Re: [syzbot] INFO: rcu detected stall in tx

[Mathias Nyman]

On 24.5.2021 22.23, Thinh Nguyen wrote:
> Alan Stern wrote:
>> 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.
>>
> 
> When the xhci can't recover a transfer with soft-retry, no outstanding
> transfer can proceed/complete for the endpoint. If the TRB pointer is 0,
> we just don't know which stream or endpoint ring it's for, but we know
> all the outstanding URBs of an endpoint. Let's may as well return an
> error status for all of them after a limited number of soft-retries.

We get the endpoint, but not the stream.

I guess we could walk through each stream of this endpoint, and return the 
first URB of every stream that has a pending URB.
xHCI spec claims to supports 65533 streams per endpoint, but in real life 
UAS probably only uses a few per endpoint?

-Mathias