Re: [RFC tip/locking/lockdep v5 08/17] lockdep: Fix recursive read lock related safe->unsafe detection

Peter Zijlstra
2018-02-22T17:41:58Z

On Thu, Feb 22, 2018 at 03:08:55PM +0800, Boqun Feng wrote:
> There are four cases for recursive read lock realted deadlocks:
>
> (--(X..Y)--> means a strong dependency path starts with a --(X*)-->
> dependency and ends with a --(*Y)-- dependency.)
>
> 1. An irq-safe lock L1 has a dependency --(*..*)--> to an
> irq-unsafe lock L2.
>
> 2. An irq-read-safe lock L1 has a dependency --(N..*)--> to an
> irq-unsafe lock L2.
>
> 3. An irq-safe lock L1 has a dependency --(*..N)--> to an
> irq-read-unsafe lock L2.
>
> 4. An irq-read-safe lock L1 has a dependency --(N..N)--> to an
> irq-read-unsafe lock L2.
>
> The current check_usage() only checks 1) and 2), so this patch adds
> checks for 3) and 4) and makes sure when find_usage_{back,for}wards find
> an irq-read-{,un}safe lock, the traverse path should ends at a
> dependency --(*N)-->. Note when we search backwards, --(*N)--> indicates
> a real dependency --(N*)-->.
>
> Signed-off-by: Boqun Feng <boqun.feng@gmail.com>
> ---
> kernel/locking/lockdep.c | 17 ++++++++++++++++-
> 1 file changed, 16 insertions(+), 1 deletion(-)
>
> diff --git a/kernel/locking/lockdep.c b/kernel/locking/lockdep.c
> index 0b0ad3db78b4..bd3eef664f9d 100644
> --- a/kernel/locking/lockdep.c
> +++ b/kernel/locking/lockdep.c
> @@ -1504,7 +1504,14 @@ check_redundant(struct lock_list *root, struct held_lock *target,
>
> static inline int usage_match(struct lock_list *entry, void *bit)
> {
> - return entry->class->usage_mask & (1 << (enum lock_usage_bit)bit);
> + enum lock_usage_bit ub = (enum lock_usage_bit)bit;
> +
> +
> + if (ub & 1)
> + return entry->class->usage_mask & (1 << ub) &&
> + !entry->is_rr;
> + else
> + return entry->class->usage_mask & (1 << ub);
> }

The whole is_rr/have_xr thing and backwards hurts my brain. That really
wants more than a little 'Note'.

Also, the above is unreadable, something like:

unsigned long usage_mask = entry->class->usage_mask;
enum lock_usage_bit ub = (enum lock_usage_bit)bit;
unsigned long mask = 1ULL << ub;

if (ub & 1) /* __STATE_RR */
return !entry->have_xr && (usage_mask & mask);

return !!(usage_mask & mask);

maybe. Also, perhaps we should make __bfs(.match) have a bool return
value.