On Mon, Feb 01, 2010 at 10:53:41PM -0800, Paul E. McKenney wrote: > Here is an approximation that might inspire someone to come up with a > real solution. > > One approach would be to store the name length with the name, so that > struct qstr loses the "len" field, and so that its "name" field points > to a struct that has a "len" field followed by an array of const > unsigned char. That way, the name and length are closely associated. > When you pick up a struct qstr's "name" pointer, you are guaranteed to > get a length that matches the name. > > Unfortunately: > > o In theory, this leaves the length of the dentry unchanged, but > alignment is a problem on 64-bit systems. Also, the long names > gain an extra four bytes. That one is not a big deal. > o If you get a pointer to the d_iname small-name field, rename > might still change the name out from under you. This could in > theory be fixed by refusing to re-use the d_iname field until > an RCU grace period had elapsed (using an external structure > instead). In practice, not sure if this is really a reasonable > approach. That, OTOH, is - most of dentries use inline name and external one is really a rarely used fallback. Making it a common case isn't nice. There's another practical problem - a lot of code uses qstr fields and patch will be painful; I couldn't care less about the out-of-tree code, but it's a flagday change and in-tree patch size is not something to sneeze at - I've been crawling through all that code for the last couple of days and there's a lot of it. Trying to play with seqlock-based solutions sounds more promising; I've missed it completely and I'm half-asleep right now, so I'll try to take a look at that after I get some sleep. -- To unsubscribe from this list: send the line "unsubscribe linux-fsdevel" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
