linux-kernel - Re: [PATCH v2 2/2] io_uring: Improve exception handling in io_ring_ctx

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Date: Thu, 11 Jan 2024 13:23:31 +0000
From: Pavel Begunkov <asml.silence@...il.com>
To: Markus Elfring <Markus.Elfring@....de>, io-uring@...r.kernel.org,
 kernel-janitors@...r.kernel.org, Gabriel Krisman Bertazi <krisman@...e.de>,
 Jens Axboe <axboe@...nel.dk>
Cc: LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v2 2/2] io_uring: Improve exception handling in
 io_ring_ctx_alloc()

On 1/10/24 20:50, Markus Elfring wrote:
> From: Markus Elfring <elfring@...rs.sourceforge.net>
> Date: Wed, 10 Jan 2024 21:15:48 +0100
> 
> The label “err” was used to jump to a kfree() call despite of
> the detail in the implementation of the function “io_ring_ctx_alloc”
> that it was determined already that a corresponding variable contained
> a null pointer because of a failed memory allocation.

It's _much_ simpler the way it currently is, compare it with maintaining
a bunch of labels. That is the advantage of being able to distinguish
un-allocated state like NULL, just kfree them and don't care about
jumping to a wrong one or keeping them in order.

  
> 1. Thus use more appropriate labels instead.
> 
> 2. Reorder jump targets at the end.
> 
> Signed-off-by: Markus Elfring <elfring@...rs.sourceforge.net>
> ---
> 
> See also:
> https://wiki.sei.cmu.edu/confluence/display/c/MEM12-C.+Consider+using+a+goto+chain+when+leaving+a+function+on+error+when+using+and+releasing+resources
> 
> 
>   io_uring/io_uring.c | 15 ++++++++++-----
>   1 file changed, 10 insertions(+), 5 deletions(-)
> 
> diff --git a/io_uring/io_uring.c b/io_uring/io_uring.c
> index c9a63c39cdd0..7727cdd505ae 100644
> --- a/io_uring/io_uring.c
> +++ b/io_uring/io_uring.c
> @@ -295,12 +295,14 @@ static __cold struct io_ring_ctx *io_ring_ctx_alloc(struct io_uring_params *p)
>   	hash_bits = ilog2(p->cq_entries) - 5;
>   	hash_bits = clamp(hash_bits, 1, 8);
>   	if (io_alloc_hash_table(&ctx->cancel_table, hash_bits))
> -		goto err;
> +		goto destroy_io_bl_xa;
> +
>   	if (io_alloc_hash_table(&ctx->cancel_table_locked, hash_bits))
> -		goto err;
> +		goto free_cancel_table_hbs;
> +
>   	if (percpu_ref_init(&ctx->refs, io_ring_ctx_ref_free,
>   			    0, GFP_KERNEL))
> -		goto err;
> +		goto free_cancel_table_locked_hbs;
> 
>   	ctx->flags = p->flags;
>   	init_waitqueue_head(&ctx->sqo_sq_wait);
> @@ -341,9 +343,12 @@ static __cold struct io_ring_ctx *io_ring_ctx_alloc(struct io_uring_params *p)
>   	INIT_WQ_LIST(&ctx->submit_state.compl_reqs);
>   	INIT_HLIST_HEAD(&ctx->cancelable_uring_cmd);
>   	return ctx;
> -err:
> -	kfree(ctx->cancel_table.hbs);
> +
> +free_cancel_table_locked_hbs:
>   	kfree(ctx->cancel_table_locked.hbs);
> +free_cancel_table_hbs:
> +	kfree(ctx->cancel_table.hbs);
> +destroy_io_bl_xa:
>   	xa_destroy(&ctx->io_bl_xa);
>   	kfree(ctx);
>   	return NULL;
> --
> 2.43.0
> 

-- 
Pavel Begunkov