[PATCH v2 1/2] srcu: Add list_splice_init_srcu()

[Qiliang Yuan <odys.yuan@gmail.com>]

list_splice_init_rcu() makes a list look empty to new readers, waits for
the current ones with the given sync function, and only then links the
entries into the new list. The sync function takes no argument, so it
can wait for RCU readers but not for the readers of an srcu_struct.

Passing synchronize_rcu() for a list read under SRCU still builds, but
leaves SRCU readers on the entries while the last one's next pointer is
moved onto the new list head. nvme_remove_namespaces() does this today.

Add list_splice_init_srcu(), which waits for the readers of the given
srcu_struct instead.

Cc: stable@vger.kernel.org
Suggested-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Qiliang Yuan <odys.yuan@gmail.com>
---
 include/linux/srcu.h | 36 ++++++++++++++++++++++++++++++++++++
 1 file changed, 36 insertions(+)

diff --git a/include/linux/srcu.h b/include/linux/srcu.h
index 1a8a465a5650d..d4dcaeb83e2b6 100644
--- a/include/linux/srcu.h
+++ b/include/linux/srcu.h
@@ -18,6 +18,7 @@
 
 #include <linux/mutex.h>
 #include <linux/rcupdate.h>
+#include <linux/rculist.h>
 #include <linux/workqueue.h>
 #include <linux/rcu_segcblist.h>
 
@@ -118,6 +119,41 @@ void cleanup_srcu_struct(struct srcu_struct *ssp);
 void synchronize_srcu(struct srcu_struct *ssp);
 void synchronize_srcu_atomic(struct srcu_struct *ssp);
 
+/**
+ * list_splice_init_srcu - splice an SRCU-protected list into an existing list
+ * @list:	the SRCU-protected list to splice
+ * @head:	the place in the existing list to splice the first list into
+ * @ssp:	the srcu_struct that readers of @list use
+ *
+ * Like list_splice_init_rcu(), but wait for the SRCU readers of @list
+ * instead of RCU ones, so that none of them can follow a next pointer
+ * onto @head.
+ *
+ * Note that this function blocks, and that the caller must prevent any
+ * other updates to @list.
+ */
+static inline void list_splice_init_srcu(struct list_head *list,
+					 struct list_head *head,
+					 struct srcu_struct *ssp)
+{
+	struct list_head *first = list->next;
+	struct list_head *last = list->prev;
+	struct list_head *next = head->next;
+
+	if (list_empty(list))
+		return;
+
+	INIT_LIST_HEAD_RCU(list);
+	synchronize_srcu(ssp);
+	ASSERT_EXCLUSIVE_ACCESS(*first);
+	ASSERT_EXCLUSIVE_ACCESS(*last);
+
+	last->next = next;
+	rcu_assign_pointer(list_next_rcu(head), first);
+	first->prev = head;
+	next->prev = last;
+}
+
 #define SRCU_GET_STATE_COMPLETED 0x1
 
 /**

-- 
2.43.0