linux-kernel - Re: [PATCH] percpu-refcount: relax limit on percpu_ref

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Message-ID: <20180911163443.GD10082@ming.t460p>
Date:   Wed, 12 Sep 2018 00:34:44 +0800
From:   Ming Lei <ming.lei@...hat.com>
To:     Tejun Heo <tj@...nel.org>
Cc:     linux-kernel@...r.kernel.org,
        Jianchao Wang <jianchao.w.wang@...cle.com>,
        Kent Overstreet <kent.overstreet@...il.com>,
        linux-block@...r.kernel.org
Subject: Re: [PATCH] percpu-refcount: relax limit on percpu_ref_reinit()

On Tue, Sep 11, 2018 at 09:30:32AM -0700, Tejun Heo wrote:
> On Wed, Sep 12, 2018 at 12:05:33AM +0800, Ming Lei wrote:
> > On Tue, Sep 11, 2018 at 08:49:59AM -0700, Tejun Heo wrote:
> > > On Tue, Sep 11, 2018 at 11:45:41PM +0800, Ming Lei wrote:
> > > > > So, this part seems wrong.  The function is called percpu_ref_reinit()
> > > > > - the refcnt is expected to be in its initial state with just the base
> > > > > ref once this function returns.  If you're removing the restriction on
> > > > 
> > > > But the comment says that 'Re-initialize @ref so that it's in the same
> > > > state as when it finished', and this invariant isn't changed with this
> > > > patch.
> > > 
> > > The comment goes "when perpcu_ref_init() finished".  The function is
> > > called re _init_.  It should put the ref in the initial state, right?
> > 
> > OK, I am fine to keep the behaviour for this API, will introduce a new helper
> > for NVMe.
> 
> Why aren't switch_to_atomic/percpu enough?

The blk-mq's use case is this _reinit is done on one refcount which was
killed via percpu_ref_kill(), so the DEAD flag has to be cleared.

> 
> > However, it is doable to switch to percpu mode from atomic mode when it
> > doesn't drop to zero, looks like sort of perpcu_ref_reinit(inherit_old_refcount).
> 
> Isn't that way more contorted than just switching operating modes?

As explained above.

Thanks,
Ming