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 <[email protected]>
> Reported-by: Fengguang Wu <[email protected]>
> Cc: Dipankar Sarma <[email protected]>
> Cc: "Paul E. McKenney" <[email protected]>
> Cc: "David S. Miller" <[email protected]>
> Cc: Li Zefan <[email protected]>
> Cc: Patrick McHardy <[email protected]>
> Cc: [email protected]
> ---
> 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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