[Xen-devel] [PATCH v3 11/39] Add list_sort() routine from Linux

[Andre Przywara <andre.przywara@xxxxxxxxxx>]

This pulls in Linux' list_sort.c, which is a merge sort implementation
for linked lists. Apart from adding a full featured license header and
adjusting the #include file, nothing has been changed in this code.
Define a promptless Kconfig which configurations can select when they
need this code and add it to the Makefile.

This is from Linux' lib/list_sort.c, as of commit e327fd7c8667
("lib: add module support to linked list sorting tests").

Signed-off-by: Andre Przywara <andre.przywara@xxxxxxxxxx>
---
Changelog v2 ... v3:
- introduce promptless Kconfig
- add Makefile line
- note Linux commit ID

 xen/common/Kconfig          |   3 +
 xen/common/Makefile         |   1 +
 xen/common/list_sort.c      | 157 ++++++++++++++++++++++++++++++++++++++++++++
 xen/include/xen/list_sort.h |  11 ++++
 4 files changed, 172 insertions(+)
 create mode 100644 xen/common/list_sort.c
 create mode 100644 xen/include/xen/list_sort.h

diff --git a/xen/common/Kconfig b/xen/common/Kconfig
index 68abf7a5e5..986f6c4149 100644
--- a/xen/common/Kconfig
+++ b/xen/common/Kconfig
@@ -44,6 +44,9 @@ config HAS_GDBSX
 config HAS_IOPORTS
        bool
 
