On Wed, Jul 11, 2007 at 09:28:06AM -0500, Dave Kleikamp wrote: > On Wed, 2007-07-11 at 15:05 +1000, Neil Brown wrote: > > It just occurred to me: > > > > If i_version is 64bit, then knfsd would need to be careful when > > reading it on a 32bit host. What are the locking rules? > > How does knfsd use i_version? I would think that if all it was doing > was to compare (i_version == previous_version) That's correct. (Though it's the client that's doing the comparison, actually--the server is just reporting the value.) > then locking wouldn't really matter. Well, theoretically, > previous_version could be 0x100000000, and i_version could be > 0x1ffffffff, knfsd checks the high word, then ext4 updates i_version > to 0x200000000, then knfsd checks the low word, detecting no change. > How likely is this? The choice of upper word in your example is arbitrary, but other than that I believe your example is essentially the only one. So this would only happen when *both* - the read of the new value of the low word happens precisely 2^32 i_version updates after the word was read on the client's previous cache revalidation, and - the value of i_version itself is close enough to a 32-bit boundary that wraparound can happen between the reads of the high and low words. > (I don't understand why i_version even needs to be 64 bits in the > first place.) A 32-bit i_version could in theory wrap pretty quickly, couldn't it? That's not a problem in itself--the problem would only arise if two subsequent client queries of the change attribute happened a multiple of 2^32 i_version increments apart. This is more likely than the previous scenario, but still very unlikely. I would have guessed that even in situations with a very high rate of updates and a low rate of client revalidations, the chance of two revalidations happening exactly 2^32 updates apart would still be no more than 1 in 2^32. (Could odd characteristics of the workloads (like updates that tend to happen in power-of-2 groups?) make it any more likely?) I'd be happier if ext4 at least allowed the possibility of 64 bits in the future. And there's always the chance someone would find a use for an i_version that was nondecreasing, even if nfs didn't care. > > Presumably it is only updated under i_mutex protection, but having to > > get i_mutex to read it would seem a little heavy handed. > > How does knfsd protect itself from the inode changing after i_version is > checked? Is any locking being done otherwise? If the client always requests the change attribute before reading, and the i_version is always updated after data is modified, I think we're OK. Admittedly this is a little subtle. --b. - To unsubscribe from this list: send the line "unsubscribe linux-ext4" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
