On Tue, Feb 02, 2010 at 07:09:08AM +0000, Al Viro wrote: > 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. K. > > 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. It is possible to move it back inline after a grace period, so that the external name would be in use for only a few milliseconds after the rename, but that of course adds more complexity. > 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. Indeed!!! > 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. Certainly sounds worth a try. Thanx, Paul -- 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
