[PATCH v2] lib/plist: add a plist_test case for plist_requeue() on the last priority bucket

[Nickolai Zeldovich <nickolai@csail.mit.edu>]

The plist_requeue() shortcut inserted the node at the head of the list
when the node was the first of the last priority bucket and had a
successor of the same priority, with an earlier bucket present (fixed
by "lib/plist: fix plist_requeue() corrupting order in the last
bucket").  The CONFIG_DEBUG_PLIST self-test did not catch it: it only
requeues nodes it has just added, which are always the last of their
bucket, so plist_requeue() returned early every time.

Add a deterministic case for the failing shape: A(1) B(2) C(2), requeue
B.  The expected result is A(1) C(2) B(2), with B at the end of the
list; the broken shortcut produced B(2) A(1) C(2).  Check it with a
WARN() rather than a BUG_ON(), and skip plist_test_check() when the
warning fires, since its BUG_ON()s would stop the boot on the misordered
list; the node_list and prio_list links of the result are still verified
by plist_test_check() when the order is right.

Boot-tested on QEMU virt with a riscv64 defconfig kernel plus
CONFIG_DEBUG_PLIST=y, built with LLVM=1.  With the fix applied the test
passes and the boot proceeds.  With the fix reverted the warning fires at
boot and the boot proceeds:

  [    0.864761] start plist test
  [    0.871849] ------------[ cut here ]------------
  [    0.871905] plist_requeue() put the node in front of the list
  [    0.872921] WARNING: lib/plist.c:295 at plist_test+0x33a/0x342, CPU#0: swapper/0/1
  ...
  [    0.885968] ---[ end trace 0000000000000000 ]---
  [    0.887060] end plist test

Assisted-by: LLM
Signed-off-by: Nickolai Zeldovich <nickolai@csail.mit.edu>
---
v2: per Andrew's review of v1,
    https://lore.kernel.org/lkml/20261010211804.2d8a5f6180c74b08e18ad48a@linux-foundation.org/
  - WARN() instead of BUG_ON(), and plist_test_check() is skipped when
    the warning fires, so a broken plist_requeue() no longer stops the
    boot at this test.
  - Boot-tested on QEMU virt, both with the queued fix (test passes)
    and without it (warning fires, boot continues).
  - Applies on top of "lib/plist: fix plist_requeue() corrupting order
    in the last bucket" (Adam Harshbarger).
v1: https://lore.kernel.org/lkml/20261011012245.765558-1-nickolai@csail.mit.edu/

 lib/plist.c | 21 +++++++++++++++++++++
 1 file changed, 21 insertions(+)

diff --git a/lib/plist.c b/lib/plist.c
index b0273533c04d..9591c38e8ad6 100644
--- a/lib/plist.c
+++ b/lib/plist.c
@@ -283,6 +283,27 @@ static int  __init plist_test(void)
 		plist_test_check(nr_expect);
 	}
 
+	/*
+	 * Requeue a node which is the first of the last priority run and has
+	 * a successor of the same priority, with an earlier run present: the
+	 * shortcut in plist_requeue() must append it at the end of the list,
+	 * not in front of the first run.  The random test above never
+	 * exercises this, since it only requeues nodes it has just added,
+	 * which are always the last of their run.  On failure warn and skip
+	 * plist_test_check(), which would BUG_ON() the misordered list.
+	 */
+	plist_head_init(&test_head);
+	for (i = 0; i < 3; i++) {
+		plist_node_init(test_node + i, i ? 2 : 1);
+		plist_add(test_node + i, &test_head);
+	}
+	plist_test_requeue(test_node + 1);
+	if (!WARN(plist_last(&test_head) != test_node + 1,
+		  "plist_requeue() put the node in front of the list\n"))
+		plist_test_check(3);
+	for (i = 0; i < 3; i++)
+		plist_del(test_node + i, &test_head);
+
 	printk(KERN_DEBUG "end plist test\n");
 
 	/* Worst case test for plist_add() */
-- 
2.55.0