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

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Date:   Thu, 29 Nov 2018 11:10:00 +0100
From:   Peter Zijlstra <peterz@...radead.org>
To:     Bart Van Assche <bvanassche@....org>
Cc:     mingo@...hat.com, tj@...nel.org, johannes.berg@...el.com,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH 25/27] locking/lockdep: Add support for dynamic keys

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.