We are passing to ->d_compare() instances parent, dentry itself,
and a consistent snapshot of its ->d_name.name and ->d_name.len. In all
but one instance (ncpfs one) the only thing we need the parent for is
finding the superblock. Which is available as dentry->d_sb. ncpfs one
is weird, but it actually wants parent's ->d_inode, so it has to be
careful about the RCU case anyway.
Do you have any objections to trimming the arguments list? I want
to kill the 'parent' argument there and let ncpfs carefully walk
dentry->d_parent->d_inode.
Another thing in the same area: __d_lookup_rcu() does
hlist_bl_for_each_entry_rcu(dentry, node, b, d_hash) {
unsigned seq;
seq = raw_seqcount_begin(&dentry->d_seq);
if (dentry->d_parent != parent)
continue;
and raw_seqcount_begin() contains smp_rmb(). Seeing that we hit ->d_parent
mismatch often enough and that we are fine with false negatives anyway,
let's turn that into
if (dentry->d_parent != parent)
continue;
seq = raw_seqcount_begin(&dentry->d_seq);
if (unlikely(dentry->d_parent != parent))
continue;
and cut down on the number of smp_rmb() per __d_lookup_rcu(). We do need the
second check (to make sure that ->d_seq guarantees a consistent state of
everything), but it's going to trigger _very_ rarely - basically, only if
we step on dentry with the right parent just as it's being hit with
cross-directory rename. That's a very slow case, since we are quite likely
to search the tail of the wrong hash chain and eventually bugger off with
NULL, switching to non-RCU codepath. So in the cases of parent mismatch
we end up doing one fetch instead of two fetches + skip smp_rmb(), while in
case of parent match we do extra fetch from hot cacheline + branch not
taken. AFAICS, it's going to be a win even on architectures with trivial
smp_rmb(); on something with costly smp_rmb() the win could be considerable.
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