[PATCH RESEND] serial: imx: Fix potential deadlock on sport->port.lock

[Uwe Kleine-König]

[Cc += Vinod Koul, dmaengine at vger.kernel.org]

Hello,

On Wed, Sep 27, 2023 at 06:19:39PM +0000, Chengfeng Ye wrote:
> As &sport->port.lock is acquired under irq context along the following
> call chain from imx_uart_rtsint(), other acquisition of the same lock
> inside process context or softirq context should disable irq avoid double
> lock.
> 
> <deadlock #1>
> 
> imx_uart_dma_rx_callback()
> --> spin_lock(&sport->port.lock)
> <interrupt>
>    --> imx_uart_rtsint()
>    --> spin_lock(&sport->port.lock)
> 
> This flaw was found by an experimental static analysis tool I am
> developing for irq-related deadlock.

Ah, I understood before that you really experienced that deadlock (or a
lockdep splat). I didn't test anything, but I think the
imx_uart_dma_rx_callback() is called indirectly by
sdma_update_channel_loop() which is called in irq context. I don't know
if this is the case for all dma drivers?!

@Vinod: Maybe you can chime in here: Is a dma callback always called in
irq context?

If yes, this patch isn't needed. Otherwise it might be a good idea to
not use the special knowledge and switch to spin_lock_irqsave() as
suggested.

> To prevent the potential deadlock, the patch uses spin_lock_irqsave()
> on the &sport->port.lock inside imx_uart_dma_rx_callback() to prevent
> the possible deadlock scenario.
> 
> Signed-off-by: Chengfeng Ye <dg573847474 at gmail.com>

If we agree this patch is a good idea, we can add:

Fixes: 496a4471b7c3 ("serial: imx: work-around for hardware RX flood")

Thanks
Uwe

-- 
Pengutronix e.K.                           | Uwe Kleine-König            |
Industrial Linux Solutions                 | https://www.pengutronix.de/ |
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