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

From: Tejun Heo
Date: Thu Jun 20 2013 - 20:32:56 EST


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@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
---
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; \
})

/**
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