Update llist code

This fixes a bug where the compiler was dropping a null ptr check

2 files changed, 66 insertions, 50 deletions

diff --git a/include/linux/llist.h b/include/linux/llist.h
index 8abc2e02..2e9c7215 100644
--- a/include/linux/llist.h
+++ b/include/linux/llist.h
@@ -1,31 +1,36 @@
-#ifndef __TOOLS_LINUX_LLIST_H
-#define __TOOLS_LINUX_LLIST_H
-
+/* SPDX-License-Identifier: GPL-2.0-only */
+#ifndef LLIST_H
+#define LLIST_H
 /*
  * Lock-less NULL terminated single linked list
  *
- * If there are multiple producers and multiple consumers, llist_add
- * can be used in producers and llist_del_all can be used in
- * consumers.  They can work simultaneously without lock.  But
- * llist_del_first can not be used here.  Because llist_del_first
- * depends on list->first->next does not changed if list->first is not
- * changed during its operation, but llist_del_first, llist_add,
- * llist_add (or llist_del_all, llist_add, llist_add) sequence in
- * another consumer may violate that.
- *
- * If there are multiple producers and one consumer, llist_add can be
- * used in producers and llist_del_all or llist_del_first can be used
- * in the consumer.
- *
- * This can be summarized as follow:
+ * Cases where locking is not needed:
+ * If there are multiple producers and multiple consumers, llist_add can be
+ * used in producers and llist_del_all can be used in consumers simultaneously
+ * without locking. Also a single consumer can use llist_del_first while
+ * multiple producers simultaneously use llist_add, without any locking.
+ *
+ * Cases where locking is needed:
+ * If we have multiple consumers with llist_del_first used in one consumer, and
+ * llist_del_first or llist_del_all used in other consumers, then a lock is
+ * needed.  This is because llist_del_first depends on list->first->next not
+ * changing, but without lock protection, there's no way to be sure about that
+ * if a preemption happens in the middle of the delete operation and on being
+ * preempted back, the list->first is the same as before causing the cmpxchg in
+ * llist_del_first to succeed. For example, while a llist_del_first operation
+ * is in progress in one consumer, then a llist_del_first, llist_add,
+ * llist_add (or llist_del_all, llist_add, llist_add) sequence in another
+ * consumer may cause violations.
+ *
+ * This can be summarized as follows:
  *
  *           |   add    | del_first |  del_all
  * add       |    -     |     -     |     -
  * del_first |          |     L     |     L
  * del_all   |          |           |     -
  *
- * Where "-" stands for no lock is needed, while "L" stands for lock
- * is needed.
+ * Where, a particular row's operation can happen concurrently with a column's
+ * operation, with "-" being no lock needed, while "L" being lock is needed.
  *
  * The list entries deleted via llist_del_all can be traversed with
  * traversing function such as llist_for_each etc.  But the list
@@ -41,19 +46,6 @@
  *
  * Copyright 2010,2011 Intel Corp.
  *   Author: Huang Ying <ying.huang@intel.com>
- *
- * This program is free software; you can redistribute it and/or
- * modify it under the terms of the GNU General Public License version
- * 2 as published by the Free Software Foundation;
- *
- * This program is distributed in the hope that it will be useful,
- * but WITHOUT ANY WARRANTY; without even the implied warranty of
- * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
- * GNU General Public License for more details.
- *
- * You should have received a copy of the GNU General Public License
- * along with this program; if not, write to the Free Software
- * Foundation, Inc., 59 Temple Place, Suite 330, Boston, MA  02111-1307  USA
  */
 
 #include <linux/atomic.h>
@@ -89,6 +81,23 @@ static inline void init_llist_head(struct llist_head *list)
 	container_of(ptr, type, member)
 
 /**
+ * member_address_is_nonnull - check whether the member address is not NULL
+ * @ptr:	the object pointer (struct type * that contains the llist_node)
+ * @member:	the name of the llist_node within the struct.
+ *
+ * This macro is conceptually the same as
+ *	&ptr->member != NULL
+ * but it works around the fact that compilers can decide that taking a member
+ * address is never a NULL pointer.
+ *
+ * Real objects that start at a high address and have a member at NULL are
+ * unlikely to exist, but such pointers may be returned e.g. by the
+ * container_of() macro.
+ */
+#define member_address_is_nonnull(ptr, member)	\
+	((uintptr_t)(ptr) + offsetof(typeof(*(ptr)), member) != 0)
+
+/**
  * llist_for_each - iterate over some deleted entries of a lock-less list
  * @pos:	the &struct llist_node to use as a loop cursor
  * @node:	the first entry of deleted list entries
@@ -106,6 +115,25 @@ static inline void init_llist_head(struct llist_head *list)
 	for ((pos) = (node); pos; (pos) = (pos)->next)
 
