62 lines
2.5 KiB
Diff
62 lines
2.5 KiB
Diff
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From 0000000000000000000000000000000000000000 Mon Sep 17 00:00:00 2001
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From: Martin Wilck <mwilck@suse.com>
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Date: Tue, 22 Aug 2023 22:30:16 +0200
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Subject: [PATCH] libmultipath: lookup_binding: add comment about the algorithm
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When I read this code, I always get confused. Adding comments to
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explain the algorithm.
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Signed-off-by: Martin Wilck <mwilck@suse.com>
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Reviewed-by: Benjamin Marzinski <bmarzins@redhat.com>
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Signed-off-by: Benjamin Marzinski <bmarzins@redhat.com>
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---
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libmultipath/alias.c | 35 +++++++++++++++++++++++++++++++++++
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1 file changed, 35 insertions(+)
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diff --git a/libmultipath/alias.c b/libmultipath/alias.c
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index 3e3dfe98..9e9ac563 100644
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--- a/libmultipath/alias.c
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+++ b/libmultipath/alias.c
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@@ -172,6 +172,41 @@ lookup_binding(FILE *f, const char *map_wwid, char **map_alias,
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alias = strtok_r(buf, " \t", &saveptr);
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if (!alias) /* blank line */
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continue;
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+
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+ /*
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+ * Find an unused index - explanation of the algorithm
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+ *
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+ * ID: 1 = mpatha, 2 = mpathb, ...
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+ *
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+ * We assume the bindings are unsorted. The only constraint
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+ * is that no ID occurs more than once. IDs that occur in the
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+ * bindings are called "used".
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+ *
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+ * We call the list 1,2,3,..., exactly in this order, the list
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+ * of "expected" IDs. The variable "id" always holds the next
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+ * "expected" ID, IOW the last "expected" ID encountered plus 1.
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+ * Thus all IDs below "id" are known to be used. However, at the
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+ * end of the loop, the value of "id" isn't necessarily unused.
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+ *
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+ * "smallest_bigger_id" is the smallest used ID that was
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+ * encountered while it was larger than the next "expected" ID
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+ * at that iteration. Let X be some used ID. If all IDs below X
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+ * are used and encountered in the right sequence before X, "id"
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+ * will be > X when the loop ends. Otherwise, X was encountered
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+ * "out of order", the condition (X > id) holds when X is
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+ * encountered, and "smallest_bigger_id" will be set to X; i.e.
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+ * it will be less or equal than X when the loop ends.
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+ *
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+ * At the end of the loop, (id < smallest_bigger_id) means that
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+ * the value of "id" had been encountered neither in order nor
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+ * out of order, and is thus unused. (id >= smallest_bigger_id)
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+ * means that "id"'s value is in use. In this case, we play safe
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+ * and use "biggest_id + 1" as the next value to try.
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+ *
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+ * biggest_id is always > smallest_bigger_id, except in the
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+ * "perfectly ordered" case.
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+ */
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+
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curr_id = scan_devname(alias, prefix);
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if (curr_id == id) {
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if (id < INT_MAX)
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