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

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Message-ID: <20180911163032.GA2966370@devbig004.ftw2.facebook.com>
Date:   Tue, 11 Sep 2018 09:30:32 -0700
From:   Tejun Heo <tj@...nel.org>
To:     Ming Lei <ming.lei@...hat.com>
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 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?

> 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?

Thanks.

-- 
tejun