[RFC] dmaengine: dw: Prevent tx-status calling desc callback (Fix UART deadlock!)

The dmaengine_tx_status() method updating the DMA-descriptors state and
eventually calling the Tx-descriptors callback may potentially cause
problems. In particular the deadlock was discovered in DW UART 8250 device
interacting with DW DMA controller channels. The call-trace causing the
deadlock is:

serial8250_handle_irq()
  uart_port_lock_irqsave(port); ----------------------+
  handle_rx_dma()                                     |
    serial8250_rx_dma_flush()                         |
      __dma_rx_complete()                             |
        dmaengine_tx_status()                         |
          dwc_scan_descriptors()                      |
            dwc_complete_all()                        |
              dwc_descriptor_complete()               |
                dmaengine_desc_callback_invoke()      |
                  cb->callback(cb->callback_param);   |
                  ||                                  |
                  dma_rx_complete();                  |
                    uart_port_lock_irqsave(port); ----+ <- Deadlock!

So in case if the DMA-engine finished working at some point before the
serial8250_rx_dma_flush() invocation and the respective tasklet hasn't
been executed yet, then calling the dmaengine_tx_status() will cause the
DMA-descriptors status update and the Tx-descriptor callback invocation.
Generalizing the case up: if the dmaengine_tx_status() method callee and
the Tx-descriptor callback refer to the related critical section, then
calling dmaengine_tx_status() from the Tx-descriptor callback will
inevitably cause a deadlock around the guarding lock as it happens in the
Serial 8250 DMA implementation above. (Note the deadlock doesn't happen
very often, but can be eventually discovered if the being received data
size is greater than the Rx DMA-buffer size defined in the 8250_dma.c
driver. In my case reducing the Rx DMA-buffer size increased the deadlock
probability.)

The easiest way to fix the deadlock was to just remove the Tx-descriptors
state update from the DW DMA-engine Tx-descriptor status method
implementation, as the most of the DMA-engine drivers imply. After this
fix is applied the Tx-descriptors status will be only updated in the
framework of the dwc_scan_descriptors() method called from the tasklet
handling the deferred DMA-controller IRQ.

Signed-off-by: Serge Semin <fancer.lancer@gmail.com

---

Note I have doubts whether it's the best possible solution of the problem
since the client-driver deadlock is resolved here by fixing the DMA-engine
provider code. But I failed to find any reasonable solution in the 8250
DMA implementation.

Moreover the suggested fix cause a weird outcome - under the high-speed
and heavy serial transfers the next error is printed to the log sometimes:

> dma dma0chan0: BUG: XFER bit set, but channel not idle!

That's why the patch submitted as RFC. Do you have any better idea in mind
to prevent the nested lock?

Cc: Viresh Kumar <vireshk@kernel.org>
Cc: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
CC: Andy Shevchenko <andy@kernel.org>
Cc: Vinod Koul <vkoul@kernel.org>
Cc: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Cc: Jiri Slaby <jirislaby@kernel.org>
Cc: "Ilpo Järvinen" <ilpo.jarvinen@linux.intel.com>
Cc: dmaengine@vger.kernel.org
Cc: linux-serial@vger.kernel.org
Cc: linux-kernel@vger.kernel.org
---
 drivers/dma/dw/core.c | 6 ------
 1 file changed, 6 deletions(-)

On Fri, Aug 23, 2024 at 12:48 PM Serge Semin <fancer.lancer@gmail.com> wrote:
>
> Hi folks
>
> Any comments or suggestion about the change? The kernel occasionally
> _deadlocks_ without it for the DW UART + DW DMAC hardware setup.

I have no time to look at that, but FWIW with a stress tests on older
machines I have seen something similar from time to time (less than
10% reproducibility ration IIRC).

P.S. Is there is any possibility to have a step-by-step reproducer?
Also can we utilise (and update if needed) the open source project
https://github.com/cbrake/linux-serial-test?

Hi Andy

On Fri, Aug 23, 2024 at 04:21:24PM +0300, Andy Shevchenko wrote:
> On Fri, Aug 23, 2024 at 12:48 PM Serge Semin <fancer.lancer@gmail.com> wrote:
> >
> > Hi folks
> >
> > Any comments or suggestion about the change? The kernel occasionally
> > _deadlocks_ without it for the DW UART + DW DMAC hardware setup.
> 
> I have no time to look at that, but FWIW with a stress tests on older
> machines I have seen something similar from time to time (less than
> 10% reproducibility ration IIRC).

Thanks for the response. I also used to have the system hanging up at
the very rare occasion, but after I decreased the size of the Rx
DMA-buffer (to fix a platform-specific problem) and sped up the port
the deadlock probability dramatically increased so I managed to debug
the hanging ups.

> 
> P.S. Is there is any possibility to have a step-by-step reproducer?

There is a good chance that my approach might be platform-specific
(but the problem is general for sure), but here is what I did to
make the deadlock reproducible at the reasonable time:
1. Revert the patch in the subject (if it's applied)
2. Decrease the Rx DMA-buffer size:
drivers/tty/serial/8250/8250_dma.c: dma->rx_size = SZ_512;
3. Increase the serial communication baud-rate:
stty -F /dev/ttyS1 1500000 raw -echo -echok -echoe;
4. Loopback the ttyS1 interface: connect Tx and Rx pins.
5. Start pushing data to the ttS1 interface by the chunks someway
greater than 512 bytes. Like this:
while :; do echo -n "-"; head -c 65536 /dev/zero > /dev/ttyS1; done

In my case the system almost always hangs up after 10-50-100-200
iterations of the one-liner above.

> Also can we utilise (and update if needed) the open source project
> https://github.com/cbrake/linux-serial-test?

I guess the utility can be used to reproduce the problem, but the
data integrity check wasn't required in my case. I am also not sure
whether the loopback-test is required to reproduce the denoted
deadlock, since only the Rx code-path causes it. So most likely the
heavy inbound traffic shall be enough.

-Serge(y)

> 
> -- 
> With Best Regards,
> Andy Shevchenko