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

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Message-ID: <20130625230938.GB30407@mtj.dyndns.org>
Date:	Tue, 25 Jun 2013 16:09:38 -0700
From:	Tejun Heo <tj@...nel.org>
To:	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
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()

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.

Thanks.

-- 
tejun
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