On Tue, Apr 15, 2025 at 03:09:12PM +0530, Nilay Shroff wrote:
>
>
> On 4/14/25 6:54 AM, Ming Lei wrote:
> > 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.
> >
> Okay, so in that case do we now really need e->sysfs_lock protection while accessing
> elevator attributes?
Yeah, I think so, elevator_exit() is always called after elevator kobject
is deleted.
However, this patchset moves elv_unregister_queue() after blk_mq_exit_sched(),
we may need this lock for failing elevator's attribute ->show() & ->store()
by adding one '->exiting' flag to 'struct elevator_queue'.
Thanks,
Ming
