[PATCH 2/2] io_uring: fix tw list mess-up by adding tw while it's already in tw list

[Hao Xu <haoxu@linux.alibaba.com>]

For multishot mode, there may be cases like:
io_poll_task_func()
-> add_wait_queue()
                            async_wake()
                            ->io_req_task_work_add()
                            this one mess up the running task_work list
                            since req->io_task_work.node is in use.

similar situation for req->io_task_work.fallback_node.
Fix it by set node->next = NULL before we run the tw, so that when we
add req back to the wait queue in middle of tw running, we can safely
re-add it to the tw list.

Fixes: 7cbf1722d5fc ("io_uring: provide FIFO ordering for task_work")
Signed-off-by: Hao Xu <haoxu@linux.alibaba.com>
---
 fs/io_uring.c | 11 ++++++++---
 1 file changed, 8 insertions(+), 3 deletions(-)

diff --git a/fs/io_uring.c b/fs/io_uring.c
index d0b358b9b589..f667d6286438 100644
--- a/fs/io_uring.c
+++ b/fs/io_uring.c
@@ -1250,13 +1250,17 @@ static void io_fallback_req_func(struct work_struct *work)
 	struct io_ring_ctx *ctx = container_of(work, struct io_ring_ctx,
 						fallback_work.work);
 	struct llist_node *node = llist_del_all(&ctx->fallback_llist);
-	struct io_kiocb *req, *tmp;
+	struct io_kiocb *req;
 	bool locked = false;
 
 	percpu_ref_get(&ctx->refs);
-	llist_for_each_entry_safe(req, tmp, node, io_task_work.fallback_node)
+	req = llist_entry(node, struct io_kiocb, io_task_work.fallback_node);
+	while (member_address_is_nonnull(req, io_task_work.fallback_node)) {
+		node = req->io_task_work.fallback_node.next;
+		req->io_task_work.fallback_node.next = NULL;
 		req->io_task_work.func(req, &locked);
-
+		req = llist_entry(node, struct io_kiocb, io_task_work.fallback_node);
+	}
 	if (locked) {
 		io_submit_flush_completions(ctx);
 		mutex_unlock(&ctx->uring_lock);
@@ -2156,6 +2160,7 @@ static void tctx_task_work(struct callback_head *cb)
 				locked = mutex_trylock(&ctx->uring_lock);
 				percpu_ref_get(&ctx->refs);
 			}
+			node->next = NULL;
 			req->io_task_work.func(req, &locked);
 			node = next;
 		} while (node);
-- 
2.24.4