On Fri, May 15, 2015 at 08:37:20PM -0700, Linus Torvalds wrote: > On May 15, 2015 8:17 PM, "Al Viro" <viro@xxxxxxxxxxxxxxxxxx> wrote: > > > > What for? All we need is a flag, waitqueue and being woken > > up when the flag gets cleared. > > You need to have the flag somewhere. > > The child dentry doesn't exist yet. > > That's the point of the hashed entry. It approximates the not-yet-existing > child dentry that we have *not* added to the parent until after lookup. Point, but... A lot of our problems comes from the fact that ->i_mutex doubles as protection against the addition to the list of children, on top of protection of directory itself. What if we do the following: have the normal case of __lookup_hash() (and other callers of lookup_real()) * allocate dentry, marked "in-lookup" * do dcache lookup, likely to come up empty, _without_ touching potential matches' d_lock, i.e. based on __d_lookup_rcu() (under rcu_read_lock(), with rename_lock loop around it). Hold parent's ->d_lock while walking the chain, grab refcount in the unlikely case the match had been found. If nothing's found *and* rename_lock hadn't been touched, insert the new dentry into hash and list of children before dropping ->d_lock. * call ->lookup() (still under ->i_mutex, shared) * clear "in-lookup" bit on _original_ dentry (we might very well have returned a different one) * kick the wait queue of parent's ->i_mutex I'll need to think about that after I get some sleep, but it smells like that could be feasible. Of course, that assumes we'll be able to cope with hashed-but-currently-in-lookup dentries, but I think it might be doable with some massage... -- 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
