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

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Date:	Wed, 10 Oct 2012 19:02:02 +0800
From:	Ming Lei <ming.lei@...onical.com>
To:	Oliver Neukum <oneukum@...e.de>
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 Wed, Oct 10, 2012 at 6:08 PM, Oliver Neukum <oneukum@...e.de> wrote:

>
> A reset always applies to the whole device. Resets are used in error
> handling of block devices (storage and uas). If you reset a device,
> pre_reset() and post_reset() of all interfaces need to be called. So they
> are part of the SCSI error handler. SCSI error handlers can allocate memory
> only with GFP_NOIO (or GFP_ATOMIC) because any IO for paging
> can cause the SCSI layer to wait for the error handling to finish. The error
> handling can only finish when pre/post_reset() have finished. Catch-22

IMO, it is not practical to obey the rule for drivers, because driver may
call many other kernel component API which may allocate memory
via GFP_KERNEL in the path easily.

Same with runtime PM case.

>
> So any control messages in block error handling need to use GFP_NOIO.
> If you look at the control message helpers in usbcore you will find a lot
> of GFP_NOIO. That is the reason.

If one driver has no .pre_reset or .post_reset, usb_reset_device() will
try to unbind&bind the interface, so you mean these usb drivers have
to use GFP_NOIO to allocate memory in its probe() and disconnect()?
Unfortunately, that is not true, and no way to make sure all memory
allocations in the path via GFP_NOIO, IMO.

Also, only very few drivers have implemented .pre_reset and .post_reset.

Thanks,
--
Ming Lei
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