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