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

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Message-ID: <20250314093714.GW5880@noisy.programming.kicks-ass.net>
Date: Fri, 14 Mar 2025 10:37:14 +0100
From: Peter Zijlstra <peterz@...radead.org>
To: Thomas Gleixner <tglx@...utronix.de>
Cc: LKML <linux-kernel@...r.kernel.org>, Marc Zyngier <maz@...nel.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.

So no objection per se, but one way to solve this is by handing
ownership to bar_create(), such that it is responsible for freeing f on
failure.

Anyway, I suspect the __must_check came from Linus, OTOH take_fd(), the
equivalent for file descriptors	also don't have that __must_check. So
clearly we have precedent here.