Re: [RFC tip/locking/lockdep v5 07/17] lockdep: Adjust check_redundant() for recursive read change

Peter Zijlstra
2018-02-22T17:29:11Z

On Thu, Feb 22, 2018 at 03:08:54PM +0800, Boqun Feng wrote:
> As we have four kinds of dependencies now, check_redundant() should only
> report redundant if we have a dependency path which is equal or
> _stronger_ than the current dependency. For example if in
> check_prev_add() we have:
>
> prev->read == 2 && next->read != 2
>
> , we should only report redundant if we find a path like:
>
> prev--(RN)-->....--(*N)-->next
>
> and if we have:
>
> prev->read == 2 && next->read == 2
>
> , we could report redundant if we find a path like:
>
> prev--(RN)-->....--(*N)-->next
>
> or
>
> prev--(RN)-->....--(*R)-->next
>
> To do so, we need to pass the recursive-read status of @next into
> check_redundant().

Very hard to read that.

> Signed-off-by: Boqun Feng <boqun.feng@gmail.com>
> ---
> kernel/locking/lockdep.c | 13 ++++++++-----
> 1 file changed, 8 insertions(+), 5 deletions(-)
>
> diff --git a/kernel/locking/lockdep.c b/kernel/locking/lockdep.c
> index e1be088a34c4..0b0ad3db78b4 100644
> --- a/kernel/locking/lockdep.c
> +++ b/kernel/locking/lockdep.c
> @@ -1338,9 +1338,12 @@ print_circular_bug_header(struct lock_list *entry, unsigned int depth,
> return 0;
> }
>
> -static inline int class_equal(struct lock_list *entry, void *data)
> +static inline int hlock_equal(struct lock_list *entry, void *data)
> {
> - return entry->class == data;
> + struct held_lock *hlock = (struct held_lock *)data;
> +
> + return hlock_class(hlock) == entry->class &&
> + (hlock->read == 2 || !entry->is_rr);
> }

So I guess @data = @next, and we're checking if @prev -> @next already
exists.

Since we only care about forward dependencies, @next->read==2 means *R
and if so, any existing link is equal or stronger. If @next->read!=2, it
means *N and we must regard *R as weaker and not match.

OK, that seems to be fine, but again, that function _really_ could do
with a comment.