Re: 8250: Possible race between console message vs DMA?

[Andy Shevchenko <andy.shevchenko@gmail.com>]

On Fri, Apr 7, 2017 at 2:08 PM, Vignesh R <vigneshr@ti.com> wrote:
> Hi All,
>
> I seem to be hitting a race condition using 8250_dma (and 8250_omap
> specific dma) support:
>
> Kernel writes log messages to console via
> serial8250_console_write()->serial8250_console_putchar() which directly
> accesses UART_TX register with port->lock acquired.
>
> Now, if the same UART instance is being used by systemd/userspace,
> characters are written to UART_TX register by serial8250_tx_chars(). The
> concurrent access by serial8250_console_write() and
> serial8250_tx_chars() is serialized by the use of port->lock spinlock
> and hence there is no issue with` non DMA case.
>
> But when using DMA with 8250 UART, I see that port->lock is held before
> scheduling of DMA TX transfer and released as soon as the transfer is
> submitted. The lock is not held until the transfer actually completes
> See,
> uart_start()
>   ->serial8250_start_tx()->
>      __start_tx()
>        ->up->dma->tx_dma(up)
> Or
> __dma_tx_complete() in 8250_dma.c that acquires and releases port->lock
> once TX DMA transfer is submitted in serial8250_tx_dma()
>
> So, when the port->lock is released, it is quite possible that DMA is
> still transferring data to UART TX FIFO and UART FIFO might be almost full.
> I see that when DMA is writing to UART TX FIFO,
> serial8250_console_write() may also write kernel log messages to UART TX
> FIFO(as port->lock is now free to be acquired), which is leading to
> overflow and lose of data. serial8250_console_write() checks for
> UART_LSR_THRE to check if Transmit hold register is empty but that may
> not be enough as DMA might put data before CPU write.
>
> It seems that both DMA and CPU might simultaneously put data to UART
> FIFO and lead to potential loss of data.
> Is the expectation that UART instance used to print kernel log messages
> is not intended to use DMA? Or am I missing something?
>
>
> Any help appreciated!

I have one patch in my tree for a long time already:
https://bitbucket.org/andy-shev/linux/commits/9f86c648e53bd25b8ec374933764577b2a340468?at=topic/uart/rpm

Besides that I have patch to disable power management on kernel
console (and non-hackish implementation of runtime PM for UART is
there in case you are wondering what that repository for).


-- 
With Best Regards,
Andy Shevchenko