linux-kernel - Re: [PATCH v2 21/24] locking/lockdep: Verify whether lock objects are small enough to be used as class keys

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Date:   Tue, 4 Dec 2018 16:08:22 -0500
From:   Waiman Long <longman@...hat.com>
To:     Bart Van Assche <bvanassche@....org>, mingo@...hat.com
Cc:     peterz@...radead.org, tj@...nel.org, johannes.berg@...el.com,
        linux-kernel@...r.kernel.org,
        Johannes Berg <johannes@...solutions.net>
Subject: Re: [PATCH v2 21/24] locking/lockdep: Verify whether lock objects are
 small enough to be used as class keys

On 12/03/2018 07:28 PM, Bart Van Assche wrote:
> Cc: Peter Zijlstra <peterz@...radead.org>
> Cc: Waiman Long <longman@...hat.com>
> Cc: Johannes Berg <johannes@...solutions.net>
> Signed-off-by: Bart Van Assche <bvanassche@....org>
> ---
>  kernel/locking/lockdep.c | 9 +++++++++
>  1 file changed, 9 insertions(+)
>
> diff --git a/kernel/locking/lockdep.c b/kernel/locking/lockdep.c
> index c936fce5b9d7..b4772e5fc176 100644
> --- a/kernel/locking/lockdep.c
> +++ b/kernel/locking/lockdep.c
> @@ -727,6 +727,15 @@ static bool assign_lock_key(struct lockdep_map *lock)
>  {
>  	unsigned long can_addr, addr = (unsigned long)lock;
>  
> +	/*
> +	 * lockdep_free_key_range() assumes that struct lock_class_key
> +	 * objects do not overlap. Since we use the address of lock
> +	 * objects as class key for static objects, check whether the
> +	 * size of lock_class_key objects does not exceed the size of
> +	 * the smallest lock object.
> +	 */
> +	BUILD_BUG_ON(sizeof(struct lock_class_key) > sizeof(raw_spinlock_t));
> +
>  	if (__is_kernel_percpu_address(addr, &can_addr))
>  		lock->key = (void *)can_addr;
>  	else if (__is_module_percpu_address(addr, &can_addr))

I don't understand what this check is for. lock_class_key and spinlock
are different objects. Their relative size shouldn't matter.

Cheers,
Longman