Re: [PATCH] io_uring: kill dead code in io_req_complete_post

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

On 3/29/24 15:47, Ming Lei wrote:
> Since commit 8f6c829491fe ("io_uring: remove struct io_tw_state::locked"),
> io_req_complete_post() is only called from io-wq submit work, where the
> request reference is guaranteed to be grabbed and won't drop to zero
> in io_req_complete_post().
> 
> Kill the dead code, meantime add req_ref_put() to put the reference.

Interesting... a nice clean up. The assumption is too implicit to
my taste, but should be just fine if we add

if (WARN_ON_ONCE(!(issue_flags & IO_URING_F_IOWQ)))
	return;

at the beginning of io_req_complete_post(), it's a slow path.
And with this change:

Reviewed-by: Pavel Begunkov <asml.silence@gmail.com>


> Cc: Pavel Begunkov <asml.silence@gmail.com>
> Signed-off-by: Ming Lei <ming.lei@redhat.com>
> ---
>   io_uring/io_uring.c | 37 ++-----------------------------------
>   io_uring/refs.h     |  7 +++++++
>   2 files changed, 9 insertions(+), 35 deletions(-)
> 
> diff --git a/io_uring/io_uring.c b/io_uring/io_uring.c
> index 104899522bc5..ac2e5da4558a 100644
> --- a/io_uring/io_uring.c
> +++ b/io_uring/io_uring.c
> @@ -929,7 +929,6 @@ bool io_req_post_cqe(struct io_kiocb *req, s32 res, u32 cflags)
>   static void io_req_complete_post(struct io_kiocb *req, unsigned issue_flags)
>   {
>   	struct io_ring_ctx *ctx = req->ctx;
> -	struct io_rsrc_node *rsrc_node = NULL;
>   
>   	/*
>   	 * Handle special CQ sync cases via task_work. DEFER_TASKRUN requires
> @@ -946,42 +945,10 @@ static void io_req_complete_post(struct io_kiocb *req, unsigned issue_flags)
>   		if (!io_fill_cqe_req(ctx, req))
>   			io_req_cqe_overflow(req);
>   	}
> -
> -	/*
> -	 * If we're the last reference to this request, add to our locked
> -	 * free_list cache.
> -	 */
> -	if (req_ref_put_and_test(req)) {
> -		if (req->flags & IO_REQ_LINK_FLAGS) {
> -			if (req->flags & IO_DISARM_MASK)
> -				io_disarm_next(req);
> -			if (req->link) {
> -				io_req_task_queue(req->link);
> -				req->link = NULL;
> -			}
> -		}
> -		io_put_kbuf_comp(req);
> -		if (unlikely(req->flags & IO_REQ_CLEAN_FLAGS))
> -			io_clean_op(req);
> -		io_put_file(req);
> -
> -		rsrc_node = req->rsrc_node;
> -		/*
> -		 * Selected buffer deallocation in io_clean_op() assumes that
> -		 * we don't hold ->completion_lock. Clean them here to avoid
> -		 * deadlocks.
> -		 */
> -		io_put_task_remote(req->task);
> -		wq_list_add_head(&req->comp_list, &ctx->locked_free_list);
> -		ctx->locked_free_nr++;
> -	}
>   	io_cq_unlock_post(ctx);
>   
> -	if (rsrc_node) {
> -		io_ring_submit_lock(ctx, issue_flags);
> -		io_put_rsrc_node(ctx, rsrc_node);
> -		io_ring_submit_unlock(ctx, issue_flags);
> -	}
> +	/* called from io-wq submit work only, the ref won't drop to zero */
> +	req_ref_put(req);
>   }
>   
>   void io_req_defer_failed(struct io_kiocb *req, s32 res)
> diff --git a/io_uring/refs.h b/io_uring/refs.h
> index 1336de3f2a30..63982ead9f7d 100644
> --- a/io_uring/refs.h
> +++ b/io_uring/refs.h
> @@ -33,6 +33,13 @@ static inline void req_ref_get(struct io_kiocb *req)
>   	atomic_inc(&req->refs);
>   }
>   
> +static inline void req_ref_put(struct io_kiocb *req)
> +{
> +	WARN_ON_ONCE(!(req->flags & REQ_F_REFCOUNT));
> +	WARN_ON_ONCE(req_ref_zero_or_close_to_overflow(req));
> +	atomic_dec(&req->refs);
> +}
> +
>   static inline void __io_req_set_refcount(struct io_kiocb *req, int nr)
>   {
>   	if (!(req->flags & REQ_F_REFCOUNT)) {

-- 
Pavel Begunkov