From 43c91b1522435429a63d175f5a953e6f727a4f88 Mon Sep 17 00:00:00 2001 From: Adam Harshbarger Date: Thu, 3 Sep 2026 17:24:56 -0500 Subject: [PATCH] lib/plist: fix plist_requeue() corrupting order in the last bucket plist_requeue() is meant to move a node to the end of its own priority run. When the node heads the *last* priority bucket it is instead placed at the head of the whole list, leaving the plist unsorted: built: A(prio 0) B(prio 1) C(prio 1) requeue(B): B(prio 1) A(prio 0) C(prio 1) expected: A(prio 0) C(prio 1) B(prio 1) prio_list is a *headless* circular ring of the nodes that lead each priority bucket. The shortcut added by commit 95d4b3450ebe ("lib/plist.c: add shortcut for plist_requeue()") takes iter = list_entry(iter->prio_list.next, struct plist_node, prio_list); node_next = &iter->node_list; which from the last bucket wraps round to the *first* bucket, so node_next ends up pointing at the head of the list rather than at its end. The plist_for_each_continue() loop immediately below it computes the correct answer (&head->node_list) for that case. With any bucket after it the shortcut is correct, which is why this went unnoticed: the benchmark in that commit measured elapsed time and never checked the resulting order. Keep the shortcut -- it is a real win -- but exclude the case where iter's bucket is the last one, which is exactly when its ring successor is the first bucket again. Reachable from mm/swapfile.c, which rotates swap_avail_heads[] with plist_requeue(). It takes three or more swap devices: at least two distinct priorities, so that a later bucket exists for the ring to wrap round from, and two or more devices sharing the lowest priority, so that plist_requeue() does not return early. One device per priority returns early at the node->prio != iter->prio test. A single priority is also safe, but for a different reason worth stating: with one bucket no node is ever linked onto prio_list at all -- plist_add() skips it for the first node and for every node whose predecessor shares its priority -- so list_empty(&iter->prio_list) holds and the shortcut is never entered. Tested by driving three implementations -- the pre-95d4b3450ebe code, current mainline, and this patch -- through 1,084,492 identical random add/del/requeue operations over 24 nodes and 1..5 distinct priorities, comparing the resulting node_list node for node after every operation: variant differs from pre-95d4b3450ebe left list unsorted pre-95d4b3450ebe -- (reference) 0 mainline 289,297 276,660 this patch 0 0 Link: https://lore.kernel.org/20260903222456.1881786-1-handyhandyman.adam@gmail.com Fixes: 95d4b3450ebe ("lib/plist.c: add shortcut for plist_requeue()") Signed-off-by: Adam Harshbarger Signed-off-by: Andrew Morton Reviewed-by: Nickolai Zeldovich Assisted-by: Claude:claude-opus-5 Cc: I Hsin Cheng Cc: # v6.15+ --- lib/plist.c | 7 +++++++ 1 file changed, 7 insertions(+) diff --git a/lib/plist.c b/lib/plist.c index a5bef38add43..b0273533c04d 100644 --- a/lib/plist.c +++ b/lib/plist.c @@ -174,8 +174,15 @@ void plist_requeue(struct plist_node *node, struct plist_head *head) /* * After plist_del(), iter is the replacement of the node. If the node * was on prio_list, take shortcut to find node_next instead of looping. + * + * prio_list is a headless ring, so from the LAST bucket ->next wraps + * round to the first one; in that case node_next is the list head. */ if (!list_empty(&iter->prio_list)) { + struct plist_node *first = plist_first(head); + + if (iter->prio_list.next == &first->prio_list) + goto queue; iter = list_entry(iter->prio_list.next, struct plist_node, prio_list); node_next = &iter->node_list;