx()
From: Jerome Marchand
Date: Thu Jun 06 2013 - 05:37:38 EST
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@xxxxxxxxxx>
>> 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).
> 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
> and is/should be protected by down_write(&zram->lock).
> 2) __zram_make_request() modifies some 64-bit counters without
> 3) zram_bvec_write() modifies some 64-bit counters and it's protected
I assume you mean down_write().
> 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
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?
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