padata: use integer wrap around to prevent deadlock on seq_nr overflow

When submitting more than 2^32 padata objects to padata_do_serial, the
current sorting implementation incorrectly sorts padata objects with
overflowed seq_nr, causing them to be placed before existing objects in
the reorder list. This leads to a deadlock in the serialization process
as padata_find_next cannot match padata->seq_nr and pd->processed
because the padata instance with overflowed seq_nr will be selected
next.

To fix this, we use an unsigned integer wrap around to correctly sort
padata objects in scenarios with integer overflow.

Fixes: bfde23ce ("padata: unbind parallel jobs from specific CPUs")
Cc: <stable@vger.kernel.org>
Co-developed-by: Christian Gafert <christian.gafert@rohde-schwarz.com>
Signed-off-by: Christian Gafert <christian.gafert@rohde-schwarz.com>
Co-developed-by: Max Ferger <max.ferger@rohde-schwarz.com>
Signed-off-by: Max Ferger <max.ferger@rohde-schwarz.com>
Signed-off-by: Van Giang Nguyen <vangiang.nguyen@rohde-schwarz.com>
Acked-by: Daniel Jordan <daniel.m.jordan@oracle.com>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>

kernel/padata.c

@@ -404,7 +404,8 @@ void padata_do_serial(struct padata_priv *padata)
 	/* Sort in ascending order of sequence number. */
 	list_for_each_prev(pos, &reorder->list) {
 		cur = list_entry(pos, struct padata_priv, list);
-		if (cur->seq_nr < padata->seq_nr)
+		/* Compare by difference to consider integer wrap around */
+		if ((signed int)(cur->seq_nr - padata->seq_nr) < 0)
 			break;
 	}
 	list_add(&padata->list, pos);