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

[Pavel Begunkov <asml.silence@gmail.com>]

On 9/27/21 1:36 PM, Hao Xu wrote:
> 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.

By the time req->io_task_work.func() is called, node->next is undefined
and free to use. io_req_task_work_add() will override it without looking
at a prior value and that's fine, as the calling code doesn't touch it
after the callback.

> 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.

I might be missing what is the problem you're trying to fix. Does the
above makes sense? It doesn't sound like node->next=NULL can solve
anything.

> 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);
> 

-- 
Pavel Begunkov