linux-kernel - Re: [PATCH 2/4] lib/percpu-refcount: introduce percpu_ref

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Message-ID: <20180919080059.GB23172@ming.t460p>
Date:   Wed, 19 Sep 2018 16:01:01 +0800
From:   Ming Lei <ming.lei@...hat.com>
To:     "jianchao.wang" <jianchao.w.wang@...cle.com>
Cc:     linux-kernel@...r.kernel.org, Tejun Heo <tj@...nel.org>,
        Kent Overstreet <kent.overstreet@...il.com>,
        linux-block@...r.kernel.org
Subject: Re: [PATCH 2/4] lib/percpu-refcount: introduce percpu_ref_resurge()

On Wed, Sep 19, 2018 at 03:55:07PM +0800, Ming Lei wrote:
> On Wed, Sep 19, 2018 at 01:19:10PM +0800, jianchao.wang wrote:
> > Hi Ming
> > 
> > On 09/18/2018 06:19 PM, Ming Lei wrote:
> > > +		unsigned long __percpu *percpu_count;
> > > +
> > > +		WARN_ON_ONCE(__ref_is_percpu(ref, &percpu_count));
> > > +
> > > +		/* get one extra ref for avoiding race with .release */
> > > +		rcu_read_lock_sched();
> > > +		atomic_long_add(1, &ref->count);
> > > +		rcu_read_unlock_sched();
> > > +	}
> > 
> > The rcu_read_lock_sched here is redundant. We have been in the critical section
> > of a spin_lock_irqsave.
> 
> Right.
> 
> > 
> > The atomic_long_add(1, &ref->count) may have two result.
> > 1. ref->count > 1
> >    it will not drop to zero any more.
> > 2. ref->count == 1
> >    it has dropped to zero and .release may be running.
> 
> IMO, both the two cases are fine and supported, or do you have other
> concern about this way?

It is too quick, :-)

Yeah, the .release() may be running.

For blk-mq/NVMe's use case, it won't be an issue. We may comment on this
race and let user handle it if it is a problem.


thanks,
Ming