 /**
+ * llist_for_each_safe - iterate over some deleted entries of a lock-less list
+ *			 safe against removal of list entry
+ * @pos:	the &struct llist_node to use as a loop cursor
+ * @n:		another &struct llist_node to use as temporary storage
+ * @node:	the first entry of deleted list entries
+ *
+ * In general, some entries of the lock-less list can be traversed
+ * safely only after being deleted from list, so start with an entry
+ * instead of list head.
+ *
+ * If being used on entries deleted from lock-less list directly, the
+ * traverse order is from the newest to the oldest added entry.  If
+ * you want to traverse from the oldest to the newest, you must
+ * reverse the order by yourself before traversing.
+ */
+#define llist_for_each_safe(pos, n, node)			\
+	for ((pos) = (node); (pos) && ((n) = (pos)->next, true); (pos) = (n))
+
+/**
  * llist_for_each_entry - iterate over some deleted entries of lock-less list of given type
  * @pos:	the type * to use as a loop cursor.
  * @node:	the fist entry of deleted list entries.
@@ -122,7 +150,7 @@ static inline void init_llist_head(struct llist_head *list)
  */
 #define llist_for_each_entry(pos, node, member)				\
 	for ((pos) = llist_entry((node), typeof(*(pos)), member);	\
-	     &(pos)->member != NULL;					\
+	     member_address_is_nonnull(pos, member);			\
 	     (pos) = llist_entry((pos)->member.next, typeof(*(pos)), member))
 
 /**
@@ -144,7 +172,7 @@ static inline void init_llist_head(struct llist_head *list)
  */
 #define llist_for_each_entry_safe(pos, n, node, member)			       \
 	for (pos = llist_entry((node), typeof(*pos), member);		       \
-	     &pos->member != NULL &&					       \
+	     member_address_is_nonnull(pos, member) &&			       \
 	        (n = llist_entry(pos->member.next, typeof(*n), member), true); \
 	     pos = n)
 
@@ -158,7 +186,7 @@ static inline void init_llist_head(struct llist_head *list)
  */
 static inline bool llist_empty(const struct llist_head *head)
 {
-	return ACCESS_ONCE(head->first) == NULL;
+	return READ_ONCE(head->first) == NULL;
 }
 
 static inline struct llist_node *llist_next(struct llist_node *node)
@@ -198,4 +226,4 @@ extern struct llist_node *llist_del_first(struct llist_head *head);
 
 struct llist_node *llist_reverse_order(struct llist_node *head);
 
-#endif /* __TOOLS_LINUX_LLIST_H */
+#endif /* LLIST_H */
diff --git a/linux/llist.c b/linux/llist.c
index ae5872b1..611ce488 100644
--- a/linux/llist.c
+++ b/linux/llist.c
@@ -1,3 +1,4 @@
+// SPDX-License-Identifier: GPL-2.0-only
 /*
  * Lock-less NULL terminated single linked list
  *
@@ -8,19 +9,6 @@
  *
  * Copyright 2010,2011 Intel Corp.
  *   Author: Huang Ying <ying.huang@intel.com>
- *
- * This program is free software; you can redistribute it and/or
- * modify it under the terms of the GNU General Public License version
- * 2 as published by the Free Software Foundation;
- *
- * This program is distributed in the hope that it will be useful,
- * but WITHOUT ANY WARRANTY; without even the implied warranty of
- * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
- * GNU General Public License for more details.
- *
- * You should have received a copy of the GNU General Public License
- * along with this program; if not, write to the Free Software
- * Foundation, Inc., 59 Temple Place, Suite 330, Boston, MA  02111-1307  USA
  */
 #include <linux/kernel.h>
 #include <linux/export.h>
@@ -41,7 +29,7 @@ bool llist_add_batch(struct llist_node *new_first, struct llist_node *new_last,
 	struct llist_node *first;
 
 	do {
-		new_last->next = first = ACCESS_ONCE(head->first);
+		new_last->next = first = READ_ONCE(head->first);
 	} while (cmpxchg(&head->first, first, new_first) != first);
 
 	return !first;