Re: [PATCH v2] rculist: list_first_or_null_rcu() should uselist_entry_rcu()

[Paul E. McKenney]

On Wed, Jun 26, 2013 at 10:27:53AM -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 several 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.
> 
> * While ->next dereference passes through list_next_rcu(), the
>   compiler is still free to fetch ->next more than once and thus
>   nullify the "__ptr != __next" condition check.
> 
> Fix it by making list_first_or_null_rcu() dereference ->next directly
> using ACCESS_ONCE() and then use list_entry_rcu() on it like other
> rculist accessors.
> 
> v2: Paul pointed out that the compiler may fetch the pointer more than
>     once nullifying the condition check.  ACCESS_ONCE() added on
>     ->next dereference.
> 
> Signed-off-by: Tejun Heo <tj@xxxxxxxxxx>
> Reported-by: Fengguang Wu <fengguang.wu@xxxxxxxxx>
> Cc: Dipankar Sarma <dipankar@xxxxxxxxxx>
> Cc: "Paul E. McKenney" <paulmck@xxxxxxxxxxxxxxxxxx>
> Cc: "David S. Miller" <davem@xxxxxxxxxxxxx>
> Cc: Li Zefan <lizefan@xxxxxxxxxx>
> Cc: Patrick McHardy <kaber@xxxxxxxxx>
> Cc: stable@xxxxxxxxxxxxxxx
> ---
> Paul, I was mistaken.  For list_first_or_null_rcu(), @ptr is constant.
> It's a value which can't change and usually not even a l-value.
> ACCESS_ONCE() is necessary when dereferencing @ptr->next, which may
> change while list_first_or_null_rcu() is in progress.
> 
> Thanks.

Fair enough!

But why drop the parens around "ptr"?

							Thanx, Paul

>  include/linux/rculist.h |    7 ++++---
>  1 file changed, 4 insertions(+), 3 deletions(-)
> 
> --- a/include/linux/rculist.h
> +++ b/include/linux/rculist.h
> @@ -266,9 +266,10 @@ static inline void list_splice_init_rcu(
>   * primitives such as list_add_rcu() as long as it's guarded by rcu_read_lock().
>   */
>  #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 *__ptr = ptr; \
> +	  struct list_head *__next = ACCESS_ONCE(__ptr->next); \
> +	  likely(__ptr != __next) ? \
> +		list_entry_rcu(__next, type, member) : NULL; \
>  	})
> 
>  /**
> 

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