On Mon, Apr 10, 2017 at 11:16 AM, Vignesh R <vigneshr@xxxxxx> wrote: > 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! 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 -- To unsubscribe from this list: send the line "unsubscribe linux-omap" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
