On Mon, Feb 01, 2010 at 02:37:32PM -0800, Linus Torvalds wrote: > > * don't use %pd under dentry->d_lock, use dentry->d_name.name instead; in > > that case it *is* safe. Incidentally, ->d_lock isn't held a lot. > > I realize we can just call it a rule, and yes, d_lock is held much less > than something like console_lock etc that we've had ABBA issues with, but > still.. > Quite frankly, I'd _much_ rather see something like just always freeing > the dentry names (when they aren't inlined) using RCU. The VFS layer quite > possibly would want to do that anyway at some point (eg Nick's VFS > scalability patches), and then we could make it just a RCU read-lock or > whatever (interrupt disable, what-not) instead. > > And I'm much happier with printk doing that kind of thing, and wouldn't > have issues with that kind of much weaker locking. Ehh... RCU will save you from stepping on freed memory, but it still will leave the joy of half-updated string with length out of sync with it, etc. We probably can get away with that, but we'll have to be a lot more careful with the order of updating these suckers in d_move_locked et.al. I don't know... Note that if we end up adding something extra to struct dentry, we might as well just add *another* spinlock, taken only under ->d_lock and only in two places in dcache.c that change d_name. That kind of thing is trivial to enforce (just grep over the tree once in a while) and if it shares the cacheline with d_lock, we shouldn't get any real overhead in d_move()/d_materialise_unique(). I'm not particulary fond of that variant, but it's at least guaranteed to be devoid of subtleties. If RCU folks can come up with a sane suggestions that would be robust and wouldn't bloat dentry - sure, I'm all for it. If not... -- 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
