On Mon, Jun 6, 2016 at 5:40 PM, Al Viro <v...@zeniv.linux.org.uk> wrote:
>
> static struct dentry *next_positive(struct dentry *parent,
> struct dentry *child, int count)
> {
> struct list_head *p = child ? &child->d_child : &parent->d_subdirs;
>From your description, you seem to be very confused about what "child
== NULL" means. Here it means that it's a cursor to the beginning, but
in your commentary on move_cursor(), you say "moves cursor immediately
past child *or* to the very end if child is NULL".
That's very confusing. Is NULL beginning or end?
I really think you'd be better off having a special ERR_PTR value for
end, possibly as a flag value in the cursor dentry.
The whole "what does NULL mean" confusion exists inside that
"next_positive" too:
> unsigned *seq = &parent->d_inode->i_dir_seq, n;
> do {
> int i = count;
> n = smp_load_acquire(seq) & ~1;
> rcu_read_lock();
> do {
> p = p->next;
> if (p == &parent->d_subdirs) {
> child = NULL;
> break;
> }
look, here you return NULL for "end" again. Even though it meant
beginning at the start of the function. Nasty.
Also, may I suggest that there is a very trivial special case for
"next_positive()" that needs no barriers or sequence checking or
anything else: at the very beginning, just load the "->next" pointer,
and if it's a positive entry, you're done. That's going to be the
common case when there _isn't_ crazy multi-threaded readdirs going on,
so it's worth handling separately.
In fact, if you have a special value for the case of "cursor is at
end" situation, then for the small directory case that can be handled
with a single getdents call, you'll *never* set the cursor in the
child list at all, which means that the above special case for
next_positive() is actually the common case even for the threaded
situation.
Linus
