linux-kernel - [PATCH v2 01/26] rculist: introduce list_next_or_null_rr

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Message-Id: <20180518130413.16997-2-roman.penyaev@profitbricks.com>
Date:   Fri, 18 May 2018 15:03:48 +0200
From:   Roman Pen <roman.penyaev@...fitbricks.com>
To:     linux-block@...r.kernel.org, linux-rdma@...r.kernel.org
Cc:     Jens Axboe <axboe@...nel.dk>,
        Christoph Hellwig <hch@...radead.org>,
        Sagi Grimberg <sagi@...mberg.me>,
        Bart Van Assche <bart.vanassche@...disk.com>,
        Or Gerlitz <ogerlitz@...lanox.com>,
        Doug Ledford <dledford@...hat.com>,
        Swapnil Ingle <swapnil.ingle@...fitbricks.com>,
        Danil Kipnis <danil.kipnis@...fitbricks.com>,
        Jack Wang <jinpu.wang@...fitbricks.com>,
        Roman Pen <roman.penyaev@...fitbricks.com>,
        "Paul E . McKenney" <paulmck@...ux.vnet.ibm.com>,
        linux-kernel@...r.kernel.org
Subject: [PATCH v2 01/26] rculist: introduce list_next_or_null_rr_rcu()

Function is going to be used in transport over RDMA module
in subsequent patches.

Function returns next element in round-robin fashion,
i.e. head will be skipped.  NULL will be returned if list
is observed as empty.

Signed-off-by: Roman Pen <roman.penyaev@...fitbricks.com>
Cc: Paul E. McKenney <paulmck@...ux.vnet.ibm.com>
Cc: linux-kernel@...r.kernel.org
---
 include/linux/rculist.h | 19 +++++++++++++++++++
 1 file changed, 19 insertions(+)

diff --git a/include/linux/rculist.h b/include/linux/rculist.h
index 127f534fec94..b0840d5ab25a 100644
--- a/include/linux/rculist.h
+++ b/include/linux/rculist.h
@@ -339,6 +339,25 @@ static inline void list_splice_tail_init_rcu(struct list_head *list,
 })
 
 /**
+ * list_next_or_null_rr_rcu - get next list element in round-robin fashion.
+ * @head:	the head for the list.
+ * @ptr:        the list head to take the next element from.
+ * @type:       the type of the struct this is embedded in.
+ * @memb:       the name of the list_head within the struct.
+ *
+ * Next element returned in round-robin fashion, i.e. head will be skipped,
+ * but if list is observed as empty, NULL will be returned.
+ *
+ * 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_next_or_null_rr_rcu(head, ptr, type, memb) \
+({ \
+	list_next_or_null_rcu(head, ptr, type, memb) ?: \
+		list_next_or_null_rcu(head, READ_ONCE((ptr)->next), type, memb); \
+})
+
+/**
  * list_for_each_entry_rcu	-	iterate over rcu list of given type
  * @pos:	the type * to use as a loop cursor.
  * @head:	the head for your list.
-- 
2.13.1