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

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Date:   Mon, 24 May 2021 14:55:20 -0400
From:   Alan Stern <stern@...land.harvard.edu>
To:     Mathias Nyman <mathias.nyman@...ux.intel.com>
Cc:     Thinh Nguyen <Thinh.Nguyen@...opsys.com>,
        Mathias Nyman <mathias.nyman@...el.com>,
        Guido Kiener <Guido.Kiener@...de-schwarz.com>,
        dave penkler <dpenkler@...il.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: [syzbot] INFO: rcu detected stall in tx

On Mon, May 24, 2021 at 06:18:59PM +0300, Mathias Nyman wrote:
> On 20.5.2021 23.30, Thinh Nguyen wrote:
> > As for the xhci driver, there maybe a case where the stream URB never
> > gets to complete because the transaction err_count is not properly
> > updated. The err_count for transaction error is stored in ep_ring, but
> > the xhci driver may not be able to lookup the correct ep_ring based on
> > TRB address for streams. There are cases for streams where the event
> > TRBs have their TRB pointer field cleared to '0' (xhci spec section
> > 4.12.2). If the xhci driver doesn't see ep_ring for transaction error,
> > it automatically does a soft-retry. This is seen from one of our
> > testings that the driver was repeatedly doing soft-retry until the class
> > driver timed out.
> > 
> > Hi Mathias, maybe you have some comment on this? Thanks.
> 
> This is true, if TRB pointer is 0 then there is no retry limit for soft retry.
> We should add one and prevent a loop. after e few soft resets we can end with a
> hard reset to clear the host side endpoint halt.
> 
> We don't know the URB that was being tansferred during the error, and can't 
> give it back with a proper error code.
> In that sense we still end up waiting for a timeout and someone to cancel
> the urb.

That's not good.  There may not be a timeout; drivers expect transfers 
to complete with a failure, not to be retried indefinitely.

However, if you do know which endpoint/stream the error is connected to, 
you should be able to get the URB.  It will be the first one queued for 
that endpoint/stream.

Alan Stern