Re: [PATCH 3/4] io-wq: fix worker->refcount when creating worker in work exit

[Jens Axboe <axboe@kernel.dk>]

On 9/11/21 1:40 PM, Hao Xu wrote:
> We may enter the worker creation path from io_worker_exit(), and
> refcount is already zero, which causes definite failure of worker
> creation.
> io_worker_exit
>                               ref = 0
> ->io_wqe_dec_running
>   ->io_queue_worker_create
>     ->io_worker_get           failure since ref is 0
> 
> Signed-off-by: Hao Xu <haoxu@linux.alibaba.com>
> ---
>  fs/io-wq.c | 2 ++
>  1 file changed, 2 insertions(+)
> 
> diff --git a/fs/io-wq.c b/fs/io-wq.c
> index 0e1288a549eb..75e79571bdfd 100644
> --- a/fs/io-wq.c
> +++ b/fs/io-wq.c
> @@ -188,7 +188,9 @@ static void io_worker_exit(struct io_worker *worker)
>  	list_del_rcu(&worker->all_list);
>  	acct->nr_workers--;
>  	preempt_disable();
> +	refcount_set(&worker->ref, 1);
>  	io_wqe_dec_running(worker);
> +	refcount_set(&worker->ref, 0);

That kind of refcount manipulation is highly suspect. And in fact it
should not be needed, io_worker_exit() clears ->flags before going on
with worker teardown. Hence you can't hit worker creation from
io_wqe_dec_running().

-- 
Jens Axboe