Re: [PATCH] rculist: list_first_or_null_rcu() should use list_entry_rcu()

["Paul E. McKenney" <paulmck@linux.vnet.ibm.com>]

On Thu, Jun 20, 2013 at 05:32:44PM -0700, Tejun Heo wrote:
> list_first_or_null() should test whether the list is empty and return
> pointer to the first entry if not in a RCU safe manner.  It's broken
> in two ways.
> 
> * It compares __kernel @__ptr with __rcu @__next triggering the
>   following sparse warning.
> 
>   net/core/dev.c:4331:17: error: incompatible types in comparison expression (different address spaces)
> 
> * It doesn't perform rcu_dereference*() and computes the entry address
>   using container_of() directly from the __rcu pointer which is
>   inconsitent with other rculist interface.  As a result, all three
>   in-kernel users - net/core/dev.c, macvlan, cgroup - are buggy.  They
>   dereference the pointer w/o going through read barrier.
> 
> Fix it by making list_first_or_null_rcu() dereference ->next directly
> and then use list_entry_rcu() on it like other rculist accessors.
> 
> Signed-off-by: Tejun Heo <tj@kernel.org>
> Reported-by: Fengguang Wu <fengguang.wu@intel.com>
> Cc: Dipankar Sarma <dipankar@in.ibm.com>
> Cc: "Paul E. McKenney" <paulmck@linux.vnet.ibm.com>
> Cc: "David S. Miller" <davem@davemloft.net>
> Cc: Li Zefan <lizefan@huawei.com>
> Cc: Patrick McHardy <kaber@trash.net>
> Cc: stable@vger.kernel.org
> ---
>  include/linux/rculist.h |    5 +++--
>  1 file changed, 3 insertions(+), 2 deletions(-)
> 
> --- a/include/linux/rculist.h
> +++ b/include/linux/rculist.h
> @@ -267,8 +267,9 @@ static inline void list_splice_init_rcu(
>   */
>  #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?

							Thanx, Paul