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

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Message-ID: <20200222004656.GA459391@carbon.DHCP.thefacebook.com>
Date:   Fri, 21 Feb 2020 16:46:56 -0800
From:   Roman Gushchin <guro@...com>
To:     Dennis Zhou <dennis@...nel.org>
CC:     <ira.weiny@...el.com>, <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

On Fri, Feb 21, 2020 at 06:56:27PM -0500, Dennis Zhou wrote:
> Hi Ira,
> 
> On Fri, Feb 21, 2020 at 03:16:07PM -0800, ira.weiny@...el.com wrote:
> > From: Ira Weiny <ira.weiny@...el.com>
> > 
> > The comment for percpu_ref_init() implies that using
> > PERCPU_REF_ALLOW_REINIT will cause the refcount to start at 0.  But
> > this is not true.  PERCPU_REF_ALLOW_REINIT starts the count at 1 as
> > if the flags were zero.  Add this fact to the kernel doc comment.
> > 
> > Signed-off-by: Ira Weiny <ira.weiny@...el.com>
> > ---
> > RESEND:
> > 	Add more people on the CC list to see if I'm wrong here.
> > 	https://lore.kernel.org/lkml/20200206042810.GA29917@iweiny-DESK2.sc.intel.com/
> > ---
> > 
> >  lib/percpu-refcount.c | 6 +++---
> >  1 file changed, 3 insertions(+), 3 deletions(-)
> > 
> > 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!