linux-kernel - Re: [PATCH] vfio-pci: Use fdget() rather than eventfd

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Date:	Tue, 20 Aug 2013 22:18:14 +0100
From:	Al Viro <viro@...IV.linux.org.uk>
To:	Alex Williamson <alex.williamson@...hat.com>
Cc:	linux-kernel@...r.kernel.org, kvm@...r.kernel.org
Subject: Re: [PATCH] vfio-pci: Use fdget() rather than eventfd_fget()

On Tue, Aug 20, 2013 at 01:18:07PM -0600, Alex Williamson wrote:
> eventfd_fget() tests to see whether the file is an eventfd file, which
> we then immediately pass to eventfd_ctx_fileget(), which again tests
> whether the file is an eventfd file.  Simplify slightly by using
> fdget() so that we only test that we're looking at an eventfd once.
> fget() could also be used, but fdget() makes use of fget_light() for
> another slight optimization.

Umm...

> @@ -210,8 +210,8 @@ fail:
>  	if (ctx && !IS_ERR(ctx))
>  		eventfd_ctx_put(ctx);
>  
> -	if (file && !IS_ERR(file))
> -		fput(file);
> +	if (irqfd.file)
> +		fdput(irqfd);
>  
>  	kfree(virqfd);

IMO it's a bad style; you have three failure exits leading here, and those
ifs are nothing but "how far did we get before we'd failed".

fail3:
	eventfd_ctx_put(ctx);
fail2:
	fdput(irqfd);
fail1:
  	kfree(virqfd);

is much easier to analyse.  It's a very common pattern and IME it's more
robust than this kind of "flexibility"...
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