Re: [PATCH v3 11/14] perf/hw_breakpoint: Reduce contention with large number of tasks

[Peter Zijlstra <peterz@infradead.org>]

On Mon, Jul 04, 2022 at 05:05:11PM +0200, Marco Elver wrote:
> +static bool bp_constraints_is_locked(struct perf_event *bp)
> +{
> +	struct mutex *tsk_mtx = get_task_bps_mutex(bp);
> +
> +	return percpu_is_write_locked(&bp_cpuinfo_sem) ||
> +	       (tsk_mtx ? mutex_is_locked(tsk_mtx) :
> +			  percpu_is_read_locked(&bp_cpuinfo_sem));
> +}

> @@ -426,18 +521,28 @@ static int modify_bp_slot(struct perf_event *bp, u64 old_type, u64 new_type)
>   */
>  int dbg_reserve_bp_slot(struct perf_event *bp)
>  {
> -	if (mutex_is_locked(&nr_bp_mutex))
> +	int ret;
> +
> +	if (bp_constraints_is_locked(bp))
>  		return -1;
>  
> -	return __reserve_bp_slot(bp, bp->attr.bp_type);
> +	/* Locks aren't held; disable lockdep assert checking. */
> +	lockdep_off();
> +	ret = __reserve_bp_slot(bp, bp->attr.bp_type);
> +	lockdep_on();
> +
> +	return ret;
>  }
>  
>  int dbg_release_bp_slot(struct perf_event *bp)
>  {
> -	if (mutex_is_locked(&nr_bp_mutex))
> +	if (bp_constraints_is_locked(bp))
>  		return -1;
>  
> +	/* Locks aren't held; disable lockdep assert checking. */
> +	lockdep_off();
>  	__release_bp_slot(bp, bp->attr.bp_type);
> +	lockdep_on();
>  
>  	return 0;
>  }

Urggghhhh... this is horrible crap. That is, the current code is that
and this makes it worse :/