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@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()". 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. 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 >>> >> > > _______________________________________________ devel mailing list devel@xxxxxxxxxxxxxxxxxxxxxx http://driverdev.linuxdriverproject.org/mailman/listinfo/devel
