linux-kernel - Re: [Xen-devel] [PATCH] PCI: Mark expected switch fall-throughs

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Date:   Mon, 25 Mar 2019 07:25:09 -0600
From:   "Jan Beulich" <JBeulich@...e.com>
To:     "Gustavo A.R.Silva" <gustavo@...eddedor.com>
Cc:     <keescook@...omium.org>, "Bjorn Helgaas" <bhelgaas@...gle.com>,
        "Stefano Stabellini" <sstabellini@...nel.org>,
        "xen-devel" <xen-devel@...ts.xenproject.org>,
        "Boris Ostrovsky" <boris.ostrovsky@...cle.com>,
        "Konrad Rzeszutek Wilk" <konrad.wilk@...cle.com>,
        "Juergen Gross" <jgross@...e.com>, <linux-kernel@...r.kernel.org>,
        <linux-pci@...r.kernel.org>
Subject: Re: [Xen-devel] [PATCH] PCI: Mark expected switch
 fall-throughs

>>> On 20.03.19 at 19:27, <gustavo@...eddedor.com> wrote:
> In preparation to enabling -Wimplicit-fallthrough, mark switch
> cases where we are expecting to fall through.
> 
> This patch fixes the following warnings:
> 
> drivers/pci/proc.c: In function ‘proc_bus_pci_ioctl’:
> drivers/pci/proc.c:216:6: warning: this statement may fall through 
> [-Wimplicit-fallthrough=]
>    if (arch_can_pci_mmap_wc()) {
>       ^
> drivers/pci/proc.c:225:2: note: here
>   default:
>   ^~~~~~~
> 
> drivers/pci/xen-pcifront.c: In function ‘pcifront_backend_changed’:
> drivers/pci/xen-pcifront.c:1105:6: warning: this statement may fall through 
> [-Wimplicit-fallthrough=]
>    if (xdev->state == XenbusStateClosed)
>       ^
> drivers/pci/xen-pcifront.c:1108:2: note: here
>   case XenbusStateClosing:
>   ^~~~
> 
> Notice that, in this particular case, the /* fall through */
> comment is placed at the very bottom of the case statement,
> which is what GCC is expecting to find.
> 
> Warning level 3 was used: -Wimplicit-fallthrough=3
> 
> This patch is part of the ongoing efforts to enable
> -Wimplicit-fallthrough.

None of the above explains why ...

> --- a/drivers/pci/xen-pcifront.c
> +++ b/drivers/pci/xen-pcifront.c
> @@ -1104,7 +1104,7 @@ static void __ref pcifront_backend_changed(struct xenbus_device *xdev,
>  	case XenbusStateClosed:
>  		if (xdev->state == XenbusStateClosed)
>  			break;
> -		/* Missed the backend's CLOSING state -- fallthrough */
> +		/* fall through - Missed the backend's CLOSING state. */

... the original comment here wasn't good enough.

Jan