Messages in this thread Patch in this message | | | Date | Fri, 28 Jun 2013 10:34:48 -0700 | From | Tejun Heo <> | Subject | [PATCH v3] rculist: list_first_or_null_rcu() should use list_entry_rcu() |
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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 --- 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; \ }) /**
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