Re: [PATCH V2 20/20] block: move wbt_enable_default() out of queue freezing from sched ->exit()

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

On Sat, Apr 19, 2025 at 08:09:04PM +0530, Nilay Shroff wrote:
> 
> 
> On 4/18/25 10:07 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().
> > 
> > Signed-off-by: Ming Lei <ming.lei@redhat.com>
> > ---
> >  block/bfq-iosched.c | 2 +-
> >  block/elevator.c    | 5 +++++
> >  block/elevator.h    | 1 +
> >  3 files changed, 7 insertions(+), 1 deletion(-)
> > 
> > diff --git a/block/bfq-iosched.c b/block/bfq-iosched.c
> > index 40e4106a71e7..310ce1d8c41e 100644
> > --- a/block/bfq-iosched.c
> > +++ b/block/bfq-iosched.c
> > @@ -7211,7 +7211,7 @@ static void bfq_exit_queue(struct elevator_queue *e)
> >  
> >  	blk_stat_disable_accounting(bfqd->queue);
> >  	blk_queue_flag_clear(QUEUE_FLAG_DISABLE_WBT, bfqd->queue);
> > -	wbt_enable_default(bfqd->queue->disk);
> > +	set_bit(ELEVATOR_FLAG_ENABLE_WBT_ON_EXIT, &e->flags);
> >  
> >  	kfree(bfqd);
> >  }
> > diff --git a/block/elevator.c b/block/elevator.c
> > index 8652fe45a2db..378553fce5d8 100644
> > --- a/block/elevator.c
> > +++ b/block/elevator.c
> > @@ -687,8 +687,13 @@ int elevator_change_done(struct request_queue *q, struct elv_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);
> >  	}
> >  	if (ctx->new) {
> >  		ret = elv_register_queue(q, ctx->new, ctx->uevent);
> > diff --git a/block/elevator.h b/block/elevator.h
> > index 486be0690499..b14c611c74b6 100644
> > --- a/block/elevator.h
> > +++ b/block/elevator.h
> > @@ -122,6 +122,7 @@ struct elevator_queue
> >  
> >  #define ELEVATOR_FLAG_REGISTERED	0
> >  #define ELEVATOR_FLAG_DYING		1
> > +#define ELEVATOR_FLAG_ENABLE_WBT_ON_EXIT	2
> >  
> >  /* Holding context data for changing elevator */
> >  struct elv_change_ctx {
> 
> It seems invoking wbt_enable_default from elevator_change_done could probably
> still race with ioc_qos_write or queue_wb_lat_store. Both ioc_qos_write and 
> queue_wb_lat_store run with ->freeze_lock and ->elevator_lock protection.

Actually wbt_enable_default() and wbt_init() needn't the above protection,
especially since the patch 2/20 removes q->elevator use in
wbt_enable_default().

Thanks,
Ming