Re: [PATCH] locking/qspinlock: Add bug check for exceeding MAX_NODES

[Will Deacon <will.deacon@arm.com>]

On Tue, Jan 15, 2019 at 04:55:44PM -0500, Waiman Long wrote:
> On some architectures, it is possible to have nested NMIs taking
> spinlocks nestedly. Even though the chance of having more than 4 nested
> spinlocks with contention is extremely small, there could still be a
> possibility that it may happen some days leading to system panic.
> 
> What we don't want is a silent corruption with system panic somewhere
> else. So add a BUG_ON() check to make sure that a system panic caused
> by this will show the correct root cause.
> 
> Signed-off-by: Waiman Long <longman@redhat.com>
> ---
>  kernel/locking/qspinlock.c | 10 ++++++++++
>  1 file changed, 10 insertions(+)
> 
> diff --git a/kernel/locking/qspinlock.c b/kernel/locking/qspinlock.c
> index 8a8c3c2..f823221 100644
> --- a/kernel/locking/qspinlock.c
> +++ b/kernel/locking/qspinlock.c
> @@ -412,6 +412,16 @@ void queued_spin_lock_slowpath(struct qspinlock *lock, u32 val)
>  	idx = node->count++;
>  	tail = encode_tail(smp_processor_id(), idx);
>  
> +	/*
> +	 * 4 nodes are allocated based on the assumption that there will
> +	 * not be nested NMIs taking spinlocks. That may not be true in
> +	 * some architectures even though the chance of needing more than
> +	 * 4 nodes will still be extremely unlikely. Adding a bug check
> +	 * here to make sure there won't be a silent corruption in case
> +	 * this condition happens.
> +	 */
> +	BUG_ON(idx >= MAX_NODES);
> +

Hmm, I really don't like the idea of putting a BUG_ON() on the spin_lock()
path. I'd prefer it if (a) we didn't add extra conditional code for the
common case and (b) didn't bring down the machine. Could we emit a
lockdep-style splat, instead?

Will