Re: [PATCH 11/15] block: move elv_register[unregister]_queue out of elevator_lock

[Ming Lei <ming.lei@redhat.com>]

On Sat, Apr 12, 2025 at 12:50:10AM +0530, Nilay Shroff wrote:
> 
> 
> On 4/10/25 7:00 PM, Ming Lei wrote:
> > +int elevator_change_done(struct request_queue *q, struct elev_change_ctx *ctx)
> > +{
> > +	int ret = 0;
> > +
> > +	if (ctx->old) {
> > +		elv_unregister_queue(q, ctx->old);
> > +		kobject_put(&ctx->old->kobj);
> > +	}
> > +	if (ctx->new) {
> > +		ret = elv_register_queue(q, ctx->new, ctx->uevent);
> > +		if (ret) {
> > +			unsigned memflags = blk_mq_freeze_queue(q);
> > +
> > +			mutex_lock(&q->elevator_lock);
> > +			elevator_exit(q);
> > +			mutex_unlock(&q->elevator_lock);
> > +			blk_mq_unfreeze_queue(q, memflags);
> > +		}
> > +	}
> > +	return 0;
> > +}
> > +
> We could have sysf elevator attributes simultaneously accessed while this function 
> adds/removes sysfs elevator attributes without any protection. In fact, the show/store
> methods of elevator attributes runs with e->sysfs_lock held. So it seems moving 
> the above function out of lock protection might cause crash or other side effects?

sysfs/kobject provides such protection, and kobject_del() will drain any
in-flight attribute access.


Thanks,
Ming