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Date:   Wed, 21 Jul 2021 10:22:16 -0400
From:   Alan Stern <stern@rowland.harvard.edu>
To:     dave penkler <dpenkler@gmail.com>
Cc:     Greg KH <gregkh@linuxfoundation.org>, qiang.zhang@windriver.com,
        Dmitry Vyukov <dvyukov@google.com>, paulmck@kernel.org,
        Guido <guido.kiener@rohde-schwarz.com>,
        USB <linux-usb@vger.kernel.org>
Subject: Re: [PATCH] USB: usbtmc: Fix RCU stall warning
Message-ID: <20210721142216.GA633399@rowland.harvard.edu>
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On Wed, Jul 21, 2021 at 11:44:23AM +0200, dave penkler wrote:
> Sorry, the issue this patch is trying to fix occurs because the
> current usbtmc driver resubmits the URB when it gets an EPROTO return.
> The dummy usb host controller driver used in the syzbot tests keeps
> returning the resubmitted URB with EPROTO causing a loop that starves
> RCU. With an actual HCI driver it either recovers or returns an EPIPE.

Are you sure about that?  Have you ever observed a usbtmc device recovering 
and continuing to operate after an EPROTO error?

An EPIPE error also seems rather unlikely -- particularly if the device is 
not plugged into a high-speed hub.

Alan Stern

> In either case no loop occurs. So for my part as a user and maintainer
> this patch is not ok.