netdev - Re: [PATCH 01/12] usbnet: introduce usbnet 3 command helpers

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Date:	Wed, 10 Oct 2012 07:51:37 +0200
From:	Oliver Neukum <oneukum@...e.de>
To:	Ming Lei <ming.lei@...onical.com>
Cc:	"David S. Miller" <davem@...emloft.net>,
	Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
	netdev@...r.kernel.org, linux-usb@...r.kernel.org
Subject: Re: [PATCH 01/12] usbnet: introduce usbnet 3 command helpers

On Wednesday 10 October 2012 11:19:09 Ming Lei wrote:
> On Tue, Oct 9, 2012 at 4:47 PM, Oliver Neukum <oneukum@...e.de> wrote:
> >
> > Using GFP_KERNEL you preclude using those in resume() and error handling.
> > Please pass a gfp_t parameter.
> 
> IMO, it is not a big deal because generally only several bytes are to be
> allocated inside these helpers.

Any allocation can do it, if the VM layer decides to block.

> If you still think the problem should be considered, another candidate fix
> is to take GFP_NOIO during system suspend/resume, and take GFP_KERNEL
> in other situations.

No, the problem is autoresume.

Suppose we have a device with two interface. Interface A be usbnet; interface B
something you page on. Now consider that you can only resume both interfaces
and this is (and needs to be) done synchronously.

Now we can have this code path:

autoresume of device -> resume() -> kmalloc(..., GFP_KERNEL) ->
VM layer decides to start paging out -> IO to interface B -> autoresume of device
--> DEADLOCK

We need to use GFP_NOIO in situations the helper cannot know about.
Please add a gfp_t parameter. Then the caller will solve that.

	Regards
		Oliver

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