Re: [PATCH bpf-next v2 11/26] rqspinlock: Add deadlock detection and recovery

[Peter Zijlstra]

On Thu, Feb 06, 2025 at 02:54:19AM -0800, Kumar Kartikeya Dwivedi wrote:
> +	/*
> +	 * Find the CPU holding the lock that we want to acquire. If there is a
> +	 * deadlock scenario, we will read a stable set on the remote CPU and
> +	 * find the target. This would be a constant time operation instead of
> +	 * O(NR_CPUS) if we could determine the owning CPU from a lock value, but
> +	 * that requires increasing the size of the lock word.
> +	 */

Is increasing the size of rqspinlock_t really a problem? For the kernel
as a whole there's very little code that really relies on spinlock_t
being u32 (lockref is an example that does care).

And it seems to me this thing might benefit somewhat significantly from
adding this little extra bit.