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@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). >>> >>> 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()". Read side is protected by down_read(&zram->lock). 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 >> > _______________________________________________ devel mailing list devel@xxxxxxxxxxxxxxxxxxxxxx http://driverdev.linuxdriverproject.org/mailman/listinfo/devel
