linux-kernel - Re: [patch V2 01/10] cleanup: Provide retain

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Message-ID: <Z9Q3eCjNfD0WQ4H5@lizhi-Precision-Tower-5810>
Date: Fri, 14 Mar 2025 10:04:40 -0400
From: Frank Li <Frank.li@....com>
To: Thomas Gleixner <tglx@...utronix.de>
Cc: LKML <linux-kernel@...r.kernel.org>, Marc Zyngier <maz@...nel.org>,
	Peter Zijlstra <peterz@...radead.org>, Nishanth Menon <nm@...com>,
	Jonathan Cameron <Jonathan.Cameron@...wei.com>,
	Dhruva Gole <d-gole@...com>, Tero Kristo <kristo@...nel.org>,
	Santosh Shilimkar <ssantosh@...nel.org>,
	Logan Gunthorpe <logang@...tatee.com>,
	Dave Jiang <dave.jiang@...el.com>, Jon Mason <jdmason@...zu.us>,
	Allen Hubbe <allenbh@...il.com>, ntb@...ts.linux.dev,
	Bjorn Helgaas <bhelgaas@...gle.com>, linux-pci@...r.kernel.org,
	Michael Kelley <mhklinux@...look.com>, Wei Liu <wei.liu@...nel.org>,
	Haiyang Zhang <haiyangz@...rosoft.com>,
	linux-hyperv@...r.kernel.org, Wei Huang <wei.huang2@....com>,
	Manivannan Sadhasivam <manivannan.sadhasivam@...aro.org>,
	"James E.J. Bottomley" <James.Bottomley@...senpartnership.com>,
	"Martin K. Petersen" <martin.petersen@...cle.com>,
	linux-scsi@...r.kernel.org,
	Jonathan Cameron <Jonathan.Cameron@...ei.com>
Subject: Re: [patch V2 01/10] cleanup: Provide retain_ptr()

On Thu, Mar 13, 2025 at 02:03:38PM +0100, Thomas Gleixner wrote:
> In cases where an allocation is consumed by another function, the
> allocation needs to be retained on success or freed on failure. The code
> pattern is usually:
>
> 	struct foo *f = kzalloc(sizeof(*f), GFP_KERNEL);
> 	struct bar *b;
>
> 	,,,
> 	// Initialize f
> 	...
> 	if (ret)
> 		goto free;
>         ...
> 	bar = bar_create(f);
> 	if (!bar) {
> 		ret = -ENOMEM;
> 	   	goto free;
> 	}
> 	...
> 	return 0;
> free:
> 	kfree(f);
> 	return ret;
>
> This prevents using __free(kfree) on @f because there is no canonical way
> to tell the cleanup code that the allocation should not be freed.
>
> Abusing no_free_ptr() by force ignoring the return value is not really a
> sensible option either.
>
> Provide an explicit macro retain_ptr(), which NULLs the cleanup
> pointer. That makes it easy to analyze and reason about.
>
> Signed-off-by: Thomas Gleixner <tglx@...utronix.de>
> Cc: Peter Zijlstra <peterz@...radead.org>
> ---
>  include/linux/cleanup.h |   17 +++++++++++++++++
>  1 file changed, 17 insertions(+)
>
> --- a/include/linux/cleanup.h
> +++ b/include/linux/cleanup.h
> @@ -216,6 +216,23 @@ const volatile void * __must_check_fn(co
>
>  #define return_ptr(p)	return no_free_ptr(p)
>
> +/*
> + * Only for situations where an allocation is handed in to another function
> + * and consumed by that function on success.
> + *
> + *	struct foo *f __free(kfree) = kzalloc(sizeof(*f), GFP_KERNEL);
> + *
> + *	setup(f);
> + *	if (some_condition)
> + *		return -EINVAL;
> + *	....
> + *	ret = bar(f);
> + *	if (!ret)
> + *		retain_ptr(f);
> + *	return ret;

Is it better like

	ret = bar(f);
	if (ret)
		return ret;

	retain_ptr(f);
	return 0;

If there are more than one f, like f1, f2, f3....

	ret= bar(f1, f2, ....)
	if (ret)
		return ret;

	retain_ptr(f1);
	retain_ptr(f2);
	...

	return 0;


Or define a macro like
#defne no_free_ptr_on_ok(ret, p) ret ? ret : __get_and_null(p, NULL), 0

	ret = bar (f);
	return no_free_ptr_on_ok(ret, f);

Frank

> + */
> +#define retain_ptr(p)				\
> +	__get_and_null(p, NULL)
>
>  /*
>   * DEFINE_CLASS(name, type, exit, init, init_args...):
>