[PATCH 3.2 029/149] rculist: list_first_or_null_rcu() should uselist_entry_rcu()

[Ben Hutchings]

3.2.52-rc1 review patch.  If anyone has any objections, please let me know.

------------------

From: Tejun Heo <tj@xxxxxxxxxx>

commit c34ac00caefbe49d40058ae7200bd58725cebb45 upstream.

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@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>
Signed-off-by: Paul E. McKenney <paulmck@xxxxxxxxxxxxxxxxxx>
Reviewed-by: Josh Triplett <josh@xxxxxxxxxxxxxxxx>
Signed-off-by: Ben Hutchings <ben@xxxxxxxxxxxxxxx>
---
 include/linux/rculist.h | 5 +++--
 1 file changed, 3 insertions(+), 2 deletions(-)

--- a/include/linux/rculist.h
+++ b/include/linux/rculist.h
@@ -254,8 +254,9 @@ 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 = ACCESS_ONCE(__ptr->next); \
+	  likely(__ptr != __next) ? \
+		list_entry_rcu(__next, type, member) : NULL; \
 	})
 
 /**

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