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

[Tejun Heo]

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.

 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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