[PATCH] workqueue: avoid usage of list iterator after loop in __flush_workqueue()

[Jakob Koschel <jkl820.git@gmail.com>]

If the list_for_each_entry_safe() iteration never breaks, 'next' would
contain an invalid pointer past the iterator loop. To ensure 'next' is
always valid, we only set it if the correct element was found. That
allows adding a WARN_ON_ONCE in case the code works incorrectly,
exposing currently undetectable potential bugs.

Additionally, Linus proposed to avoid any use of the list iterator
variable after the loop, in the attempt to move the list iterator
variable declaration into the macro to avoid any potential misuse after
the loop [1].

Link: https://lore.kernel.org/all/CAHk-=wgRr_D8CB-D9Kg-c=EHreAsk5SqXPwr9Y7k9sA6cWXJ6w@mail.gmail.com/ [1]
Signed-off-by: Jakob Koschel <jkl820.git@gmail.com>
---
 kernel/workqueue.c | 17 ++++++++++-------
 1 file changed, 10 insertions(+), 7 deletions(-)

diff --git a/kernel/workqueue.c b/kernel/workqueue.c
index b8b541caed48..73aff7b42903 100644
--- a/kernel/workqueue.c
+++ b/kernel/workqueue.c
@@ -2982,14 +2982,16 @@ void __flush_workqueue(struct workqueue_struct *wq)
 	WARN_ON_ONCE(wq->flush_color != this_flusher.flush_color);
 
 	while (true) {
-		struct wq_flusher *next, *tmp;
+		struct wq_flusher *next = NULL, *iter, *tmp;
 
 		/* complete all the flushers sharing the current flush color */
-		list_for_each_entry_safe(next, tmp, &wq->flusher_queue, list) {
-			if (next->flush_color != wq->flush_color)
+		list_for_each_entry_safe(iter, tmp, &wq->flusher_queue, list) {
+			if (iter->flush_color != wq->flush_color) {
+				next = iter;
 				break;
-			list_del_init(&next->list);
-			complete(&next->done);
+			}
+			list_del_init(&iter->list);
+			complete(&iter->done);
 		}
 
 		WARN_ON_ONCE(!list_empty(&wq->flusher_overflow) &&
@@ -3006,8 +3008,8 @@ void __flush_workqueue(struct workqueue_struct *wq)
 			 * flusher_queue.  This is the start-to-wait
 			 * phase for these overflowed flushers.
 			 */
-			list_for_each_entry(tmp, &wq->flusher_overflow, list)
-				tmp->flush_color = wq->work_color;
+			list_for_each_entry(iter, &wq->flusher_overflow, list)
+				iter->flush_color = wq->work_color;
 
 			wq->work_color = work_next_color(wq->work_color);
 
@@ -3025,6 +3027,7 @@ void __flush_workqueue(struct workqueue_struct *wq)
 		 * Need to flush more colors.  Make the next flusher
 		 * the new first flusher and arm pwqs.
 		 */
+		WARN_ON_ONCE(!next);
 		WARN_ON_ONCE(wq->flush_color == wq->work_color);
 		WARN_ON_ONCE(wq->flush_color != next->flush_color);
 

---
base-commit: c0927a7a5391f7d8e593e5e50ead7505a23cadf9
change-id: 20230302-workqueue-avoid-iter-after-loop-9d26db48654c

Best regards,
-- 
Jakob Koschel <jkl820.git@gmail.com>