Date: Thu, 4 Jan 2018 15:47:32 -0800
From: "Paul E. McKenney" <paulmck@linux.vnet.ibm.com>
To: Matthew Wilcox <willy@infradead.org>
Cc: Rao Shoaib <rao.shoaib@oracle.com>,
        Boqun Feng <boqun.feng@gmail.com>,
        linux-kernel@vger.kernel.org, brouer@redhat.com, linux-mm@kvack.org
Subject: Re: [PATCH 1/2] Move kfree_call_rcu() to slab_common.c
Reply-To: paulmck@linux.vnet.ibm.com
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On Thu, Jan 04, 2018 at 03:13:07PM -0800, Matthew Wilcox wrote:
> On Thu, Jan 04, 2018 at 02:18:50PM -0800, Rao Shoaib wrote:
> > > > > > +#define kfree_rcu(ptr, rcu_head_name) \
> > > > > > +??? do { \
> > > > > > +??????? typeof(ptr) __ptr = ptr;??? \
> > > > > > +??????? unsigned long __off = offsetof(typeof(*(__ptr)), \
> > > > > > +????????????????????????????? rcu_head_name); \
> > > > > > +??????? struct rcu_head *__rptr = (void *)__ptr + __off; \
> > > > > > +??????? __kfree_rcu(__rptr, __off); \
> > > > > > +??? } while (0)
> > > > > why do you want to open code this?
> > > But why are you changing this macro at all?  If it was to avoid the
> > > double-mention of "ptr", then you haven't done that.
> > I have -- I do not get the error because ptr is being assigned only one. If
> > you have a better way than let me know and I will be happy to make the
> > change.
> 
> But look at the original:
> 
> #define kfree_rcu(ptr, rcu_head)                                        \
>         __kfree_rcu(&((ptr)->rcu_head), offsetof(typeof(*(ptr)), rcu_head))
>                        ^^^                                ^^^
> 
> versus your version:
> 
> +#define kfree_rcu(ptr, rcu_head_name) \
> +??? do { \
> +??????? typeof(ptr) __ptr = ptr;??? \
>                 ^^^          ^^^
> +??????? unsigned long __off = offsetof(typeof(*(__ptr)), \
> +????????????????????????????? rcu_head_name); \
> +??????? struct rcu_head *__rptr = (void *)__ptr + __off; \
> +??????? __kfree_rcu(__rptr, __off); \
> +??? } while (0)
> 
> I don't see the difference.

I was under the impression that typeof did not actually evaluate its
argument, but rather only returned its type.  And there are a few macros
with this pattern in mainline.

Or am I confused about what typeof does?

							Thanx, Paul