+config NEEDS_LIST_SORT
+        bool
+
 config HAS_BUILD_ID
        string
        option env="XEN_HAS_BUILD_ID"
diff --git a/xen/common/Makefile b/xen/common/Makefile
index 3a349f478b..24d4752ccc 100644
--- a/xen/common/Makefile
+++ b/xen/common/Makefile
@@ -19,6 +19,7 @@ obj-y += keyhandler.o
 obj-$(CONFIG_KEXEC) += kexec.o
 obj-$(CONFIG_KEXEC) += kimage.o
 obj-y += lib.o
+obj-$(CONFIG_NEEDS_LIST_SORT) += list_sort.o
 obj-$(CONFIG_LIVEPATCH) += livepatch.o livepatch_elf.o
 obj-y += lzo.o
 obj-$(CONFIG_HAS_MEM_ACCESS) += mem_access.o
diff --git a/xen/common/list_sort.c b/xen/common/list_sort.c
new file mode 100644
index 0000000000..af2b2f6519
--- /dev/null
+++ b/xen/common/list_sort.c
@@ -0,0 +1,157 @@
+/*
+ * list_sort.c: merge sort implementation for linked lists
+ * Copied from the Linux kernel (lib/list_sort.c)
+ *
+ * This program is free software; you can redistribute it and/or modify it
+ * under the terms and conditions of the GNU General Public License,
+ * version 2, as published by the Free Software Foundation.
+ *
+ * This program is distributed in the hope 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, see <http://www.gnu.org/licenses/>.
+ */
+
+#include <xen/lib.h>
+#include <xen/list.h>
+
+#define MAX_LIST_LENGTH_BITS 20
+
+/*
+ * Returns a list organized in an intermediate format suited
+ * to chaining of merge() calls: null-terminated, no reserved or
+ * sentinel head node, "prev" links not maintained.
+ */
+static struct list_head *merge(void *priv,
+                               int (*cmp)(void *priv, struct list_head *a,
+                                       struct list_head *b),
+                               struct list_head *a, struct list_head *b)
+{
+       struct list_head head, *tail = &head;
+
+       while (a && b) {
+               /* if equal, take 'a' -- important for sort stability */
+               if ((*cmp)(priv, a, b) <= 0) {
+                       tail->next = a;
+                       a = a->next;
+               } else {
+                       tail->next = b;
+                       b = b->next;
+               }
+               tail = tail->next;
+       }
+       tail->next = a?:b;
+       return head.next;
+}
+
+/*
+ * Combine final list merge with restoration of standard doubly-linked
+ * list structure.  This approach duplicates code from merge(), but
+ * runs faster than the tidier alternatives of either a separate final
+ * prev-link restoration pass, or maintaining the prev links
+ * throughout.
+ */
+static void merge_and_restore_back_links(void *priv,
+                               int (*cmp)(void *priv, struct list_head *a,
+                                       struct list_head *b),
+                               struct list_head *head,
+                               struct list_head *a, struct list_head *b)
+{
+       struct list_head *tail = head;
+       u8 count = 0;
+
+       while (a && b) {
+               /* if equal, take 'a' -- important for sort stability */
+               if ((*cmp)(priv, a, b) <= 0) {
+                       tail->next = a;
+                       a->prev = tail;
+                       a = a->next;
+               } else {
+                       tail->next = b;
+                       b->prev = tail;
+                       b = b->next;
+               }
+               tail = tail->next;
+       }
+       tail->next = a ? : b;
+
+       do {
+               /*
+                * In worst cases this loop may run many iterations.
+                * Continue callbacks to the client even though no
+                * element comparison is needed, so the client's cmp()
+                * routine can invoke cond_resched() periodically.
+                */
+               if (unlikely(!(++count)))
+                       (*cmp)(priv, tail->next, tail->next);
+
+               tail->next->prev = tail;
+               tail = tail->next;
+       } while (tail->next);
+
+       tail->next = head;
+       head->prev = tail;
+}
+
+/**
+ * list_sort - sort a list
+ * @priv: private data, opaque to list_sort(), passed to @cmp
+ * @head: the list to sort
+ * @cmp: the elements comparison function
+ *
+ * This function implements "merge sort", which has O(nlog(n))
+ * complexity.
+ *
+ * The comparison function @cmp must return a negative value if @a
+ * should sort before @b, and a positive value if @a should sort after
+ * @b. If @a and @b are equivalent, and their original relative
+ * ordering is to be preserved, @cmp must return 0.
+ */
+void list_sort(void *priv, struct list_head *head,
+               int (*cmp)(void *priv, struct list_head *a,
+                       struct list_head *b))
+{
+       struct list_head *part[MAX_LIST_LENGTH_BITS+1]; /* sorted partial lists
+                                               -- last slot is a sentinel */
+       int lev;  /* index into part[] */
+       int max_lev = 0;
+       struct list_head *list;
+
+       if (list_empty(head))
+               return;
+
+       memset(part, 0, sizeof(part));
+
+       head->prev->next = NULL;
+       list = head->next;
+
+       while (list) {
+               struct list_head *cur = list;
+               list = list->next;
+               cur->next = NULL;
+
+               for (lev = 0; part[lev]; lev++) {
+                       cur = merge(priv, cmp, part[lev], cur);
+                       part[lev] = NULL;
+               }
+               if (lev > max_lev) {
+                       if (unlikely(lev >= ARRAY_SIZE(part)-1)) {
+                               dprintk(XENLOG_DEBUG,
+                                       "list too long for efficiency\n");
+                               lev--;
+                       }
+                       max_lev = lev;
+               }
+               part[lev] = cur;
+       }
+
+       for (lev = 0; lev < max_lev; lev++)
+               if (part[lev])
+                       list = merge(priv, cmp, part[lev], list);
+
+       merge_and_restore_back_links(priv, cmp, head, part[max_lev], list);
+}
+EXPORT_SYMBOL(list_sort);
diff --git a/xen/include/xen/list_sort.h b/xen/include/xen/list_sort.h
new file mode 100644
index 0000000000..13ce0a55ec
--- /dev/null
+++ b/xen/include/xen/list_sort.h
@@ -0,0 +1,11 @@
+#ifndef _LINUX_LIST_SORT_H
+#define _LINUX_LIST_SORT_H
+
+#include <xen/types.h>
+
+struct list_head;
+
+void list_sort(void *priv, struct list_head *head,
+              int (*cmp)(void *priv, struct list_head *a,
+                         struct list_head *b));
+#endif
-- 
2.14.1


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