Re: [PATCH v2 4/5] blk-iocost: fix sleeping in atomic context warnning

[Yu Kuai]

Hi,

在 2022/11/15 6:07, Tejun Heo 写道:
> On Fri, Nov 04, 2022 at 10:39:37AM +0800, Yu Kuai wrote:
> > From: Yu Kuai <yukuai3@xxxxxxxxxx>
> >
> > match_u64() is called inside ioc->lock, which causes smatch static
> > checker warnings:
> >
> > block/blk-iocost.c:3211 ioc_qos_write() warn: sleeping in atomic context
> > block/blk-iocost.c:3240 ioc_qos_write() warn: sleeping in atomic context
> > block/blk-iocost.c:3407 ioc_cost_model_write() warn: sleeping in atomic
> > context
> >
> > Fix the problem by introducing a mutex and using it while prasing input
> > params.
>
> It bothers me that parsing an u64 string requires a GFP_KERNEL memory
> allocation.
>
> > @@ -2801,9 +2806,11 @@ static void ioc_rqos_queue_depth_changed(struct rq_qos *rqos)
> >   {
> >   	struct ioc *ioc = rqos_to_ioc(rqos);
> >   
> > +	mutex_lock(&ioc->params_mutex);
> >   	spin_lock_irq(&ioc->lock);
> >   	ioc_refresh_params(ioc, false);
> >   	spin_unlock_irq(&ioc->lock);
> > +	mutex_unlock(&ioc->params_mutex);
> >   }
>
> Aren't the params still protected by ioc->lock? Why do we need to grab both?

Yes, the params is updated inside ioc->lock, but they can be read
without the lock before updating them, which is protected by mutex
instead.

> Any chance I can persuade you into updating match_NUMBER() helpers to not
> use match_strdup()? They can easily disable irq/preemption and use percpu
> buffers and we won't need most of this patchset.

Do you mean preallocated percpu buffer? Is there any example I can
learn? Anyway, replace match_strdup() to avoid memory allocation sounds
good.

Thanks,
Kuai
> Thanks.