linux-kernel - [PATCH] rculist: list_first_or_null_rcu() should use list_entry

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Message-ID: <20130621003244.GD11837@mtj.dyndns.org>
Date:	Thu, 20 Jun 2013 17:32:44 -0700
From:	Tejun Heo <tj@...nel.org>
To:	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>,
	Dipankar Sarma <dipankar@...ibm.com>
Cc:	Fengguang Wu <fengguang.wu@...el.com>,
	"David S. Miller" <davem@...emloft.net>,
	Li Zefan <lizefan@...wei.com>,
	Patrick McHardy <kaber@...sh.net>, linux-kernel@...r.kernel.org
Subject: [PATCH] rculist: list_first_or_null_rcu() should use list_entry_rcu()

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@...nel.org>
Reported-by: Fengguang Wu <fengguang.wu@...el.com>
Cc: Dipankar Sarma <dipankar@...ibm.com>
Cc: "Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
Cc: "David S. Miller" <davem@...emloft.net>
Cc: Li Zefan <lizefan@...wei.com>
Cc: Patrick McHardy <kaber@...sh.net>
Cc: stable@...r.kernel.org
---
 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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