From: Joel Fernandes <joelaf@google.com>
To: linux-kernel@vger.kernel.org
Cc: Joel Fernandes <joelaf@google.com>,
Huang Ying <ying.huang@intel.com>, Ingo Molnar <mingo@kernel.org>,
Will Deacon <will.deacon@arm.com>,
Paul McKenney <paulmck@linux.vnet.ibm.com>
Subject: [RFC] llist: Fix code comments about llist_del_first locking
Date: Thu, 8 Dec 2016 13:54:40 -0800 [thread overview]
Message-ID: <1481234081-61472-1-git-send-email-joelaf@google.com> (raw)
Usage llist_del_first needs lock protection, however the table in the
comments of llist.h show a '-'. Correct this, and also add better
comments on top.
Cc: Huang Ying <ying.huang@intel.com>
Cc: Ingo Molnar <mingo@kernel.org>
Cc: Will Deacon <will.deacon@arm.com>
Cc: Paul McKenney <paulmck@linux.vnet.ibm.com>
Signed-off-by: Joel Fernandes <joelaf@google.com>
---
include/linux/llist.h | 19 ++++++++++---------
1 file changed, 10 insertions(+), 9 deletions(-)
diff --git a/include/linux/llist.h b/include/linux/llist.h
index fd4ca0b..15e4949 100644
--- a/include/linux/llist.h
+++ b/include/linux/llist.h
@@ -3,14 +3,15 @@
/*
* 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 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 will need to use a lock
+ * with any other operation (ABA problem). This is because llist_del_first
+ * depends on list->first->next not changing but there's no way to be sure
+ * about that and the cmpxchg in llist_del_first may succeed if list->first is
+ * the same after concurrent operations. For example, a llist_del_first,
+ * llist_add, llist_add (or llist_del_all, llist_add, llist_add) sequence in
+ * another consumer may cause violations.
*
* 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
@@ -19,7 +20,7 @@
* This can be summarized as follow:
*
* | add | del_first | del_all
- * add | - | - | -
+ * add | - | L | -
* del_first | | L | L
* del_all | | | -
*
--
2.8.0.rc3.226.g39d4020
next reply other threads:[~2016-12-08 22:03 UTC|newest]
Thread overview: 7+ messages / expand[flat|nested] mbox.gz Atom feed top
2016-12-08 21:54 Joel Fernandes [this message]
2016-12-09 0:35 ` [RFC] llist: Fix code comments about llist_del_first locking Huang, Ying
2016-12-09 0:42 ` Joel Fernandes
2016-12-09 0:43 ` Joel Fernandes
2016-12-09 2:12 ` Huang, Ying
2016-12-09 2:22 ` Joel Fernandes
2016-12-09 2:26 ` Huang, Ying
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