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

Peter Zijlstra
2025-02-10T10:36:32Z

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.