linux-kernel - Re: [PATCH] usb: hub: wrong order of putting interfaces when processing hub events

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Date:	Fri, 12 Sep 2014 10:06:03 -0400 (EDT)
From:	Alan Stern <stern@...land.harvard.edu>
To:	Petr Mladek <pmladek@...e.cz>
cc:	Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
	Tejun Heo <tj@...nel.org>,
	Joe Lawrence <joe.lawrence@...atus.com>,
	Jiri Kosina <jkosina@...e.cz>, <linux-usb@...r.kernel.org>,
	<linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] usb: hub: wrong order of putting interfaces when processing
 hub events

On Fri, 12 Sep 2014, Petr Mladek wrote:

> usb_autopm_put_interface() must be called only when usb_autopm_get_interface()
> succeeded before. There was wrong ordering of the goto targets.

No, you are wrong.  The code is correct as it stands.

> The funny thing is that the names of the goto targets and the related comments
> were correct.

That's because the whole thing is correct.  Go back and read the 
comments more carefully -- especially the comment following the "loop:" 
label.

> Signed-off-by: Petr Mladek <pmladek@...e.cz>
> ---
>  drivers/usb/core/hub.c | 4 ++--
>  1 file changed, 2 insertions(+), 2 deletions(-)
> 
> diff --git a/drivers/usb/core/hub.c b/drivers/usb/core/hub.c
> index d481c99a20d7..6afd79ee3340 100644
> --- a/drivers/usb/core/hub.c
> +++ b/drivers/usb/core/hub.c
> @@ -5132,12 +5132,12 @@ static void hub_events(void)
>  
>   loop_autopm:
>  		/* Balance the usb_autopm_get_interface() above */
> -		usb_autopm_put_interface_no_suspend(intf);
> +		usb_autopm_put_interface(intf);

There's no point to this change.  Now this call will decrement the
usage counter and try to suspend -- but the suspend is bound to fail
because the usage counter is still > 0.

>   loop:
>  		/* Balance the usb_autopm_get_interface_no_resume() in
>  		 * kick_khubd() and allow autosuspend.
>  		 */
> -		usb_autopm_put_interface(intf);
> +		usb_autopm_put_interface_no_suspend(intf);

And now this call will decrement the usage counter down to 0, but won't 
try to suspend -- so the hub won't autosuspend at all!

>   loop_disconnected:
>  		usb_unlock_device(hdev);
>  		usb_put_dev(hdev);

Alan Stern

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