linux-kernel - Re: [PATCH] rculist: list_first_or_null_rcu() should use list_entry

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Date:	Wed, 26 Jun 2013 07:17:52 -0700
From:	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
To:	Tejun Heo <tj@...nel.org>
Cc:	Dipankar Sarma <dipankar@...ibm.com>,
	Fengguang Wu <fengguang.wu@...el.com>,
	"David S. Miller" <davem@...emloft.net>,
	Li Zefan <lizefan@...wei.com>,
	Patrick McHardy <kaber@...sh.net>, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] rculist: list_first_or_null_rcu() should use
 list_entry_rcu()

On Tue, Jun 25, 2013 at 04:09:38PM -0700, Tejun Heo wrote:
> Hello, Paul.
> 
> On Tue, Jun 25, 2013 at 03:57:59PM -0700, Paul E. McKenney wrote:
> > >  #define list_first_or_null_rcu(ptr, type, member) \
> > >  	({struct list_head *__ptr = (ptr); \
> > > -	  struct list_head __rcu *__next = list_next_rcu(__ptr); \
> > > -	  likely(__ptr != __next) ? container_of(__next, type, member) : NULL; \
> > > +	  struct list_head *__next = __ptr->next; \
> > > +	  likely(__ptr != __next) ? \
> > > +		list_entry_rcu(__next, type, member) : NULL; \
> > 
> > I am a bit uneasy with this, and would feel better if the volatile
> > cast was on the very first fetch of the ->next pointer.
> > 
> > Is there some reason why my unease is ill-founded?
> 
> Do you mean something like the following?
> 
> 	  struct list_head *__next = ACCESS_ONCE(__ptr->next); \
> 	  likely(__ptr != __next) ? \
> 		list_entry_rcu(__next, type, member) : NULL; \
> 
> Yeah, that looks right to me.

I would feel much better about this!  Does it avoid warnings in your
use cases?

							Thanx, Paul

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