Re: [RESEND PATCH] percpu_ref: Fix comment regarding percpu_ref_init flags
From: Roman Gushchin
Date: Fri Feb 21 2020 - 19:47:16 EST
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@xxxxxxxxx wrote:
> > From: Ira Weiny <ira.weiny@xxxxxxxxx>
> > 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@xxxxxxxxx>
> > ---
> > RESEND:
> > Add more people on the CC list to see if I'm wrong here.
> > https://lore.kernel.org/lkml/20200206042810.GA29917@xxxxxxxxxxxxxxxxxxxxxxxxx/
> > ---
> > 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.