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

[Vignesh R]

On Sunday 09 April 2017 04:37 PM, Andy Shevchenko wrote:
> On Fri, Apr 7, 2017 at 2:08 PM, Vignesh R <vigneshr@xxxxxx> 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

I had similar patch in mind. Do you plan to submit above patch to the
mailing list? You may also consider to add the issue I mentioned above
to the commit description. Thanks!

> 
> 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).
> 
Nice!

-- 
Regards
Vignesh