linux-kernel - Re: [PATCH 1/3] rculist: Fix list_entry_rcu to read ptr with rcu_dereference

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Message-ID: <20150325143018.GZ5718@linux.vnet.ibm.com>
Date:	Wed, 25 Mar 2015 07:30:18 -0700
From:	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
To:	Patrick Marlier <patrick.marlier@...il.com>
Cc:	linux-kernel@...r.kernel.org, Josh Triplett <josh@...htriplett.org>
Subject: Re: [PATCH 1/3] rculist: Fix list_entry_rcu to read ptr with
 rcu_dereference_raw

On Tue, Mar 24, 2015 at 11:31:38AM +0100, Patrick Marlier wrote:
> Change to read effectively ptr with rcu_dereference_raw and not the
> __ptr variable on the stack.
> 
> Signed-off-by: Patrick Marlier <patrick.marlier@...il.com>

Avoiding an extra load could be worthwhile in a number of situations,
agreed.

However, won't this change cause sparse to complain if invoked on a
non-RCU-protected pointer?  The ability to use list-RCU API
members on both RCU and non-RCU pointers was one of the points
of the previous commit, right?

							Thanx, Paul

> ---
>  include/linux/rculist.h | 5 +----
>  1 file changed, 1 insertion(+), 4 deletions(-)
> 
> diff --git a/include/linux/rculist.h b/include/linux/rculist.h
> index a18b16f..9d9baea 100644
> --- a/include/linux/rculist.h
> +++ b/include/linux/rculist.h
> @@ -247,10 +247,7 @@ static inline void list_splice_init_rcu(struct
> list_head *list,
>   * primitives such as list_add_rcu() as long as it's guarded by
> rcu_read_lock().
>   */
>  #define list_entry_rcu(ptr, type, member) \
> -({ \
> -   typeof(*ptr) __rcu *__ptr = (typeof(*ptr) __rcu __force *)ptr; \
> -   container_of((typeof(ptr))rcu_dereference_raw(__ptr), type, member); \
> -})
> +   container_of((typeof(ptr))rcu_dereference_raw(ptr), type, member)
> 
>  /**
>   * Where are list_empty_rcu() and list_first_entry_rcu()?
> -- 
> 2.1.0
> 

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