From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1754576AbdDRGTP (ORCPT ); Tue, 18 Apr 2017 02:19:15 -0400 Received: from fllnx210.ext.ti.com ([198.47.19.17]:11902 "EHLO fllnx210.ext.ti.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1751675AbdDRGTN (ORCPT ); Tue, 18 Apr 2017 02:19:13 -0400 Subject: Re: 8250: Possible race between console message vs DMA? To: Andy Shevchenko References: <7c75614c-71cb-0a4e-222c-713f65a89f23@ti.com> <69f52038-f0e6-6f56-4f0b-842021152e2c@ti.com> CC: "linux-serial@vger.kernel.org" , Peter Hurley , Greg Kroah-Hartman , "linux-omap@vger.kernel.org" , "linux-kernel@vger.kernel.org" From: Vignesh R Message-ID: Date: Tue, 18 Apr 2017 11:49:21 +0530 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:45.0) Gecko/20100101 Thunderbird/45.8.0 MIME-Version: 1.0 In-Reply-To: Content-Type: text/plain; charset="utf-8" Content-Transfer-Encoding: 7bit Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Tuesday 18 April 2017 11:46 AM, Andy Shevchenko wrote: > On Mon, Apr 10, 2017 at 11:16 AM, Vignesh R wrote: >> On Sunday 09 April 2017 04:37 PM, Andy Shevchenko wrote: >>> On Fri, Apr 7, 2017 at 2:08 PM, Vignesh R 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. > Yes, I am working on a patch for 8250_omap driver. Thanks! -- Regards Vignesh