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SubjectRe: 8250: Possible race between console message vs DMA?
On Mon, Apr 10, 2017 at 11:16 AM, Vignesh R <vigneshr@ti.com> wrote:
> On Sunday 09 April 2017 04:37 PM, Andy Shevchenko wrote:
>> 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
>
> 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!

Yes, I'm planning to do so, but be aware that OMAP has its own DMA
glue layer and thus my patch doesn't affect it.

--
With Best Regards,
Andy Shevchenko

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