Re: [PATCH 25/27] locking/lockdep: Add support for dynamic keys

From: Peter Zijlstra
Date: Thu Nov 29 2018 - 05:10:07 EST

On Wed, Nov 28, 2018 at 03:43:23PM -0800, Bart Van Assche wrote:
> +/* hash_entry is used to keep track of dynamically allocated keys. */
> struct lock_class_key {
> + struct hlist_node hash_entry;
> struct lockdep_subclass_key subkeys[MAX_LOCKDEP_SUBCLASSES];
> };

One consideration; and maybe we should have a BUILD_BUG for that, is
that this object should be no larger than the smallest lock primitive.

That typically is raw_spinlock_t, which normally is 4 bytes, but with
lockdep on that at least also includes struct lockdep_map.

So what we want is:

sizeof(lock_class_key) <= sizeof(raw_spinlock_t)

Otherwise, two consecutive spinlocks could end up with key overlap in
their subclass range.

Now, I think that is still valid after this patch, but it is something
that gave me pause.