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Date:   Mon, 5 Oct 2020 21:23:33 -0400
From:   Alan Stern <stern@rowland.harvard.edu>
To:     Shuah Khan <skhan@linuxfoundation.org>
Cc:     Andrey Konovalov <andreyknvl@google.com>,
        Greg Kroah-Hartman <gregkh@linuxfoundation.org>,
        Valentina Manea <valentina.manea.m@gmail.com>,
        Shuah Khan <shuah@kernel.org>,
        USB list <linux-usb@vger.kernel.org>,
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        Dmitry Vyukov <dvyukov@google.com>,
        Nazime Hande Harputluoglu <handeharputlu@google.com>,
        syzkaller <syzkaller@googlegroups.com>
Subject: Re: Is usb_hcd_giveback_urb() allowed in task context?
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On Mon, Oct 05, 2020 at 05:38:22PM -0600, Shuah Khan wrote:
> On 10/5/20 9:25 AM, Alan Stern wrote:
> > On Mon, Oct 05, 2020 at 05:21:30PM +0200, Andrey Konovalov wrote:
> > No, no -- it won't work right if it's called in process context.  Not
> > only do the spinlock calls leave the interrupt flag unchanged, also the
> > driver callback routines may expect to be invoked with interrupts
> > disabled.  (We have tried to fix this, but I'm not at all certain that
> > all the cases have been updated.)
> > 
> 
> In the case of vhci case, usb_hcd_giveback_urb() is called from vhci's
> urb_enqueue, when it determines it doesn't need to xmit the urb and can give
> it back. This path runs in task context.
> 
> Do you have any recommendation on how this case can be handled?

Just call local_irq_disable() before usb_hcd_giveback_urb(), and 
local_irq_enable() afterward.

Alan Stern