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Date:   Sat, 18 Apr 2020 15:08:40 -0700
From:   Andrew Morton <akpm@linux-foundation.org>
To:     Matthew Wilcox <willy@infradead.org>
Cc:     linux-fsdevel@vger.kernel.org, linux-kernel@vger.kernel.org,
        Manfred Spraul <manfred@colorfullife.com>,
        Davidlohr Bueso <dave@stgolabs.net>
Subject: Re: [PATCH] ipc: Convert ipcs_idr to XArray
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        <20200418131509.fb3c19bf450d618be797c030@linux-foundation.org>
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On Sat, 18 Apr 2020 14:34:51 -0700 Matthew Wilcox <willy@infradead.org> wrote:

> On Sat, Apr 18, 2020 at 01:15:09PM -0700, Andrew Morton wrote:
> > > --- a/ipc/util.c
> > > +++ b/ipc/util.c
> > > @@ -104,12 +104,20 @@ static const struct rhashtable_params ipc_kht_params = {
> > >  	.automatic_shrinking	= true,
> > >  };
> > >  
> > > +#ifdef CONFIG_CHECKPOINT_RESTORE
> > 
> > The code grew a few additional CONFIG_CHECKPOINT_RESTORE ifdefs. 
> > What's going on here?  Why is CRIU special in ipc/?
> 
> "grew a few"?  I added (this) one and deleted two others.  From in the
> middle of functions, like we usually prefer.
> 

Oh.

> 
> @@ -17,11 +17,11 @@ struct ipc_ids {
> ...
>  #ifdef CONFIG_CHECKPOINT_RESTORE
> -       int next_id;
> +       int restore_id;
>  #endif
> 
> > > +#define set_restore_id(ids, x)	ids->restore_id = x
> > > +#define get_restore_id(ids)	ids->restore_id
> > > +#else
> > > +#define set_restore_id(ids, x)	do { } while (0)
> > > +#define get_restore_id(ids)	(-1)
> > > +#endif
> > 
> > Well these are ugly.  Can't all this be done in C?
> 
> Would you rather see it done as:
> 
> static inline void set_restore_id(struct ipc_ids *ids, int id)
> {
> #ifdef CONFIG_CHECKPOINT_RESTORE
> 	ids->restore_id = id;
> #endif
> }
> 
> static inline int get_restore_id(struct ipc_ids *ids)
> {
> #ifdef CONFIG_CHECKPOINT_RESTORE
> 	return ids->restore_id;
> #else
> 	return -1;
> #endif
> }

Looks nicer.  Has type checking regardless of Kconfig.  Doesn't have
lval-and-rval in one case, neither in the other.  Doesn't risk
unused-var warnings dependent on Kconfig.  Etc.

Could also do

#ifdef CONFIG_CHECKPOINT_RESTORE
static inline void set_restore_id(struct ipc_ids *ids, int id)
{
	ids->restore_id = id;
}

static inline int get_restore_id(struct ipc_ids *ids)
{
	return ids->restore_id;
	return -1;
}
#else
...