On Fri, Apr 11, 2025 at 12:50:42AM +0530, Nilay Shroff wrote:
>
>
> On 4/10/25 7:00 PM, Ming Lei wrote:
> > scheduler's ->exit() is called with queue frozen and elevator lock is held, and
> > wbt_enable_default() can't be called with queue frozen, otherwise the
> > following lockdep warning is triggered:
> >
> > #6 (&q->rq_qos_mutex){+.+.}-{4:4}:
> > #5 (&eq->sysfs_lock){+.+.}-{4:4}:
> > #4 (&q->elevator_lock){+.+.}-{4:4}:
> > #3 (&q->q_usage_counter(io)#3){++++}-{0:0}:
> > #2 (fs_reclaim){+.+.}-{0:0}:
> > #1 (&sb->s_type->i_mutex_key#3){+.+.}-{4:4}:
> > #0 (&q->debugfs_mutex){+.+.}-{4:4}:
> >
> > Fix the issue by moving wbt_enable_default() out of bfq's exit(), and
> > call it from elevator_change_done().
>
> [...]
>
> > diff --git a/block/elevator.c b/block/elevator.c
> > index 1cc640a9db3e..9cf78db4d6a4 100644
> > --- a/block/elevator.c
> > +++ b/block/elevator.c
> > @@ -676,8 +676,13 @@ int elevator_change_done(struct request_queue *q, struct elev_change_ctx *ctx)
> > int ret = 0;
> >
> > if (ctx->old) {
> > + bool enable_wbt = test_bit(ELEVATOR_FLAG_ENABLE_WBT_ON_EXIT,
> > + &ctx->old->flags);
> > +
> > elv_unregister_queue(q, ctx->old);
> > kobject_put(&ctx->old->kobj);
> > + if (enable_wbt)
> > + wbt_enable_default(q->disk);
> > }
> wbt_enable_default is also called from ioc_qos_write and blk_register_queue
> with ->elevator_lock protection. So avoiding protection here doesn't seem
> correct.
The only code which needs the protection should be:
```
wbt_enable_default():
...
if (q->elevator && test_bit(ELEVATOR_FLAG_DISABLE_WBT, &q->elevator->flags))
enable = false;
```
The flag can be moved to request queue from elevator queue, will do it in
V2.
Thanks,
Ming
