linux-kernel - Re: [RESEND PATCH] percpu_ref: Fix comment regarding percpu_ref

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Message-ID: <20200223143222.GA29607@iweiny-DESK2.sc.intel.com>
Date:   Sun, 23 Feb 2020 06:32:23 -0800
From:   Ira Weiny <ira.weiny@...el.com>
To:     Roman Gushchin <guro@...com>
Cc:     Dennis Zhou <dennis@...nel.org>, linux-kernel@...r.kernel.org,
        Petr Mladek <pmladek@...e.com>, Tejun Heo <tj@...nel.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Sakari Ailus <sakari.ailus@...ux.intel.com>
Subject: Re: [RESEND PATCH] percpu_ref: Fix comment regarding percpu_ref_init
 flags


[snip]

> > > diff --git a/lib/percpu-refcount.c b/lib/percpu-refcount.c
> > > index 4f6c6ebbbbde..48d7fcff70b6 100644
> > > --- a/lib/percpu-refcount.c
> > > +++ b/lib/percpu-refcount.c
> > > @@ -50,9 +50,9 @@ static unsigned long __percpu *percpu_count_ptr(struct percpu_ref *ref)
> > >   * @flags: PERCPU_REF_INIT_* flags
> > >   * @gfp: allocation mask to use
> > >   *
> > > - * Initializes @ref.  If @flags is zero, @ref starts in percpu mode with a
> > > - * refcount of 1; analagous to atomic_long_set(ref, 1).  See the
> > > - * definitions of PERCPU_REF_INIT_* flags for flag behaviors.
> > > + * Initializes @ref.  If @flags is zero or PERCPU_REF_ALLOW_REINIT, @ref starts
> > > + * in percpu mode with a refcount of 1; analagous to atomic_long_set(ref, 1).
> > > + * See the definitions of PERCPU_REF_INIT_* flags for flag behaviors.
> > 
> > Yeah. Prior we had both PERCPU_REF_INIT_ATOMIC and PERCPU_REF_INIT_DEAD
> > with the latter implying the former. So 0 meant percpu and the others
> > meant atomic. With PERCPU_REF_ALLOW_REINIT, it's probably easier to
> > understand by saying if neither PERCPU_REF_INIT_ATOMIC or
> > PERCPU_REF_INIT_DEAD is set, it starts out in percpu mode which is
> > mentioned in the comments where the flags are defined.  It's not great
> > having implied flags, but it's worked so far.
> > 
> > Also, it's not quite analagous to atomic_long_set(ref, 1) as there is a
> > bias to prevent prematurely hitting 0.
> > 
> > I can take this and massage the wording a bit.
> 
> Hello Ira! Hello Dennis!
> 
> Yeah, I'd simple say that it starts in the percpu mode, except the case when
> PERCPU_REF_INIT_ATOMIC is set, then (atomic mode, 1) and
> PERCPU_REF_INIT_DEAD is set, then (atomic mode, 0).
> 
> PERCPU_REF_ALLOW_REINIT actually doesn't affect the initial state.
> 

Thanks for the clarification.  Dennis let me know if you want me to resubmit
the patch.

Thanks!
Ira