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

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Message-ID: <20250313152413.0000581b@huawei.com>
Date: Thu, 13 Mar 2025 15:24:13 +0000
From: Jonathan Cameron <Jonathan.Cameron@...wei.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>, "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, 13 Mar 2025 14:03:38 +0100 (CET)
Thomas Gleixner <tglx@...utronix.de> 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>

Seems sensible to me and the resulting code is reasonably easy to
follow / contained in a small region.

Reviewed-by: Jonathan Cameron <Jonathan.Cameron@...wei.com>

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