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From:   Christoph Hellwig <hch@lst.de>
To:     "Darrick J . Wong" <darrick.wong@oracle.com>
Cc:     Damien Le Moal <Damien.LeMoal@wdc.com>,
        Andreas Gruenbacher <agruenba@redhat.com>,
        linux-xfs@vger.kernel.org, linux-fsdevel@vger.kernel.org,
        linux-kernel@vger.kernel.org
Subject: [PATCH 01/13] list.h: add list_pop and list_pop_entry helpers
Date:   Thu, 27 Jun 2019 12:48:24 +0200
Message-Id: <20190627104836.25446-2-hch@lst.de>
In-Reply-To: <20190627104836.25446-1-hch@lst.de>
References: <20190627104836.25446-1-hch@lst.de>
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We have a very common pattern where we want to delete the first entry
from a list and return it as the properly typed container structure.

Add two helpers to implement this behavior.

Signed-off-by: Christoph Hellwig <hch@lst.de>
---
 include/linux/list.h | 33 +++++++++++++++++++++++++++++++++
 1 file changed, 33 insertions(+)

diff --git a/include/linux/list.h b/include/linux/list.h
index e951228db4b2..ba6e27d2235a 100644
--- a/include/linux/list.h
+++ b/include/linux/list.h
@@ -500,6 +500,39 @@ static inline void list_splice_tail_init(struct list_head *list,
 	pos__ != head__ ? list_entry(pos__, type, member) : NULL; \
 })
 
+/**
+ * list_pop - delete the first entry from a list and return it
+ * @list:	the list to take the element from.
+ *
+ * Return the list entry after @list.  If @list is empty return NULL.
+ */
+static inline struct list_head *list_pop(struct list_head *list)
+{
+	struct list_head *pos = READ_ONCE(list->next);
+
+	if (pos == list)
+		return NULL;
+	list_del(pos);
+	return pos;
+}
+
+/**
+ * list_pop_entry - delete the first entry from a list and return the
+ *			containing structure
+ * @list:	the list to take the element from.
+ * @type:	the type of the struct this is embedded in.
+ * @member:	the name of the list_head within the struct.
+ *
+ * Return the containing structure for the list entry after @list.  If @list
+ * is empty return NULL.
+ */
+#define list_pop_entry(list, type, member)			\
+({								\
+	struct list_head *pos__ = list_pop(list);		\
+								\
+	pos__ ? list_entry(pos__, type, member) : NULL;		\
+})
+
 /**
  * list_next_entry - get the next element in list
  * @pos:	the type * to cursor
-- 
2.20.1