linux-kernel - [PATCH tip/core/rcu 6/9] rculist: list_first_or_null_rcu() should use list_entry

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Message-Id: <1376966534-30775-6-git-send-email-paulmck@linux.vnet.ibm.com>
Date:	Mon, 19 Aug 2013 19:42:11 -0700
From:	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
To:	linux-kernel@...r.kernel.org
Cc:	mingo@...e.hu, laijs@...fujitsu.com, dipankar@...ibm.com,
	akpm@...ux-foundation.org, mathieu.desnoyers@...ymtl.ca,
	josh@...htriplett.org, niv@...ibm.com, tglx@...utronix.de,
	peterz@...radead.org, rostedt@...dmis.org, dhowells@...hat.com,
	edumazet@...gle.com, darren@...art.com, fweisbec@...il.com,
	sbw@....edu, Tejun Heo <tj@...nel.org>,
	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>,
	"David S. Miller" <davem@...emloft.net>,
	Li Zefan <lizefan@...wei.com>,
	Patrick McHardy <kaber@...sh.net>, stable@...r.kernel.org
Subject: [PATCH tip/core/rcu 6/9] rculist: list_first_or_null_rcu() should use list_entry_rcu()

From: Tejun Heo <tj@...nel.org>

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@...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
Signed-off-by: Paul E. McKenney <paulmck@...ux.vnet.ibm.com>
Reviewed-by: Josh Triplett <josh@...htriplett.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 f4b1001..4106721 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; \
 	})
 
 /**
-- 
1.8.1.5

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