linux-kernel - [PATCH RFC tip/core/rcu 3/6] rcu: Replace list_first_entry_rcu() with list_first_or_null

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Date:	Mon, 23 Apr 2012 09:42:24 -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, Valdis.Kletnieks@...edu,
	dhowells@...hat.com, eric.dumazet@...il.com, darren@...art.com,
	fweisbec@...il.com, patches@...aro.org,
	Michel Machado <michel@...irati.com.br>,
	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
Subject: [PATCH RFC tip/core/rcu 3/6] rcu: Replace list_first_entry_rcu() with list_first_or_null_rcu()

From: Michel Machado <michel@...irati.com.br>

The list_first_entry_rcu() macro is inherently unsafe because it cannot
be applied to an empty list.  But because RCU readers do not exclude
updaters, a list might become empty between the time that list_empty()
claimed it was non-empty and the time that list_first_entry_rcu() is
invoked.  Therefore, the list_empty() test cannot be separated from the
list_first_entry_rcu() call.  This commit therefore combines these to
macros to create a new list_first_or_null_rcu() macro that replaces
the old (and unsafe) list_first_entry_rcu() macro.

This patch incorporates Paul's review comments on the previous version of
this patch available here:

https://lkml.org/lkml/2012/4/2/536

This patch cannot break any upstream code because list_first_entry_rcu()
is not being used anywhere in the kernel (tested with grep(1)), and any
external code using it is probably broken as a result of using it.

Signed-off-by: Michel Machado <michel@...irati.com.br>
CC: "Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
CC: Dipankar Sarma <dipankar@...ibm.com>
Signed-off-by: Paul E. McKenney <paulmck@...ux.vnet.ibm.com>
---
 include/linux/rculist.h |   33 +++++++++++++++++++++++++++++----
 1 files changed, 29 insertions(+), 4 deletions(-)

diff --git a/include/linux/rculist.h b/include/linux/rculist.h
index a20c050..e0f0fab 100644
--- a/include/linux/rculist.h
+++ b/include/linux/rculist.h
@@ -233,18 +233,43 @@ static inline void list_splice_init_rcu(struct list_head *list,
 	})
 
 /**
- * list_first_entry_rcu - get the first element from a list
+ * Where are list_empty_rcu() and list_first_entry_rcu()?
+ *
+ * Implementing those functions following their counterparts list_empty() and
+ * list_first_entry() is not advisable because they lead to subtle race
+ * conditions as the following snippet shows:
+ *
+ * if (!list_empty_rcu(mylist)) {
+ *	struct foo *bar = list_first_entry_rcu(mylist, struct foo, list_member);
+ *	do_something(bar);
+ * }
+ *
+ * The list may not be empty when list_empty_rcu checks it, but it may be when
+ * list_first_entry_rcu rereads the ->next pointer.
+ *
+ * Rereading the ->next pointer is not a problem for list_empty() and
+ * list_first_entry() because they would be protected by a lock that blocks
+ * writers.
+ *
+ * See list_first_or_null_rcu for an alternative.
+ */
+
+/**
+ * list_first_or_null_rcu - get the first element from a list
  * @ptr:        the list head to take the element from.
  * @type:       the type of the struct this is embedded in.
  * @member:     the name of the list_struct within the struct.
  *
- * Note, that list is expected to be not empty.
+ * Note that if the list is empty, it returns NULL.
  *
  * This primitive may safely run concurrently with the _rcu list-mutation
  * primitives such as list_add_rcu() as long as it's guarded by rcu_read_lock().
  */
-#define list_first_entry_rcu(ptr, type, member) \
-	list_entry_rcu((ptr)->next, type, member)
+#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; \
+	})
 
 /**
  * list_for_each_entry_rcu	-	iterate over rcu list of given type
-- 
1.7.8

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