linux-kernel - Re: Re: Re: Re: Re: [syzbot] INFO: rcu detected stall in tx

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Message-ID: <20210519143613.GA1165692@rowland.harvard.edu>
Date:   Wed, 19 May 2021 10:36:13 -0400
From:   Alan Stern <stern@...land.harvard.edu>
To:     dave penkler <dpenkler@...il.com>
Cc:     Guido Kiener <Guido.Kiener@...de-schwarz.com>,
        Dmitry Vyukov <dvyukov@...gle.com>,
        syzbot <syzbot+e2eae5639e7203360018@...kaller.appspotmail.com>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        "lee.jones@...aro.org" <lee.jones@...aro.org>,
        USB list <linux-usb@...r.kernel.org>,
        "bp@...en8.de" <bp@...en8.de>,
        "dwmw@...zon.co.uk" <dwmw@...zon.co.uk>,
        "hpa@...or.com" <hpa@...or.com>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "luto@...nel.org" <luto@...nel.org>,
        "mingo@...hat.com" <mingo@...hat.com>,
        "syzkaller-bugs@...glegroups.com" <syzkaller-bugs@...glegroups.com>,
        "tglx@...utronix.de" <tglx@...utronix.de>,
        "x86@...nel.org" <x86@...nel.org>
Subject: Re: Re: Re: Re: Re: [syzbot] INFO: rcu detected stall in tx

On Wed, May 19, 2021 at 10:48:29AM +0200, dave penkler wrote:
> On Sat, 8 May 2021 at 16:29, Alan Stern <stern@...land.harvard.edu> wrote:
> >
> > On Sat, May 08, 2021 at 10:14:41AM +0200, dave penkler wrote:
> > > When the host driver detects a protocol error while processing an URB
> > > it completes the URB with EPROTO status and marks the endpoint as
> > > halted.
> >
> > Not true.  It does not mark the endpoint as halted, not unless it
> > receives a STALL handshake from the device.  A STALL is not a protocol
> > error.
> >
> > > When the class driver resubmits the URB and the if the host driver
> > > finds the endpoint still marked as halted it should return EPIPE
> > > status on the resubmitted URB
> >
> > Irrelevant.
> Not at all. The point is that when an application is talking to an
> instrument over the usbtmc driver, the underlying host controller and
> its driver will detect and silence a babbling endpoint.

No, they won't.  That is, they will detect a babble error and return an 
error status, but they won't silence the endpoint.  What makes you think 
they will?

> Hence no EPROTO loop will ensue in this case and therefore no changes
> are needed in usbtmc.

Since this conclusion relies on the incorrect assumption above, it also 
is incorrect.

Alan Stern