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(). > 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? Thanks Jerome > Regards! > Gerry > _______________________________________________ devel mailing list devel@xxxxxxxxxxxxxxxxxxxxxx http://driverdev.linuxdriverproject.org/mailman/listinfo/devel
