linux-kernel - Re: [PATCH] usb/core/devio.c: Don't use GFP_KERNEL while we cannot reset a storage device

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Date:	Mon, 11 Mar 2013 12:08:46 -0400 (EDT)
From:	Alan Stern <stern@...land.harvard.edu>
To:	Ming Lei <tom.leiming@...il.com>
cc:	Alexey Khoroshilov <khoroshilov@...ras.ru>,
	Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
	Hans de Goede <hdegoede@...hat.com>,
	<linux-usb@...r.kernel.org>, <linux-kernel@...r.kernel.org>,
	<ldv-project@...uxtesting.org>
Subject: Re: [PATCH] usb/core/devio.c: Don't use GFP_KERNEL while we cannot
 reset a storage device

On Mon, 11 Mar 2013, Ming Lei wrote:

> On Mon, Mar 11, 2013 at 11:32 PM, Alan Stern <stern@...land.harvard.edu> wrote:
> >
> > Of course you have to lock the device before changing its driver.  What
> > would happen if two different threads tried to change a device's driver
> > at the same time?
> 
> Yes, claim/release interface need device lock, but the patch doesn't
> touch claim/release command handling.

Then why did you ask?  You wrote: "Looks device lock isn't required for
USB transfer of kernel driver."


> > usbdev_do_ioctl() needs to acquire the device lock in order to prevent
> > races with driver_disconnect() and usbdev_remove().
> 
> Looks the patch basically converts the allocation inside URB submit path,
> and actually I mean why we need to hold device lock in submitting
> URB path?  Device lock isn't required before submitting URBs
> in kernel driver.

In general it isn't, no.  But usbfs uses the lock to prevent races with 
driver_disconnect() -- it is invalid to submit URBs after the 
disconnect routine has returned.

Alan Stern

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