Re: [PATCH v3] rculist: list_first_or_null_rcu() should use list_entry_rcu()

Paul E. McKenney
2013-06-28T19:25:17Z

On Fri, Jun 28, 2013 at 10:34:48AM -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.
>
> v3: Restored () around macro param which was accidentally removed.
> Spotted by Paul.
>
> 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

Reviewed-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>

> ---
> include/linux/rculist.h | 5 +++--
> 1 file changed, 3 insertions(+), 2 deletions(-)
>
> diff --git a/include/linux/rculist.h b/include/linux/rculist.h
> index 8089e35..523f13c 100644
> --- a/include/linux/rculist.h
> +++ b/include/linux/rculist.h
> @@ -267,8 +267,9 @@ static inline void list_splice_init_rcu(struct list_head *list,
> */
> #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 = ACCESS_ONCE(__ptr->next); \
> + likely(__ptr != __next) ? \
> + list_entry_rcu(__next, type, member) : NULL; \
> })
>
> /**
>